Central Park’s Woodlands: Forest Community Changes after a Decade of Management
Nyssa sylvatica in four seasons, at Tupelo Meadow in the Ramble.
Alex Hodges Submitted in partial fulfillment of the Master of Natural Resources degree Oregon State University Fall 2020
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ABSTRACT
The management of the natural landscapes of New York City’s Central Park has varied over the years, but recently the Central Park Conservancy has begun to make a concerted effort to consolidate these naturalistic landscapes within one cohesive management unit. Much of the management of the plant community involves systematic removal of invasive species, planting and seeding of native species, and encouragement of native plant population regeneration and diversity in order to improve various aesthetic, cultural, community, and ecological values. The resulting Natural Areas management section currently includes the Hallett Sanctuary (a small ~4 acre ‘island’ of woodland in the southern end of the park), the Ramble (a ~36 acre woodland near the center of the park), the North Woods (a ~ 38 acre woodland in the far north section of the park) and the Dene Slope native meadow (the only non-woodland ‘natural area’, converted from lawn in 2016). Ongoing management and capital projects, which includes path/trail improvements as well as plantings and invasive species removal, currently utilizes 13 full-time employees as well as seasonal interns and weekly volunteer groups. The potential addition in the near future of another area, the Great Hill (which is adjacent to the North Woods) could expand the Natural Areas further. There are few studies which quantify the species composition of these woodlands. One such study, by Dr. Regina Alvarez nearly a decade ago (Alvarez 2012), produced valuable baseline diversity data and pointed out which woody species, both native and non-native, are most prolific in different size classes. Species such as oaks (Quercus spp.) were found to be well represented in higher size classes but not in lower size classes, and non-native species such as Ailanthus altisimma, Acer platanoides and Acer pseudoplatanus were heavily represented in lower size classes. Species such as Phellodendron amurense and Styphnolobium japonicum were also identified as showing increased recruitment, with their continual monitoring for invasive qualities suggested as well. This study aims to measure changes in the species composition of these woodlands’ woody plants greater than 1cm diameter at breast height (DBH) over the last decade, as well as determine the efficacy of management efforts over that period by applying the same point-centered quarter survey method utilized in the Alvarez study. This comprehensive survey spanned each of the three designated woodlands in the park, as well as the Great Hill. Data was collected from 1480 woody plants in the designated woodlands, with 119 species from 67 genera identified. This is compared to 1272 individuals sampled from the Alvarez study, which yielded 82 species from 50 genera. In addition to the observed increase in richness, shifts in the dominant species of the entire sample population were also noted. The primacy of Prunus serotina remains but is less pronounced, and when grouped by genus, Quercus jumped ahead of Prunus in comparison to the last survey when comparing Importance Values. In the lower quartile size range (roughly between 1cm and 2.4cm DBH, depending on the specific woodland), significantly less invasive species were found, though certain non-native and/or invasive species had increased in this size class since the last survey. From the Great Hill, 384 woody plants were tallied which included 44 species from 30 genera. While the dominant species (as measured by Importance Value) were largely native, there was a noticeably higher component of invasive species than in the other 3 woodlands. The difference in the lower quartile size range was also significant in comparison to the other landscapes, with a much larger component of invasive species, as well as noticeably lower regeneration from some dominant canopy species. Overall, these results reflect a decade of successful active management, for which the intention/need was outlined in the Alvarez study. The structure of the woody species community in these woodlands has changed, largely along the intended trajectory: increased overall diversity, increased regeneration of oaks, and decreased presence of specific invasives in particular size classes, among other insights. Another major aspect of this study is the application of this sampling method to the woodland landscapes of the Great Hill, which establishes a baseline of data describing this landscape’s structure from which to measure future management actions. The insights from the second survey of the Ramble, North Woods and Hallett also provide a measure of the efficacy of the Natural Areas team’s efforts, which creates a reasonable expectation for potential management goals at the Great Hill.
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Table of Contents Abstract…………...………………………………………………………………………………………………………………………………………………..…….....2 Table of Contents ……………………………………………………………………………………………………………………………………………….……....…3
Acknowledgements ….…………………………………………………………………………………………………………………………………….....….……..…5
Introduction ………..………………………………………………………………………………………………………………………………………...…………….6
Research Intent……………………………………………………………………………………………………………………………………...…….….7
Background ……………………………….…………………………………………………………...... …………………….…………..8
Historical Context……………………………………………………………………………………………….……………...………………….…………8
Geographic Context….…………………………………………………………………………………………….…………………………………...…...10
Site Locations ……………………………………………………………………………..…………………..…………………..……………10
Climate……………………………………....……………………………………………………………………………………….…………10
Topography/Hydrology/Soils……………………………………………………………………………………………………………...……10
Vegetation……………………...….……………………………………………………………...…………………………………….………12
Social Context ………………………...…………………………………………………………………………...………………………………………...13
Management Philosophy and Goals …….…………………………………………………………………………..……………………………………..15
Methods ……………………..………………………………………………………………………………………………………….…………………………………18
Concerning Species Nomenclature ………...………………………………………………………………………..……………………………………..24
Data Analysis …………………...………………………………………………………………………………………………………………………………………..26
Importance Value Calculation ...…………………………………...………………………………………………………………………………………26
Diversity Indices …………………………………………………………………………………………………………………………………………….29
Points per Area ……………...…………………………………………………………………………………...…………………………………………29
Results ………………………………...…………………………………………………………………………………………..………………………………………31
Diversity ……………………………………………………………………………………………………………………………………………………..31
Natives vs Exotics …………………………………………………………………………………………………………………………………………...34
Importance Values ………………………………………………………………………………………………………………………………………….35
Total Woodlands ………………………………………………………………………………………………….………………………………………...35
Invasive Species Changes …….……………………………………………………………………..…………………………………..……..36
Native Species Changes …..…………………………………………………………………………….……………………..……………….37
By Genus …...……………………………………………………………………………………………..…………………….……………...38
Lower Quartile Range………...……………………………………………………………………...…………………………...…………….38
Map of areas sampled ………………………..……………………………………………………………………………………..………….40
Importance Value rankings by species …………….…...………………………………………………………………………………………41
Descriptive statistics …..……………….…………………………………………………………………………………………..…..……….44
Importance Value rankings by Genus …………….…………….……………………………………………………………………………...47
Lower quartile range Importance Value rankings by species ………………………………………………………………………………….49
The Ramble ..……………..…………………………………………………………………………………………………………………………………51
Invasive Species Changes ….…………………………………………………………………………………………………………………..51
Native Species Changes ………………………………………………………………………………………………………………………..52
By Genus ……...………………………………………………………………………………………………………………………………..53
Lower Quartile Range ..…………………………………………………………………………………………………………...... ….………53
Ramble sample unit locations ……………………………………………………………………………………..……………...... ………….55
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Importance Value rankings by species ………………………………………………………………………………………………………....56
Descriptive statistics …...…………………………………………………...... ………………………………..59
Importance Value ranking by genus .…………………………………………………………….……………...…………………..…………62
Lower quartile range Importance Value rankings by species …………………………………….……………………...………………….....64
The North Woods ………...……………………………………………………………………………..…………………………………..………………66
Invasive Species Changes ….……………....…………………………………………………………………………………..………………66
Native Species Changes …….………………………………………………………………………………………………………….………67
By Genus .……………………...….……………………………………………………………...……………………………………….……68
Lower Quartile Range ....…………………………………………………………………………...…………………………………...……...69
North Woods sample unit locations …….……………………………………………………………..………………………………...……..71
Importance Value rankings by species ...………………………………………………………………….……………………………………72
Descriptive statistics …………..………………………………………………………………………..………………………………..……..74
Importance Value ranking by genus ……………………………………………………………………………………………………….…..76
Lower quartile range Importance Value rankings by species …..……………………………………………………………………...………78
The Hallett Sanctuary ………..…………………………………………………………………………………………………………………………….79
Invasive Species Changes ……...……………………………………………………………………………………………………..………..80
Native Species Changes .…………………………………………………………………………...………………………………………..…81
By Genus …..…………………………………………………………………………………………..………………………………….……82
Hallett sample unit locations ……………………………………………………………………….……………………………………...... 83
Importance Value ranking by species …….………………………………………………………..…………………………………………..84
Descriptive statistics …..………………………..……………………………………………………….…………..………………………….86
Importance Value ranking by genus ……..……………………………………………………………..……………………………………...87
The Great Hill: Initial Findings ….………...……………………………………………………………………...……………………………………….88
Invasive Species ……..………………………..………………………………………………………………………………….…………….89
Native Species ……………………………………….…...……………………………………………………………………….……………89
By Genus …………..…..……………….…………………………………………………………………………………………...………….90
Lower Quartile Range …………………………….…………….……………………………………………………………………………...91
The Great Hill: Recommendations …………………………………….………………………………………………………………………………….92
Great Hill sample locations .……………………………………………………………………………………………………………………94
Importance Value ranking by species ..………….……………………………………………………………………………………………..95
Descriptive statistics …..………………………………………………………………………………………………………………………..97
Importance Value ranking by genus .…………………………………………………………………………………………………………..99
Lower quartile Importance Value ranking ……………………………………………………………………………………………………100
Discussion ………………………………………………………………………………………………………………………………………………………...……...101
Recommendations .………………………………………………………………………………………………………………………………………………..…….104
Conclusions ……...………………………………………………………………………………………………………………………………………………………106
Works Cited ……………………………………………………………………………………………………………………………………………………………..107
Appendix A. Historic Maps of the Great Hill site …………………………………………………………………………………………………………………….111
Appendix B. Historic maps of Central Park …………………………………………………………………………………………………………………………..112
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Acknowledgements
This study would not have been nearly as effective at demonstrating the impacts of management in the woodlands if it were not for Regina Alvarez’ previous study of the Ecology of the Woodlands of Central Park. As such, I owe my primary gratitude to Regina, both through the documentation of her survey as well as putting the Conservancy on the path towards effective and cohesive ecological management. Likewise, John Paul Catusco and now Eric Whitaker picked up where Regina left off, and all three of them have served as mentors to me in the years since I began working in the park. I must thank Eric especially, as well as John Dillon and Gary Gentilucci, for approving of my work on this project during this past summer. Jack Rosacker was also invaluable in helping re- orient me to GIS and the park’s base map, and Fabrice Rochelemagne’s insights about tree care in the woodlands has always been appreciated, as well as that of all the Tree Crew members.
I must also acknowledge the constant support of Paul Ries, my advisor at Oregon State University. He has cheerfully and reliably shepherded me through the process of online graduate school, from my first class years ago in Arboriculture to completing my Graduate Certificate in Urban Forestry, all the way to the completion of the Master of Natural Resources program. I could always count on Dr. Ries to add clarity when I was confused or to give me confidence when I lacked it. Likewise, I also thank my committee members Meg Krawchuk and John Lambrinos. Their experience in the fields of ecology and horticulture is invaluable, and I can only hope to one day hold something close to their level of expertise. The entire OSU community was very supportive, and I can’t thank them enough.
I must of course thank all of my coworkers who not only helped me on this project, but who support me and each other in our work in the park every day. Phil Croteau, Ethan Kibbe and Lisa Kozlowski were all instrumental in helping collect data in the North Woods and Great Hill, as well as keeping me on track near the end of a long day. Aubrey Carter, Carlos Olivares, and Jerry Heinzen were each helpful in the Ramble, and James Mohn was also nice enough to lend a hand in all three of the aforementioned landscapes. His willingness to have questions bounced off of him throughout the day was perhaps even more appreciated. Also, while they might not have directly helped with the survey, Kennes Dingle, Gabriel Wilder, and Desmond Ngiam all helped spread the load in the park when my work on this project began to increase. Thomas Kain’s input and observations concerning the Great Hill was also appreciated. All of the aforementioned people, as well as numerous others (including our intrepid volunteers), are responsible for the work that has produced the outcomes observed in this study.
I would like also to show appreciation to my mother Lisa Hodges for supporting me continuously in everything I do. She will undoubtedly be satisfied that I now have more time to visit home after completing this program, and I must say I feel the same.
Last but not least, I must thank my biggest helper, supporter, and partner, Mimi Gunderson. She helped me sample over half of all points in the survey and helped give me confidence to go through with it in general. Her knowledge, good humor and tenacity are just a few traits that I was glad to be able to make use of. She was the only person to help in all 4 landscapes, even spending every single weekend over the summer with me in the park, as well as helping me stay sane during quarantine (and in general). On this project as in life, I truly could not have done it without her.
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INTRODUCTION
The woodlands of Central Park are unique landscapes that exhibit many ‘natural’ qualities which New Yorkers seek for respite from the urban environment. Different from the traditional ‘urban forest’ where trees are integrated with infrastructure to augment public spaces, these woodlands are intended to produce an impression that the visitor has entered a space entirely separate from the infrastructure of the city, even that the landscape is unmanaged or otherwise unplanned. While this effect is intentional, it can also mask the planning and management input involved in creating a naturalistic and diverse landscape in an urban setting. The park has been heavily designed since its inception, and the woodlands are no exception. However, periods of neglect and decline that impacted the park as a whole in previous decades were also particularly impactful on the woodlands. Various invasive species, some of which were deliberately planted by park gardeners, proliferated in landscapes continually disturbed by foot traffic, bikes, motor vehicles, vandalism, etc. Certain native species also proliferated in this state of constant disturbance, but in general the intention of the woodlands as peaceful sanctuaries from urban life, both for humans as well as wildlife, was compromised, as was the ability of certain native species to survive and thrive. The Central Park Conservancy, established in 1980, aimed to set the park on a trajectory of restoration and stewardship, with the guiding vision being the original design and intention set forth in the Greensward Plan by designers Frederick Law Olmsted and Calvert Vaux. Within the Conservancy is the Natural Areas team of landscape professionals who restore and maintain the four currently designated Natural Areas: The North Woods, the Ramble, the Hallett Sanctuary, and the Dene Slope Native Meadow (See Figure 1 at left, as well as Figure 5 on pg. 39).
The primary directives of the Natural Areas team are to remove invasive species, promote native species regeneration and diversity, and re-introduce suitable native plant species, some of which were once thought to have existed on the site of the Park but have subsequently been extirpated. Fallen trees and woody debris is generally left in the landscape, and hazard trees are topped and left as snags for wildlife habitat, rather than removing them completely as in other areas of the park. The topography of the Ramble was heavily manipulated and originally intended as a highly managed woodland garden. The North Woods in the northern end of the park, as well as Figure 1.) The 3 designated the adjacent Great Hill, was intended to remain similar to the Natural Areas woodlands of woodlands they once were before they were cut for firewood and Central Park, NYC: The Hallett, other land clearing efforts in the 18th and 19th centuries. The Hallett Ramble, and North Woods. Sanctuary, a 4-acre woodland located in the far southeastern corner of
6 the park, was constructed of dredged material from what became the 59th St Pond and was not originally intended to be a natural area, but after being closed off to the public between the years of 1934 and 2001 it slowly became populated with volunteer vegetation and was eventually considered one of New York City’s Forever Wild sites (NYC Parks, 2000). After over a decade of restoration efforts the Sanctuary was re-opened for daily use to the public in 2016. The Natural Areas are meant to allow for and showcase native species, harbor wildlife, and provide ecosystems services similar to what might have existed in Manhattan pre-development. These habitats in turn provide value to park users by exposing visitors to landscapes dominated by diverse flora and fauna native to the region, in addition to providing a relaxing and scenic environment.
Research Intent In the nearly 6 years since I began working with the Central Park Conservancy in the Natural Areas of Central Park (called the Woodlands until the 2018 inclusion of the Dene Slope and various water bodies) I have witnessed gradual but substantial changes in the composition of the park’s woodlands, due in large part to management actions but also influenced by biotic factors and shifts in park usage. These changes have occurred in both the herbaceous understory as well as the overstory and shrub-layer. Management efforts are primarily concentrated on the minimization of negative impacts from over-use (social trails and soil compaction, littering, vandalism, etc.) and maximization of the diversity of plants native to the greater New York City Region (roughly within 50 miles of Manhattan), including the selective and/or phased removal of species deemed invasive. The change in the woodlands over a relatively short period of time is noticeable to long-time staff members, but to many casual park users, and occasionally even others within the Conservancy, these changes might seem abstract or difficult to notice. To better relate the nature of these changes to various stakeholders, as well as provide evidence of the impact of sustained, cohesive management efforts, it became a necessity to quantify these changes in some way. The Conservancy’s previous Director of Horticulture and Woodland Management, Dr. Regina Alvarez, previously conducted a study of the ecology of the park’s woodlands, submitted as her doctoral dissertation nearly a decade ago (Alvarez, 2012). A heavy component of this study was a survey of woody plants, utilizing the point centered quarter method of sampling. This survey was the first of its kind in the park, as previously only large tree (greater than 6” diameter) censuses (Rogers, 1987) or comprehensive vascular plant species counts (Rawolle and Pilat, 1857, DeCandido et al., 2007, and more recently Atha et al., 2020) had been undertaken, but not a systematic ecological survey including saplings focused on the woodlands of the park. Dr Alvarez’s study obtained data from random population samples along transects from the three designated woodlands of the park (The Ramble, North Woods, and Hallett Sanctuary), and used these data to rank each species by their corresponding Importance Values. That survey represents a baseline of ecological data that captures a snapshot in time of the composition of the park’s woodlands. It was also intended to help the park’s woodland managers make ecologically oriented decisions, with the primary goal of increasing native plant diversity. Alvarez notes in her study that repetition of similar studies in the future would be needed to determine the impact of future management on the woodlands’ woody species composition of. The relatively high
7 presence of invasive species and depressed presence of oaks in the lower size range, the overall continued dominance of the native pioneer species Prunus serotina, and the need for monitoring the presence of certain non-native species in the event they should be managed as invasives were identified as questions whose answers could be informed by future studies such as this one. This study aims to replicate the Alverez survey as closely as practical, and in doing so obtain a dataset that can be easily compared to the previous survey. Comparative changes from the previous study in the taxa with the highest Importance Values (Importance Values are a proxy for relative species influence in a landscape and are described in detail in the Data Analysis section, pg. 26), in the composition of the lower quartile size/diameter range (LQR) of specimens, and in various diversity metrics such as richness and evenness could provide insights into the progress towards completion of management goals. Another large consideration was to design a survey that could be easily repeated in the future, using relatively affordable and accessible technologies. Finally, the potential inclusion of the Great Hill, long a wooded landscape but not formally designated as such, under the umbrella of the Natural Areas management structure offers the opportunity here to conduct a preliminary survey by which to measure future management efforts. These ecological data, along with the comparative changes shown in the Natural Areas over the last decade, could help create realistic goals and expectations for management and restoration of this landscape in the future.
BACKGROUND Historical Context The current designated woodlands of Central Park occupy a total of approximately 80 of the park’s 843 acres. Much of the park’s land was originally low-lying and swampy, with small streams, seeps, and even tidal marshes punctuated by rocky outcrops. This topography meant that much of the land was difficult to develop, though there were at least 1,600 recorded residents and 40 permanent structures (Barnard, 2016, Rosenweig & Blackmar, 1992) within the park before they were forced to leave or be bought out when the land was seized through eminent domain and razed in 1855. One settlement, Seneca Village, was a primarily Black community with over 250 tax-paying residents and multiple schools, churches, and cemeteries (Martin, 1997). Though a primary aim of the park’s creation was to facilitate shared use and recreation by all social classes (Miller, 2003), it is important to remember the context in which the park was created.
By at least the end of the Revolutionary War, during which time British forces occupied the city, the park and most of northern Manhattan had been completely cleared of all trees. George Washington, in observing the island of Manhattan from the New Jersey Palisades in 1781, noted: ‘The island is totally stripped of trees and wood of every kind; but low bushes (apparently as high as a man’s waist) appear in places which were covered in wood in 1776’ (Barnard, 2016). During the Revolutionary War and the following War of 1812, much of what is now the North Woods, Great Hill and Forts landscapes was occupied by armed forces, with the surrounding ridges and valleys cleared to maintain strategic sightlines.
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The construction of the park by its designers Frederick Law Olmsted and Calvert Vaux, who called themselves ‘land improvers’, marked considerable alteration in the topography/hydrology of the park (Miller, 2003). Most accounts of the park’s inception describe it as wholly constructed from the genius of its creators, but the creators themselves devised their winning design, the Greensward Plan, to use the underlying natural features of the land to their advantage, creating ponds and lakes from the four existing streams which ran West to East, and using the dredge material to smooth out meadows (Barnard, 2016). A notable study conducted by Chief Gardener Ignaz Pilat on the grounds of the site in 1857 attempted to catalog all current species before widespread construction and planting (Rawolle and Pilat, 1857). Each landscape, and in particular each current woodland area, was impacted differently by construction. The Ramble, located in the center of the park, is notable as the first landscape to be finished, and was regarded as one of the most popular areas in the park upon its construction. The Ramble was also one of the most modified landscapes in the park, with its soil profile heavily altered through blasting and grading. Rather than an attempt at recreating a native woodland, the Ramble was more of a stylized Victorian version of a woodland (Rogers, 1987), heavily divided by numerous winding paths which altered its hydrology significantly. This constructed landscape also included many horticultural species from around the world in addition to eastern North American species. Exotic tree and shrub specimens like Korean Evodia (Tetradium daniellii), Royal Paulownia (Paulownia tomentosa), Amur Cork Tree (Phellodendron amurense) and Chinese Scholar Tree (Styphnolobium japonicum) are a few such species that park managers monitor and occasionally selectively remove should they become invasive. Each of these species, besides the Paulownia, were also found in the survey sample of this study.
In contrast to the Ramble, the North Woods was one of the least altered landscapes in the park. By the time the City had begun work in the far northern section of the park, a second growth of native forest species had begun that Olmsted and Vaux decided to guide back into a forested landscape, with an 1873 survey showing that the current native tree stock of hickory, oak, linden, and beech were encouraged, though vines like English ivy, clematis and wisteria were planted to tumble over the rocks (Rogers, 1987). Located in the Ravine in the North Woods a water body known as the Loch was created during park construction by widening an existing stream. However, by the 1930’s erosion led to soil from the slopes of the Ravine filling in the Loch to the point that it again resembled a small stream. Recent capital projects to dredge the Loch and repair pathways represent a significant increase in management efforts in the North Woods over the last few years. During this time, large amounts of plant material, including woody species, was also planted both along the watercourse and in adjacent landscapes.
The 4-acre Hallett Sanctuary, located by the Pond in the southwest corner of the park, is different from the other woodlands in that it was never planned to be maintained as woodland. Originally called the Promontory, its intended use was as a foil to the Pond, either to be gazed upon from across the water or stood upon to see views across the park. Wisteria and other exotic species were introduced over the years, with little input of native plants. In 1934 the area was closed off to visitors, though this only perpetuated neglect of the landscape. Much of the Hallett is composed of fill soil excavated from the Pond overlain on an existing rock outcrop, resulting in less than ideal soil structure. By the early 2000’s the Hallett Sanctuary was overrun by Japanese and Chinese wisteria (Wisteria sinensis and W. floribunda) and black cherries, with occasional oak, ash and maple. The Alvarez study noted the dominance of black cherries, and
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Wisteria is still present, though in far smaller amounts. After sustained staff, volunteer, and capital restoration efforts, including re-introduction of a wide variety of native plant species, the Hallett Sanctuary was opened to the public in May 2016, from 10am to dusk.
Geographic Context
Site Locations
The woodlands of Central Park are located in relatively disparate locations. The North Woods and Great Hill landscapes are located in the northwestern portion of the park, roughly between 102nd St and 109th St, with a park drive/roadway separating them. The Ramble is located in roughly the center of the park, from 74th to 79th St., while the Hallett Sanctuary is located at the far southeastern corner of the park between 60th and 62nd St. All of these locations are somewhat distanced from the perimeter of the park, separated either by waterbodies, park drives, or both, so their locations are relatively ‘insulated’ from the dense urban streetscape in comparison to other areas of the park, but they are also quite insulated/distanced from each other as well. This means that visiting both the Ramble and North Woods in a day would be relatively uncommon for a park visitor, and also makes the exchange of species/genetic information between these natural areas less likely as well.
Climate
The climate in New York City is considered humid-subtropical (Kotteck, 2006), which corresponds to USDA Zone 7b in Manhattan (USDA). Rainfall averages between 3 and 4.5 inches each month of the year (US Climate Data), meaning that plant communities are not heavily driven by seasonality of rainfall and are not generally drought tolerant. For example, Alvarez (2012) noted that certain species of Acer and Fraxinus have been heavily impacted by droughts in the past. It should also be noted that New York City sits on a divide of sorts, with the northern latitude bringing cold winters and the coastal effects of the Atlantic Ocean bringing hot and humid summers. Many northern species find themselves at the lower end of their range in NYC, due to climate but also soils given that the region is the southern terminus of historical glaciation. A changing climate might have implications for the future management of the park’s forests, including planting decisions.
Topography/Hydrology/Soils
The topography of each woodland is heavily influenced by different levels of human alteration. The North Woods and Great Hill are generally considered to have been the least altered in terms of topography and hydrology, both having remained more or less in a forested state since before the park was created, save short periods when they were shorn of mature trees. The Loch, despite being a Scottish word for ‘lake’, is a small stream that bisects the North Woods in a sunken area called the Ravine. This stream was historically called Montayne’s
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Rivulet. As was mentioned previously, it was widened considerably during the park’s creation, but erosion, potentially caused by a decrease in understory vegetation from trampling and removal of understory leaf litter/coarse woody debris (Sauer, 1998), caused the artificially widened landscape to become silted in during the early 20th century. A recently completed capital project restored some widened pools to the Loch, while retaining some of the swampy wetland ecosystem that had been established in the decades since it had silted in, with the overall aim of restoring the original design vision of the Loch while retaining and enhancing the existing structural diversity in the landscape.
Besides the Loch, the Great Hill and North Woods consist of generally forested hillsides of up to 25% slope, with occasional exposed Manhattan schist bedrock. The site is located within the Lower Hudson watershed number 02030101 (USGS, 2019). While the Great Hill contains no waterbodies, surface rainwater water does flow to a waterbody to the south called the Pool, which subsequently flows through the Loch. The Loch connects the Pool to the Harlem Meer, which is in turn ultimately connected to tunnels which take the water to the East River.
In terms of soil structure, it is important to note that the New York City area was impacted by the most recent period of glaciation, which often translates to uplands that have relatively thin top layers of organic matter, as well as erratic distribution of weathered material. Manhattan is the southernmost tip of this glaciation effect, meaning it has a soil structure more resembling New England but a climate more resembling Southern New Jersey. A review of data from the National Cooperative Soil Survey provided by the USDA’s Natural Resource Conservation (NRCS, 2020) service shows the woodlands contain Charlton, Chatfield, Hollis, and rock outcrop soil complexes dominating the profile on much of the rolling hillsides. Chatfield and Hollis are both generally characterized by fine sandy loam till over gneiss or schist bedrock, with Hollis being thinner and rockier. Charlton soil, on the other hand, is composed of coarse-loamy supraglacial melt-out till derived from igneous and metamorphic rock (in this case, likely gneiss and/or schist), and usually extends further than 80 inches to bedrock. This complex of soil types is also generally well drained and largely acidic. On the margins of these predominant soil types, near pathways and adjacent horticultural areas, exist varying amounts of Greenbelt soil types. Greenbelt is characterized primarily by human transported or re-worked loamy soil, sometimes topsoil from an unknown location, so some variation in soil different than historical soil types does exist (NRCS, 2020). It is also known that soils in Central Park were often heavily altered through the practice of blasting and crushing bedrock to provide fill, as well as the incorporation of over 18,500 cubic yards of topsoil carted in from New Jersey to amend areas that would be planted (Rosenzweig & Blackmar, 1992), and have been altered by various usage and management practices throughout the years. The North Woods and Great Hill were less altered in this way and are more characteristic of the soil types listed above (and thus can likely be more directly compared in this study), while the Ramble and Hallett were intensively re-graded into various artificial rock outcrops and other landscape features. The soils across much of Central Park are also assumed to be acidic, with tests performed in 2013 across the Ramble and North Woods showing sometimes extremely low pH (as low as 3.97 at one site in the Ramble) and only 2 sites above pH 6 (found near areas with Greenbelt soil complex) (Great Ecology, 2013) (NRCS, 2020).
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Vegetation
All of Manhattan is in the same ecoregion. This ecoregion is defined from level I-III as 8.1.7: Northeastern Coastal Zone, within the Mixed Wood Plains in the Eastern Temperate Forest. More specifically, the level IV ecoregion is defined as 59c: Southern New England Coastal Plains and Hills (EPA, 2016). The forest coverage of the woodlands of Central Park is generally dominated by a species complex perhaps best described a Northern oak-hickory hardwood forest. This is characterized by a native canopy of Northern red oak (Quercus rubra) pin oak (Quercus palustris), bitternut hickory (Carya cordiformis) pignut hickory (Carya glara) and shagbark hickory (Carya ovata), though it has been observed that bitternut hickory is by far the most numerous hickory in the park and the only one succeeding at reproducing (Atha et al., 2020, as well as personal observation). Red maple (Acer rubrum), American elm (Ulmus americana), sweetgum (Liquidambar styraciflua), tulip tree (Liriodendron tulipifera), American basswood (Tilia Americana) and black cherry (Prunus serotina) are also present throughout each woodland, while American beech (Fagus grandifolia) is occasional. The park has noted heavy numbers of young black cherries (Prunus serotina) in the disturbed and compacted landscapes of the woodlands (Rogers, 1987) as well as significant increases in non-native canopy trees like Norway maple (Acer platanoides) and Sycamore maple (Acer pseudoplatanus), as noted in various studies (Alvarez, 2012, Andropogon, 1989, Great Ecology, 2013). Though native conifers like white pine (Pinus strobus) and pitch pine (Pinus rigida) have been reintroduced to Central Park, they do not naturalize readily in the area and neither were found in the original plant survey of the park (Andropogon, 1989, Rawolle & Pilat, 1857) though there are accounts of their occurrence in Manhattan in early colonial times (Sanderson, 2013). The herbaceous understory varies greatly between each woodland, but common native species are white wood aster (Eurybia divaricata), Virginia knotweed (Persicaria virginiana) and white snakeroot (Ageratina altissima), along with varying levels of other species depending on the landscape. There are quite a many native ferns, most of which are re-introduced but naturalizing native species (e.g., Thelypteris noveboracensis, Matteuccia struthiopteris), some of which are completely spontaneous (e.g., Asplenium platyneuron, Woodsia obtusa), along with various populations of more sensitive species like spring ephemeral wildflowers. Invasive species, primarily Japanese knotweed (Fallopia japonica), garlic mustard (Alliaria petiolata), mugwort (Artemisia vulgaris) and lesser celandine (Ficaria verna) are also present in varying amounts. Native understory shrubs such as spicebush (Lindera benzoin) and common witch hazel (Hamamelis virginiana) are relatively common, while various exotic honeysuckle species (Lonicera spp.) and multiflora rose (Rosa multiflora) are represented as well, though in much lower numbers than in years past. Invasive vines such as porcelain-berry (Ampelopsis brevipedunculata), Asiatic bittersweet (Celastrus orbiculatus), and Wisteria spp. are also variously represented, with a few of them actually being captured in the survey. Most of these invasive species are often able to flourish in landscapes constantly disturbed by human foot and vehicle traffic, and efforts to remove and limit them is ongoing.
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While this study’s survey only captures woody species (trees, shrubs, and some vines), the herbaceous/graminoid community in the understory and along pathway edges is a large focus of management efforts in the Natural Areas. These species are important to the greater forest ecosystem, and their composition can impact the process of ecological succession. Alvarez (2012) noted that high amounts of Japanese knotweed, mugwort and vines such as Asiatic bittersweet could thrive in constant disturbance, but also further hinder regeneration of certain key native trees such as oaks, while species like Norway maple could be better positioned to dominate the future canopy.
Social Context Central Park necessarily serves a wide profile of user groups, and these usage patterns have changed over time. Many locals use the woodlands throughout the year, some daily, as places for bird watching and nature appreciation. When the Central Park Conservancy tried in the mid-1980s to recreate an historic view in the Ramble by removing many large trees, the outcry from community activists was enough to cause the Conservancy to halt and reevaluate their efforts (Andropogon, 1989). Birders are still an important group of users who make their feelings known to park workers, often watching management decisions critically with an eye towards how it might affect the birds. There is certainly justification for treating these areas as bird sanctuaries: the New York City Chapter of the Audubon Society states that at least 280 species of bird have been seen in the park, with 192 considered annual regulars (NYC Audubon, 2020), and some consider urban areas to be ‘hotspots’ of bird biodiversity, with some studies showing localized increases in bird diversity in urban as opposed to periurban woodlands (Croci et al., 2008). In an effort to better respond to community concerns while restoring the woodlands, the Conservancy brought together representatives from groups like the local Linnaean Society, Audubon Society, Sierra Club, and Institute of Ecosystem Studies to form the Woodlands Advisory Board (Sauer, 1998). This group meets periodically and is informed of all aspects of the restoration and management efforts in the Woodlands, while the Conservancy is able to hear concerns and recommendations about ways to safeguard bird habitat and public accessibility. In fact, every major capital project must also go before all 6 Community District Boards bordering the park prior to approval. During early discussion concerning the future of the woodlands, members of the Woodlands Advisory Board had refused to agree to a ‘natives only’ policy of planting unless it was proven that native species could survive and offer similar benefits to wildlife (Sauer, 1998). While the Conservancy has largely gained the trust of the birding community, it is important to remember that the priorities and trust of citizen groups is essential in guiding management practices of the woodlands going forward. A 2011 survey obtained detailed information about the usage patterns of the park, and at the time estimated that the park received approximately 37 million visits per year, from nearly 9 million different visitors from around the world, though the majority of these visits were locals. When asked the primary intention of their visit, 64% responded ‘Walking or wandering’ and 15% responded ‘nature study and/or appreciation’ (CPC, 2011). This 15% figure might not seem remarkable, but it is a considerable increase over the 3.7% of visitors in 1982 who’s primary
13 purpose in visiting was to appreciate nature (Rogers, 1987). Of Central Parks 37 million visitors in 2011, it is estimated that of the many different sections of the park, the North Woods received among the least visitors, at ~1.4 million, and the Ramble among the most, at ~3.1 million. The Hallett, not open to the public at the time of the study, was not counted, though usage has been heavy since its popular re-opening to the public in 2016. For perspective, the estimated number of visitors to just the North Woods (the least visited area in the park) is still more than the entire annual visitation of the New York Botanical Garden, which has a very comparable endowment and annual expense budget (NYBG, 2020, CPC, 2020). However, when local residents were asked which parts of the park they avoid, 7% mention the Woodland areas (with another 7% mentioning the ‘North End’, which includes the North Woods and Great Hill) (CPC, 2011), a figure much higher than most other areas in the park, meaning that perception of the safety of the woods is an ongoing issue to be addressed by management. These data about the usage of the park are important in that they help guide management priorities to reflect public use. The study from 2011 is now nearly a decade old, and many large capital projects to restore vistas and structures, improve drainage and pathway surfaces have been completed in each of the 3 designated Natural Areas woodlands, as well as across the park, in the years since that survey. Another similarly comprehensive survey is planned in the coming year(s), though it remains to be seen how the ongoing COVID-19 pandemic will impact when this takes place. Before this year’s pandemic, it was widely assumed that usership numbers had made a notable increase. The observation of tenured Natural Areas staff members is that recent years have seen a steadily increasing and diverse group of visitors using the woodlands for nature appreciation, especially birdwatching. As per conversations with Regina Alvarez, in the early 2000’s fencing was perpetually difficult to maintain and there was a constant push and pull between allowing free access to the off-trail areas and protecting landscapes. While fencing will likely always remain an important tool in guiding park users to remain on the path in sensitive areas, much of the park’s usage has shifted to reflect more respect for the landscapes’ plants and wildlife value. Concurrently, there has also been a shift toward using shorter fence (2-3ft) more for guidance purposes than for restriction, as well as to minimize damage due to vandalism. However anecdotal this may be, the most recent response of park visitors in the midst of this year’s COVID-19 pandemic has been inspiring, and Natural Areas staff have witnessed increasing use of the woodlands for the purpose of nature appreciation and solitude. It should be noted that each woodland has different types of usership. The Ramble is situated between two major museums on either side of the park (the American Museum of Natural History and the Metropolitan Museum of Art) and sees the greatest number of tourists. The Hallett Sanctuary, located very near the Midtown intersection of Central Park South (59th St) and 5th Avenue, is also relatively heavily used, and has no fences to obstruct off-path usage. The North Woods and Great Hill, while increasingly popular with local residents, are still much less visited in comparison. The impending construction of a new swimming pool and ice-skating facility to replace the aging Lasker Pool near the North Woods might change that, and it will be interesting to chart future changes in park usage and demographics in the coming decades.
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The most important social context to remember about the park’s woodlands is that they are meant to serve both the community of locals and visitors as well as the wildlife community. This entails a natural tension between park users who want to explore the woodlands, and park management’s desire to limit disturbance and human impacts.
Management Philosophy and Goals
The goals that drive the management of the park’s Natural Areas attempt to serve the social priorities of its various user groups, as well as adhering to the historical and architectural vision of the park while promoting ecological resilience and complexity. Influencing these priorities are the geographic and ecological factors previously mentioned. The overarching stewardship philosophy of the Natural Areas is based loosely on the idea that these are woodlands which are meant to somewhat recreate the experience of upstate New York in regions like the Catskills and Adirondacks, but with a plant palette and ecological system more representative of the species that were previously found on the island of Manhattan before colonization and widespread development. The many meandering pathways, stone steps archways, rustic structures, viewpoints, and overlooks are carefully planned and curated architectural elements, and while ecological value, diversity and resilience are valued, aesthetics and safety are primary goals as well.
This “restoration horticulture” style of management in the Natural Areas necessarily requires nuance. The Society for Ecological Restoration defines restoration as “the process of assisting the recovery of an ecosystem that has been degraded, damaged, or destroyed”. Of course, this requires a system of reference (Restore to what? A landscape before colonization? A similar “model” woodland nearby? And what really constitutes damage or “degradation”?). In Central Park, as in many other places, the goal of completely recreating an ecosystem that existed before the creation of the park is often not realistic or even desired, but aiming to restore ecological associations and indigenous species in ways that allow for more resiliency following disturbance is achievable and desirable. One the one hand, the park’s Natural Areas team must manage for cultural and horticultural goals as in the rest of the park. This takes the form of clearance and structural pruning of woody species away from pathways, properly timing the cutback of herbaceous plants near edges for horticultural impact or ease of management, applying woodchips to pathways and tree pits, annual lawn maintenance, maintaining scenic vistas and safe pathway surfaces, as well as removing litter, maintaining fences, directing/restricting off-path foot traffic and even strategically/aesthetically scattering woody debris after a storm or tree removal. On the other hand, the team must manage for ecological and restoration goals. This is performed by minimizing harmful disturbance factors, preventing and remediating erosion and compaction, removing invasive species and planting/encouraging native species (which requires the skill to accurately identify all species), collecting and sowing seeds and providing seasonal disturbance regimes to mimic historical disturbances such as wildfire and grazing. In the middle of these two different sets of goals is the goal to provide value, both cultural and ecological, through the reintroduction and cultivation of species indigenous to the New York City area.
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This balanced hybrid of cultural/horticultural goals on one hand and ecological/restoration goals on the other is key to ongoing management success in these landscapes. This management style necessitates prioritizing which species to actively discourage (e.g., Norway maple, multiflora rose), which to passively discourage (e.g., Amur cork tree, Asian ornamental cherries), which species to keep but not actively encourage (e.g., black cherries, bitternut hickory) and which species to plant or actively encourage (e.g., chestnut oak, white oak, pawpaw, witch-hazel). This hybrid style of management naturally manifests in ways that are different from traditional management in the rest of the park. For example, unlike in much of the rest of the park ‘weedy’ native species like pokeweed (Phytolacca americana), white snakeroot (Ageratina altissima) and black cherry (Prunus serotina) are considered desirable in the Natural Areas and are not removed in most scenarios, salt and other deicers are never used on pathways, leaves are not removed, and coarse woody debris is largely left in the landscape. However, as mentioned earlier, there are also differences between the management of the park’s Natural Areas and the way a traditional wildland/natural area would be managed. Safety and aesthetic protocols call for periodic pruning or trimming, meadows are seasonally mowed or cut back to replace historic disturbance, and even some native species, (like the pokeweed mentioned earlier) are sometimes culled in favor of promoting even greater native plant diversity and/or aesthetic value.
Currently the Natural Areas have 13 full time employees, including Forepersons, Manager/Assistant Manager, and a Volunteer Coordinator in addition to the Natural Areas Technicians. These Technicians are responsible for specific zones of the park, and all maintenance and ongoing restoration activities are that person’s responsibility. This includes daily trash removal, pathway maintenance, fence repair/replacement, graffiti removal, and interacting with members of the public, in addition to weeding, planting and soil stabilization. Turf care, leaf raking and snow shoveling are also part of the job. Summer interns and dedicated weekly volunteers are thus instrumental in helping Technicians complete large projects and restoration efforts, given the wide range of other maintenance work required. The vast majority of weeding and invasive species removal is performed manually, with little usage of herbicide. It is a job that requires a diverse skillset (as well as mindset).
This team of dedicated staff is a realization of recommendations from a report by ecological consulting firm Andropogon, which was commissioned in the late 1980’s to provide guidance on how to restore the woodlands, making them both more stable ecologically but also more inviting to visitors (Andropogon, 1989). The Conservancy’s earliest management plan, detailed in Rebuilding Central Park: A Management and Restoration Plan (Rogers, 1987), first inventoried all trees over 6” diameter at breast height (DBH) and also surveyed ‘ground layer’ vegetation. This original survey alerted Conservancy management to the fact that certain native trees were less represented than they thought, and that relatively few species had come to dominate woodlands and other naturalized areas. Black cherries (Prunus serotina) were characterized as ‘rampant’, representing nearly 80% of understory stems in the Ramble (Rogers, 1987). Lack of control of various means of disturbance such as mountain bikes, off road vehicles, nightly illicit usage and uncontrolled trampling had had a major impact on the state of the woodlands throughout the 1970’s and 1980’s (Andropogon, 1989). Most plantings in the woodlands during that era were chosen primarily for their hardiness and horticultural value, or their perceived ability to attract birds. Foresters also recommended thinning out many major
16 canopy trees to promote a dense succession of regrowth, though without concurrent curtailing of invasive species and heavy trampling, dominance of black cherries and invasive species would have likely made this plan less viable than it would be in a more rural woodland. The ability to have reached the point today where the Conservancy is able to address general maintenance as well as make significant strides toward both invasive species removal and the recovery/re- introduction of native species is a significant accomplishment. All of the previous studies and efforts over the years have not only helped build capacity to better manage these landscapes, but also to fine-tune the Conservancy’s efforts to better serve the needs of park users.
The park’s Natural Areas staff use an approach where remnant exotic (non-native) but non-invasive species are maintained largely as they would be in other areas of the park, though their reproduction is monitored and kept in check by technicians, and they are replaced with similar native species, either through planting or via natural regeneration, when they die or are otherwise removed. For example, species such as exotic crabapples (Malus spp.) and cork-tree (Phellodendron amurense) can provide food and habitat for wildlife, as well as aesthetic beauty, and so their wholesale removal is not necessarily desired, at least not in the short term. However, these species have both shown increasing tendency to naturalize (as this and previous surveys show), and if the situation allows, staff actively remove them to be replaced with similar native species. Hardy native species are used to help fill niches left by removal of invasive species, and in landscapes that have stabilized staff are able to steadily increase biodiversity and structural diversity by re-introducing more sensitive species such as ferns and native spring ephemerals. By consulting an early park survey by the park’s first gardener Ignaz Pilat (Rawolle and Pilat, 1857), staff can select species that were specifically found on the grounds of the park before it was developed to reintroduce if the conditions seem favorable. Since 2011, over 400 species of native plants have been planted by Natural Areas staff in the woodlands. Increasing biodiversity is desired both in the sense of providing functional redundancy to assure a more adaptable ecosystem, but also to provide safe refuge and conservation of these species by giving the public greater access to appreciate them as part of indigenous heritage and natural history. In this sense the woodlands of the park represent a somewhat experimental landscape, in which altered and heavily visited landscapes are steadily augmented with increasingly more native species.
The tree care protocols in the woodlands are different from the rest of the park as well. Along the same lines of leaving leaf litter and coarse woody debris largely in place, the Conservancy’s team of arborists use a different approach to tree pruning and hazard mitigation than in the rest of the park. Every tree in the park is inspected very regularly (some of them many times per year given the volume of large events the park hosts annually), so hazards rarely go unnoticed or mitigated. In the woodlands, given the density of fenced landscapes and wide areas with low risk of a target, deadwood is not routinely removed from mature trees unless within proximity to a pathway or other target, and large hazard trees are often topped and left as snags (or ‘habitat trees’), rather than being completely removed, and are continually monitored. Cooperation with the tree crew is an inherently important part of managing the safety of Natural Areas, given the high density of canopy trees in the woodlands.
The changes in species diversity, both observed in the field and quantitatively represented in this dataset, should be put into greater context as it pertains to the Conservancy’s management philosophy. Changes along a trajectory towards more diversity of native species can be viewed
17 as both a measure of and a roadmap for management success. The Natural Areas of Central Park present unique management challenges, with various constraints, realities and wide-ranging values influencing how ‘natural’ these systems can effectively become. Plants within these areas are planted, removed, pruned, and sometimes watered, leading to a landscape that is at least in part ‘horticultural’ as well as ‘natural’. In this case, the process of ‘natural succession’ is planned and presided over on even small scales. Various factors impact how these systems behave and present, but it is clear that there are specific overarching management priorities, and field surveys like this one are an objective way to measure (at least in part) progress towards these priorities. Hopefully, this study can shed light on how active management practices have impacted the structure of the Natural Areas and give a look to future suggested actions for optimization of management and monitoring.
METHODS
Central Park is composed of many meandering pathways, rock outcrops, waterbodies, and other defining features. For resource inventory purposes, each individual landscape within the park has a discrete identification (ID) number to help locate resources within the park. Originally called “lawn areas” by early Conservancy geographers (Rogers, 1987), an individual landscape is one that is contiguous and delineated/bound by adjacent pathways, roadways, watercourses, rock outcrops, fences or other such clear shift in surface type or management, such as between a mowed lawn and a mulch border or designated woodland. In a Geographic Information System, a map of the entire park can be composed of these landscapes and other features (see Fig. 2 below). Various values are attached to these landscape IDs, with data describing surface type, lawn grade, area, and other information. Early tree censuses used these ID numbers to attach locations to each tree. Today, park resource data managers also collect GPS data on various other park assets and infrastructure, with most mature trees GPS located as well. The survey in this study utilized a proprietary shapefile of only the park’s landscapes, which it should be noted does not include identifying locations of any resources such as trees, benches, lampposts, etc. It may be a useful research question to overlay mature tree GPS data with the data collected in this or similar surveys, but that is beyond the scope of this study.
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Figure 2.) Map of the Ramble in Central Park (in the center of the park, just south of 79th St), created with mapping software. Landscape ID numbers are included, superimposed over each discrete landscape unit. These IDs are critical to management and inventory of park infrastructure and were useful in organizing the field work for this survey.
Alvarez (2012) previously used the point centered quarter method (subsequently abbreviated as PCQM) of plant community sampling, and for continuity and ease of comparison it was used in this survey as well. This method is attractive for use in Central Park as it is a ‘plot- less’ method of sampling, with data collected from random points rather than plots. The lack of permanent plots and the relatively rapid (roughly 4-5 points per hour during a typical day of surveying) completion of each point are both important features of this method given the public, high-use nature of the park. In this method, random points are selected within the site, with the general consensus being that at least 20 points per site are needed for sufficient statistical analysis (Alvarez, 2012). Each point is then divided into four 90° quarters, and the closest tree from the central point in each quarter (for a total of 4 trees per point) that satisfies the size requirement is measured, with the distance from the point, species, and DBH recorded for each. See Fig. 3 below:
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Figure 3.) Illustration depicting the point centered quarter method of sampling. Each randomly selected point is divided into 4 90° quarters, and the closest woody plant over 1cm at breast height (1.3m) is measured for DBH and distance from the point, with the species recorded as well.
One difference between this survey and the one conducted by Alvarez (2012) is that she selected random points along chosen transects, using a random number generator to determine the next point along the transect. As the previous transects’ locations were not known, this survey instead used a QGIS function to create random points in each landscape, to be found in the field by a personal GPS unit. QGIS is a free, open-source desktop GIS (Geographic Information System) program very similar to the popular ArcGIS application. Using the park landscape shapefile, the group of discrete individual landscapes that made up each larger landscape (i.e., North Woods, Great Hill, etc.) was selected, and any landscapes that could not be accurately characterized as ‘woodland’ (lawns, garden-beds, meadow landscapes managed largely for exclusion of woody plants, etc.) were excluded. The PCQM does not allow for not recording a tree in a given quarter (Mitchell, 2010), and it was foreseen that randomly dropped points near a path, road or lawn could create this possibility. Knowing that the chosen GPS unit (a Garmin GPSMAP 66st purchased for use on this project due to its relatively high accuracy for the cost) had a stated accuracy of within 5 meters, the selection of landscapes were buffered 5 meters from the path/edge, thus leaving a smaller, more interior area in which random points could be created. A function within QGIS that plots random points within the chosen area was then used, and it was specified that each point must be 5 meters from another point. Additionally, it was ensured that every ‘sampleable’ landscape (that is, a landscape containing area not within 5 meters of an edge) contained a minimum of one randomly generated point before generating the final number of additional points. See Figures 5, 6, 7, 8, and 9 on the respective pgs. 40, 55, 71, 83, and 94, to see sample unit locations corresponding to each separate woodland in the Results section.
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These choices for random point generation had a few intended positive effects, while also creating some unavoidable amount of impact on the nature of the sample population. Buffering 5 meters from edges means that the sample population is skewed slightly to interior specimens, but alternatively this is also designed to help avoid the influence of edge effects on the larger population data. Edge effects include cutting back/pruning plants near pathways for clearance, or removal due to negative pathway impact or obstruction of sightlines. Buffering from the edge/path also serves two other purposes. First, it helps avoid the instance of arriving to a point that the unit indicates is on or near a path or other landscape not intended for sampling (due to the 5m range of error of the unit). Second, it helps avoid the instance of a sample being so close to a path that one or more of the stems would be located on opposite sides of a path (thus distorting the distance and by extension the species density value), or worse, there would be no trees to sample in a quarter since it could be adjacent to a large lawn or road. In practice, a high percentage of the randomly generated points were usable, with all 4 quarters having a viable specimen not across a pathway (though sampling trees across a path from the point was not explicitly ruled out, doing so did not have to be resorted to). Many specimens were indeed located near to pathways anyway, given that some landscapes had less dense woody vegetation and the wide spacing of trees involved going closer to a path to find a viable stem, or the margin of error in the GPS unit meant that it occasionally indicated a point being located nearer than 5 meters from the edge, so using the buffer zone worked as intended. Concerning the minimum of 5 meters between random points, this could be also viewed as slightly skewing the randomness of the survey and by extension the sample population by limiting the number of potential points in a small area, and thus theoretically could lead to undercounting some species that might have high abundance but clumped in a relatively small area. For the purposes of this survey, this is acceptable given that 2 trees cannot be sampled twice when using the PCQM, and thus it stands to reason that points should be located far enough away as to avoid this possibility. Most other descriptions of this survey type even recommend using a method to ensure adequate distance between points (Mitchell, 2010). It is also desired that the sample population capture as much of the woodlands as possible, which is a limitation of the transect method given the shape of the woodlands in Central Park. All surveying was carried out in the summer of 2020, from late June to September. Once random points were generated the point file was uploaded to the GPS unit, and the list and locations of the points were printed out, grouped according to landscape number for ease of locating in the field. The surveyors (myself and at least one other assistant) then went into the field armed with the GPS unit, a compass, paper data sheets, a steel rod and mallet, a logger-style metric-unit tape for both distance and DBH measurements, a6” ruler, and a PVC ‘cross’ to visually delineate the four 90° quarters during sampling (see Fig. 4 below). Upon arrival at a point the steel rod was driven into the ground with the mallet to mark the location, and a compass was used to orient the PVC cross North to South (the orientation of the quarters might impact trees selected, so using a standard orientation avoids bias here), with the steel rod adjacent to the center of the lower left quarter of the PVC cross, an orientation which was also kept constant. In the case that the ground was too rocky, or it was otherwise difficult to drive the steel rod, the PVC cross was placed directly on the ground with the center of the cross indicating the point location. The distance was then measured from the steel rod/center of the cross to the 21 point at the same height on the nearest woody stem in each quarter that was at least 1cm at breast height (or 1.3m for the purposes of this study), the species recorded, and the diameter measured with the DBH tape. A small ruler and caliper were used to measure smaller stems between 1 and 5 cm DBH. After multiple attempts, it was shown that obtaining values with the ruler was both less cumbersome and similarly accurate as the caliper, so most of these smaller values were taken with the ruler. Measurements were taken with a precision level to the closest millimeter for DBH, and closest centimeter for distance from the point.
Figure 4.) Field sampling supplies: PVC cross to visually delineate quarters, steel rod and mallet to mark point location, Garmin GPSMAP 66st GPS unit, compass (phone), paper datasheets and clip board, and metric DBH/distance loggers tape.
Species identification references used were Gleason and Cronquist (1991) and Haines (2011). If a tree or shrub had multiple trunks or stems originating below the 1.3 m point, each stem was recorded. During data entry, the data from multiple stems was converted into an equivalent total DBH value, such that it would return the diameter equivalent that would produce the same basal area as the sums of all the individual stems (this was done by taking the square root of the sum of all of the squared stems). In the event that some specimens were clonal/reproducing vegetatively through underground roots, only stems with obvious connection to the root flare of the original/closest stem were included. In the case of species such as American beech (Fagus grandifolia), black tupelo (Nyssa sylvatica), Sassafras (Sassafras albidum) and various viburnums (Viburnum spp.), it was important to use this rule.
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One issue with the method of generating random points via GPS is the potential for bias when placing the point in the field. This issue was acknowledged at the outset, and steps were taken throughout the process to consciously minimize this bias potential. In theory, given the inherently limited precision of consumer grade GPS units, the arrival to/placement of a point could be subject to bias on the part of the surveyor, potentially leading to inclusion of preferred individuals or placing the point in a less treacherous location, consciously or unconsciously. To avoid this, at the outset it was decided not to let the unit ‘settle down’ upon reaching a point unless in the case of obvious error, which was usually temporary, and instead we stopped walking as soon as the unit indicated that the point had been reached. Taking measurements at an exact predefined GPS location was less important that the location itself being random, after all. It was observed that in practice, focusing closely on the electronic handheld unit while walking through the woods meant that the species of the specific few stems in the immediate vicinity were rarely noticed until after placement of the point marker. Each point was approached very slowly, in order to allow the GPS unit to follow as accurately as possible, and the field worker (myself) intentionally tried to maintain ‘tunnel-vision’ while watching the GPS unit, and not the surroundings. Most often the GPS unit performed reliably and indicated arrival to a point relatively unambiguously, despite the 5-meter stated margin of error in accuracy. If the unit indicated that a point was clearly located on a non-woodland landscape, or if a tree was to be sampled in multiple points, the point in question was thrown out. There was a total of 466 viable sampled points, out of 472 randomly plotted points. Of the total points plotted, 165 were in the North Woods (the same number as in the Alvarez survey), and 163 were viable; 165 points were plotted in the Ramble (compared to 111 in the Alvarez study) and all 165 were viable; 42 were plotted in the Hallett Sanctuary (the same as in the Alvarez study) and all 42 were viable; and 100 points were plotted in the Great Hill, with 96 being deemed viable. Reasons for removing a point or refusing to sample it included one instance of the same tree potentially being sampled in 2 points, one instance of the GPS indicating the point’s location on a wide pathway (perhaps due to a minor discrepancy between the base map and the actual landscape), one instance of a quarter not having a sampleable stem due to being relatively close to the adjacent park drive (despite the point being 5m from the edge), and 3 instances, all within the Great Hill, of the points being deemed ‘not woodland’, due to obvious evidence of ongoing tree exclusion or horticultural beds not consistent with a woodland landscape. One final piece of information was gathered at each site, and that was the estimation of whether the specimen had been planted, not planted, or ‘possibly’ planted. While not a primary metric for the purposes of this study, it was a potentially interesting question to ask: how much of the sample size were specimens that had actually been planted? It is already recognized that most plants in Central Park, even a sizable portion of those in the Natural Areas, have been planted or are directly descended from specimens that have been planted. In practice, this metric was hard to gauge given the near impossibility of determining whether older specimens had been planted or not. In many cases we could make a very close guess (either the samplers had planted them directly, or location or other factors were extremely indicative of it having been planted), and so the determination of whether a specimen had been planted was optional and only marked when we could be relatively certain. However, it became apparent that any estimate of how
23 many of the sampled individuals were planted would likely be a gross underestimation, so it was elected not to use those data in this study. Each field data collection session included me and usually one other participant. Out of 12 staff members on the Natural Areas team (not including myself), data collection assistance was received from 9 of them. Each assistant was extensively oriented to the nature of the survey, the proper use of equipment, and the potential instances for bias, and the roles of measuring and recording were often traded. The intention here was to ensure that a wide range of participants could not only take part in the project, but also to demonstrate to more people the nature of how the data was collected. Essentially, the use of many assistants (but always with my presence) was an effort to build confidence and transparency in the data collection process, as well as general awareness of what the project entailed. It was also essential for the timely collection of data. Finally, with all the data collected on physical data sheets, it was then entered into Excel spreadsheets, including information about the date of collection and the field crew involved. This dataset was then analyzed to create the information presented in the Results section.
Concerning Species Nomenclature As was mentioned previously, all species were identified with Gleason and Cronquist (1991) and/or Haines (2011). In general, Haines (2011) is more up to date with current nomenclature, though being a flora of New England proper it does omit some more southern species (along with some exotics), so cross referencing with Gleason was necessary. Most of these species were identified in the field without need to consult these manuals, except for a few notable instances. It should also be noted that certain species names/treatment in this survey differ slightly from the Alvarez study, and this section will serve as a round-up of these differences, which are important to remember when comparing the two studies. For starters, Alvarez (2012) considered all crabapples to be Malus sylvestris, or the European crabapple. However, further analysis by Daniel Atha and Regina Alvarez in the most recent flora of spontaneously occurring species in the park (Atha et al., 2020), found that these volunteer crabapples were a complex of Asian varieties, namely Malus hupehensis, the tea crabapple, and Malus toringo, Siebold’s crabapple. Each of these species are also planted for their horticultural interest throughout the park (notably in a prominent allée in the park’s Conservatory Garden), with their spread by birds such as the Cedar waxwing likely explaining their increase. For the purposes of this study, and the difficulty of differentiating between these two species after the spring season, all crabapples are considered Malus spp. in this survey, though it can be assumed that the vast majority of those found are one of the two Asian varieties noted above. In a similar vein, a few other species from the family Rosaceae also provided difficulty, notable specimens from the genera Amelanchier and Prunus. In the Alvarez study, all serviceberries were considered Amelanchier canadensis. In this survey, it was possible to identify some specimens as Amelanchier canadensis, but others were trickier. Notably, there were numerous instances where a definitive conclusion could not be made, especially when it became apparent that the specimen was either Amelanchier arborea or Amelanchier laevis.
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Differentiation between these two species (at least for me) would have required seeing them in the early spring season to compare characteristics during early flowering. As such, most Amelanchier in this survey were coded Amelanchier spp. but all specimens labeled as such in this survey can be assumed to be any of the three native species listed above. The species complex Prunus spp. is also referred to throughout the paper. For the purposes of this study, this refers to a complex of ornamental Asian cherry trees that have been planted in the Natural Areas in the past and have begun to naturalize more recently. It is likely that many of these specimens are (or are related to) Prunus serrulata, but this distinction was not made for this survey due to a lack of expertise differentiating these varieties. Another major difference between this survey and the Alvarez survey was the treatment of the genus Fraxinus. The previous study considered all ashes to be Fraxinus americana. However, more study led to the conclusion that many specimens of Fraxinus pennsylvanica were also present throughout all three woodlands. Further study of Fraxinus in Central Park has uncovered more information more recently, and now even a few specimens of Fraxinus profunda have also been verified, with some located in the floodplain of the Loch in the North Woods (Atha et al., 2020). Given this disparity, I recommend looking at Fraxinus’s Importance Value rankings grouped by genus to better compare with the previous survey. Yet another major difference was with the treatment of Quercus velutina. Many previous surveys do not consider this to be a significant species in the park, but analysis of many specimens, especially in the North Woods but also in the Ramble, indicate that it is indeed present in more than insignificant amounts in the park’s woodlands. Alvarez (2012) did not record any Quercus velutina, so the reader can choose to assume that either the treatment of the specimens in this survey are incorrect and thus belong to Quercus rubra, or that Dr. Alvarez and others have overlooked this species. At any rate, this is worth remembering when comparing the data from the two studies, so looking at rankings by genus as well as by species might be useful here. There are various other minor differences of note between the treatment of species names in each study. Ulmus procera was found in the previous survey as in this one, but in a few instances, it was chosen to identify a specimen as Ulmus glabra, given a few distinctive traits (mainly the presence of 3 acuminate lobes at the apex observed on most leaves). It is possible that either this is a misidentification, and they should all be considered Ulmus procera, or the Alvarez survey may have not captured any Ulmus glabra, given that only a few were seen during this survey. In the Alvarez study, a few specimens of Hamamelis vernalis were also recorded, though this survey recorded all witch-hazels as Hamamelis virginiana, given the difficulty of differentiating the two when not in flower. It is widely considered that most specimens in the park are Hamamelis virginiana, and the Alvarez study only found a few Hamamelis vernalis. The Alvarez study also made pains to differentiate between various Crataegus species, while this study rarely attempted to do so. While the genus is not unfamiliar, and there are a few instances where a certain declaration of a species could be made, in most cases they were all lumped within the species complex of Crataegus spp. Most of these, some of them likely cultivars, are assumed to be of ‘native’ origin. In the previous survey, the chestnut oak was named Quercus
25 prinus, in line with Gleason and Cronquist (1991). However, currently botanists have settled on Quercus montana (Haines, 2011), which is how it is referred to in this survey. There are only a few individuals of this species present currently, all planted, even though this was noted as a common and important species found in previous surveys (Rawolle and Pilat, 1857) and in Manhattan in general (Sanderson, 2013). The Korean Evodia has also changed names since the last survey, going from Evodia daniellii to Tetradium daniellii. While a horticultural introduction with relatively few specimens in this survey and in general, it occupies prominent locations in the Ramble and Hallett Sanctuary. Finally, the London Plane tree is often referred to as Platanus x hybridus, in numerous texts as well as the previous survey. Here it is instead referred to by the commonly used alternative, Platanus x acerifolia.
DATA ANALYSIS
Importance Value Calculation A primary goal of this survey was to produce similar useful indices as the Alvarez study in order to compare these against each other to measure change. The data that was collected included abundance (total individuals sampled), DBH, and distance from the sample point. From these values can be generated relative values for each species estimating density, dominance/cover, and frequency. Each of these values for each species, represented as a percentage of the total (a value between 0 and 100), can be summed to create an Importance Value unique to each species.
Importance Value = Relative Density + Relative Dominance + Relative Frequency
For example, a landscape with only one species present would return an Importance Value of 300, since it would represent 100% of the density, dominance/cover, and frequency. This relative value for each species can be used as a proxy for ‘ecological dominance’, in the sense that it assigns equal weight to the three metrics of abundance (density), size (dominance, or some call this ‘cover’), and frequency (percentage of sample points in which a species is found). It should be stressed that the ‘importance values’ used here are not meant to represent an absolute value that ascribes ‘importance’ to each species, as whether or not a species is ‘important’ is inherently subjective. It has been characterized as the relative ‘contribution’ of a species to the entire community (Barbour et al., 1999, Alvarez, 2012). The Alvarez study used the Importance Values as indicators for each woodland, and so this survey will replicate that process. A more exact description of how these values are obtained is included below: With the abundance data the relative density for each species can be estimated by taking the proportion of those individuals in relation to the total N (sample size). Taking the mean (or average) of the distance values (r̅) also allows for the approximation of absolute density
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(individuals per area, or λ), as Cottam and Curtis (1956) showed that r̅ is essentially equal to the square root of the area per individual (1/ λ) , or √1/λ . Thus:
absolute density = λ = 1/ r̅ 2 absolute density per species (or species a), with n = to number of points, is:
quarters with species a λa = ∙ λ 4n And the relative density per species (or species a) is the proportion of that species from the whole:
= quarters with species a ∙ 100 4n However, this approximation (as well as the relative value) does assume that the species/individuals are arranged randomly (Mitchell, 2010). This is probably not true in any woodland since various species clump together and create stands in some areas while being absent in others, a phenomenon often due to topographical heterogeneity. While the park’s woodlands can easily be termed diverse in terms of landscape types and features, it should be noted that each of these woodlands are relatively homogenous in terms of soil type, elevation, and microclimate, largely due to their relatively small size and close proximity. The average distance values were very stable across the woodlands, with each woodland’s mean point-to-tree distance ranging from between 2.22 and 2.28 meters, a difference of only 6 cm between the most dense (Hallett Sanctuary) and least dense (the Ramble) woodlands. As such, the total stems per hectare figure is likely very accurate, especially for the whole dataset, though species values will become less accurate as the number of sampled individuals per species goes down. Species counts of less than 7 individuals are hard to draw much hard conclusions from (Mitchell, 2010), except when combined with extensive on-the-ground observation or experience. For reference, only 40 out of the 119 species found in the combined Natural Areas dataset exceeded 7 individuals. The DBH (in this case equal to 1.3 meters) value can be used to obtain total basal (or cross-sectional) area, using the equation A (or basal area) = 휋푑2/4, with the total basal area per species simply being the total basal area of all individuals of that species, per hectare. Basal area is a good indicator of tree size and biomass, for each species as well as the total. Similar to the relative density value above, this absolute value can be converted to a relative basal area value as well, which expresses each species’ basal area as a percentage of the total. For this study’s analysis, this value is referred to as relative dominance. This is shown as:
Relative Dominance for species a = Total BA of species 푎 ∙ 100 (Mitchell, 2010) Total BA for all species Certain species will have relatively few individuals (and thus low relative density), but a large total size, which can balance out low relative density or frequency values in the total Importance Value analysis. Species such as Quercus spp. were noted to exhibit these features in the Alvarez study, and this study will highlight some gradual changes in this. This relative
27 dominance value is also one that becomes more variable when there are only a few individuals per species. For example, in the North Woods only two individuals of Platanus occidentalis (American sycamore) were sampled, though the extreme size of one of the specimens (119.5cm) contributed to that species ranking #12 in terms of Importance Value. Since incremental increases in DBH values correspond with exponential increases in basal area values, even seemingly small differences in size between two large trees can result in much different basal areas, so it is important to remember the significance of the total individuals sampled per species when interpreting these aggregate values. Finally, the absolute frequency is merely a percentage value of the total absolute number of sample units (in this case points, with 4 individuals each) in which a species is found, or:
# or sample points with a species ∙ 100. Total # of sample points
The relative frequency is represented as: Absolute frequency of a species ∙ 100. (Mitchell, 2010). Total frequency of all species
This value helps approximate the physical ‘spread’ of each species. For example, in the Ramble dataset Nyssa sylvatica and Sassafras albidum were found in relatively high abundance (more than 30 each). However, these species have tendencies to form dense monodominant stands, especially at the sample sites in the Ramble, and many of the plots where these species were found contained 2 or more individuals of that species. This results in a relatively lower relative frequency value in relation to the density value, when compared to a species with the same number of individuals but less individuals per point. For example, Carpinus caroliniana or the genus Fraxinus were found in nearly as many plots as their total species counts, and as such have relatively higher frequency in relation to their density. Of course, a species could have very high values in both density and frequency relative to the other species. These values for each species in each of the 4 woodlands surveyed, as well as the combined values for the 3 woodlands designated as Natural Areas, are given in the Results section.
Diversity Indices A common intention for many studies such as this one is to interpret collected data in such a way as to quantify ‘diversity’ in some way, and there are a number of commonly used metrics that attempt to do so. While the value of these metrics and how they are calculated is a topic of debate in modern ecology (Palmer, 2016), for some purposes they can indeed be useful in their reflection of the number and evenness of species. In this case, the Central Park Conservancy (and by extension the user groups it aims to represent) has decided that the Natural Areas should present to the park user a naturalistic experience similar to what would be found in more ‘pristine’ landscapes outside of the city. As part of this decision, the promotion of native
28 plant diversity by the infusion of native species and management of invasive species has been deemed a worthy goal. Descriptive statistics such as richness (total number of species) and evenness (the relative individual representation of each species within the community) and the use of indices such as the Shannon-Weiner index and the Simpson index can all be useful in helping describe a plant community (Gurevitch, 2002). The Shannon-Weiner index, represented by H, incorporates species richness, abundance, and evenness, and the larger the number the more diverse a landscape is considered to be. Increases in either richness or evenness result in an increase in H, with numbers above 3 considered relatively diverse (Barbour, 1999, Gurevich, 2002). The Shannon evenness value itself can also be obtained, allowing for a comparison of both richness and evenness discretely. The Simpson D index is less sensitive to the addition of more species, and more sensitive to the more abundant species. This number ranges between 0 and 1, with smaller values correlating to higher diversity, and larger values meaning more dominance by a few species (Gurevich, 2002). The Simpson 1 – D and Simpson 1/D values are merely different attempts to show the Simpson D value more intuitively, with a larger number (closer to 1) representing diversity for 1 – D (pronounced 1 minus D), and a larger number in general representing diversity for 1/D. Given the relatively small numbers for Simpson D in this study, the Simpson 1/D value might be the one easiest to interpret by the reader. These values, along with other descriptive statistics, are included in Table 1 in the Results section:
Points per Area It might also be interesting to note the differences in how many points per area each landscape had. The point totals were chosen in order to ensure that at least as many points were taken in this survey as the previous survey, and in the case of the Ramble, 165 points were sampled vs. 111 previously. In the North Woods and Hallett, both woodlands had the same number of points (165 and 42, respectively) as the previous survey, though 2 of the points in the North Woods were not viable for this survey. Still, this amount of points per area in the North Woods from the previous survey was used as a sort of baseline minimum from which to make sure that other landscapes had at least as many points per total area. As we can see in Table 1 below, of the area minus the buffer zone, the North Woods did have the least points per hectare at 30, with about 15 points per hectare if buffered area is included (as there were numerous specimens sampled within 5m of the edge, because some points were relatively close to the 5 m point). The Hallett had by far the most, and the Ramble and Great Hill were in the middle of the pack. This disparity could be viewed in two different ways: if the points per area in the North Woods was deemed sufficient, then perhaps effort was wasted by oversampling the other woodlands, or alternatively perhaps all of the woodlands should have been sampled at least as much as the Hallett. The explanation for this is that this survey was heavily limited by time and taking more points did not seem feasible. However, it was important to attempt to mirror the
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efforts of the previous survey as much as possible, while trying to increase the points in the Ramble to better reflect the sample point total in the similarly sized North Woods. One interesting statistic to include is the percentage of each woodland’s area that was ‘sampleable’. In other words, it is known that the Ramble has far more pathways that the North Woods despite their overall similar size, and showing how this difference impacted the amount of available area for which to locate sample points could be worthy of note. Indeed, the difference in the amount of available area after buffering 5m from the edges/pathways was even larger in the Ramble than intuitively expected, with only 35% of the Ramble’s woodland area not within 5m from an edge. For the North Woods, this effect was the most muted, but even still only 50% of the total woodland area was not within 5m from a path. Two things should also be noted about this. First, there are many small, often peripheral landscape ID’s that make up each woodland, and some of them are indeed so small that they become completely unsamplable after buffering 5m from the path, thus causing most points to be located proportionately in the larger landscapes. Second, ‘rock outcrops’ and/or boulders are also included in the base map layer and are buffered around just the same as would be buffered around a path or lawn. This decreased the sampleable area a bit, and though woody plants are less likely to occur on these outcrops, there are certainly many trees and shrubs that do grow on them. It should also be noted that the total (no buffer) area of the woodlands listed below does not reflect pathway area, non-woodland landscapes (like lawns) or water bodies (such as the Loch in the North Woods). I mention this because the stated values below are noticeably lower than the commonly stated total areas for each woodland (~38ac/15.4ha for the North Woods, ~36ac/14.6ha acres for the Ramble, and ~4ac/1.6ha for the Hallett), which would include all area, woodland or otherwise.
Ramble North Woods Hallett Great Hill Hectares (no buffer) 9.40 10.76 1.18 4.72 Hectares (after buffer) 3.30 5.43 0.48 2.14 Sample points per hectare (total area) 17.55 15.33 140.05 34.96 Sample points per hectare (not incl. buffered area) 50.03 30.03 87.82 44.89 % area sampleable (>5m from edge) 35.08 50.43 40.59 45.30
Table 1.) Total ‘woodland’ area and points per area for each woodland, with and without 5m buffer from edge. Color coding is to help visualize differences between woodlands, with darker green indicating higher relative values.
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RESULTS
Diversity As can be seen in Table 2 below, total species richness for the combined Natural Areas dataset increased substantially from 82 species in the last survey to 119 total species, though the number sampled did increase from 1272 to 1480, with 216 added individuals taken in the Ramble and 8 less taken in the North Woods. This high overall richness value was surprising, though perhaps it should not be a shock considering the increase in species brought in via planting. Indeed, records show that the Natural Areas team has planted 155 species of native trees and shrubs since 2011 alone, a number which does not include non-native species or common natives such as Prunus serotina and Ulmus americana. The Ramble had the most species in the survey at 84, with the North Woods coming second at 74. Given that the sample size for each woodland was very similar for this survey (differing only by 8 individuals), this is notable. In the last survey, the Ramble was sampled significantly less than the North Woods, and the result showed a slight lead for the North Woods over the Ramble in species richness (57 vs 50). It is also surprising that the Hallett managed to capture 41 species, when only 42 points were visited (totaling 168 individuals).
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Natural Areas Total Ramble North Woods Hallett Great Hill Sample Units (points) 370 165 163 42 96 Mean Distance (r) 2.25 (2.18, 2.32) 2.28 (2.17, 2.38) 2.22 (2.12, 2.33) 2.23 (2.02, 2.43) 2.23 (2.07, 2.38) Absolute Density (trees/hectare) 1978.69 1931.28 2020.42 2009.29 2014.99 Total Genera 67 54 44 29 30 Mean DBH, w/95% Confidence Intervals (cm) 11.32 (10.43, 12.22) 12.29 (11.77, 13.81) 10.63 (9.43, 11.83) 10.23 (8.01, 12.45) 9.94 (8.36, 11.53) DBH St. Dev. (cm) 17.53 19.86 15.57 14.57 15.80 Median DBH (cm) 5.00 5.10 5.10 4.50 4.90 Max DBH 123.50 123.50 119.5 97.50 125.60 Native Species Richness 81 60 53 26 27 Exotic Species Richness 38 24 21 15 17 Total Species Richness 119 84 74 41 44 Total Species Richness (Alvarez 2012) 82 50 57 27 - Total Sampled (N) 1480 660 652 168 384 Total Sampled (N) (Alvarez 2012) 1272 444 660 168 Shannon Index (Natives) 2.89 2.86 2.68 2.36 1.79 Shannon Index (Total) 3.76 3.70 3.44 3.14 2.99 Shannon Index (Total) (Alvarez 2012) 2.51 3.08 3.09 2.51 - Shannon Evenness (Natives) 0.60 0.65 0.62 0.63 0.47 Shannon Evenness (Total) 0.79 0.84 0.80 0.85 0.79 Shannon Evenness (Total) (Alvarez 2012) 0.76 0.79 0.76 0.76 - Simpson D Index (Natives) 0.06 0.05 0.07 0.10 0.11 Simpson D Index (Total) 0.04 0.04 0.05 0.07 0.07 Simpson D Index (Total) (Alvarez 2012) 0.17 0.08 0.09 0.17 - Simpson 1 - D Index (Natives) 0.94 0.95 0.93 0.90 0.89 Simpson 1 - D Index (Total) 0.96 0.96 0.95 0.93 0.93 Simpson 1 - D Index (Total) (Alvarez 2012) 0.83 0.92 0.91 0.83 - Simpson 1/D Index (Natives) 16.73 18.41 13.54 10.25 9.12 Simpson 1/D Index (Total) 23.61 26.03 18.64 15.00 13.93 Simpson 1/D Index (Total) (Alvarez 2012) 6.03 11.89 11.48 6.03 - Table 2.) Diversity indices and other statistics for each woodland in Central Park, New York City, including comparisons with data from Alvarez (2012). The color coding is intended to help visually depict differences in values between woodlands, as well as highlight changes from the previous survey. DBH = Diameter at Breast Height, N = number of individual plants sampled.
The Shannon diversity index for each woodland was over 3, except for the Great Hill which was close at 2.99 (though its native-only Shannon index was much lower than the others, at 1.79). The Ramble took the lead among woodlands, with the North Woods close behind. The Shannon evenness values are notable in that they show a relatively small increase from the last survey in comparison to other metrics, though the Hallett showed a marked increase in evenness (going from 0.76 to 0.85), even coming first in this category overall. Perhaps the starkest difference was in the Shannon index’s change between this survey and the last, going from 2.51 to 3.76. The Simpson diversity index (the 1/D index is perhaps the best visual indicator) also shows a noticeable increase, which is encouraging especially given the number of species that
32 were only represented by a single individual per woodland, meaning that perhaps despite this, the evenness among more highly represented species (those ‘at the top’) has improved. One aspect included in this study that was not included in the Alvarez study is the inclusion and differentiation of native species’ diversity indices, in addition to the diversity indices that included all species, native or not. Of course, the distinction of what is considered “native” or not is binary for the purposes of this study, though in the real world the “native-ness” of a species might fall along a scale of sorts, with some species native only a few hundred miles away (or less) perhaps facilitating more ecological associations and “native” features than species from other continents. For the purposes of this study, species that are generally considered native to within a few hundred miles for New York City are included in the “native” category (e.g., Gleditsia triacanthos, Gymnocladus dioicus, etc.). Of course, these values are up for debate and the value of a plant’s associations/functions can be measured in various ways, as well as depend on what historical time period is referenced in determining if it is “native”. For the purposes of this study it can help to visualize the overall comparative presence of native species versus non-native species, in this case when comparing diversity indices. For example, in general about two thirds of species in the Natural Areas are native, with a bit less than that in the Hallett. Also notice that the Shannon and Simpson indices are each lower when only native species are included, given that native species only make up a portion of the total sample, and the indices are calculated using the total individuals sampled (N). However, since the Simpson index is less impacted by total number of species, the drop off there between the total dataset and only natives is lower than the Shannon indices. As the Conservancy’s Natural Areas team goes forward in its plan to prioritize and improve native plant diversity, it is important to keep these distinctions in mind when evaluating data such as these. It should be pointed out that every single index in the dataset improved upon the previous survey, and the Great Hill underperformed in every index except species richness, which came in at 44 over the Hallett’s 41, though the Great Hill included more than twice the sampled individuals as the Hallett (384 vs 168). When looking at each index value of the Great Hill when only native species are included, the differences are even more stark, with the Shannon index and Shannon evenness values dropping off considerably. This is quite different from the Natural Areas datasets, including the Hallett, and could highlight differences in management over the years. While improved values of diversity indices in the Natural Areas were expected, such a clear difference is striking. The native species richness for the Natural Areas nearly matched the total richness from the previous dataset (81 vs 82), and the native Shannon index and Simpson index values even overperformed the values from the total dataset of the previous study. When speaking with current and previous park workers (including Regina Alvarez), the consensus is that the woodlands have changed dramatically (though steadily), with greatly reduced understory invasive species such as Japanese knotweed and vines such as Wisteria. Since Regina’s departure the subsequent work led by John-Paul Catusco after the North Woods was included in the Natural Areas in 2015, and continued by Natural Areas Manager Eric Whitaker’s team, efforts to reduce invasive species in the North Woods in particular have increased drastically. Even those who have worked in the Natural Areas for relatively few years can see large changes as a result of the ongoing work, and it can be noted when comparing datasets that many new species are
33 ones that have been planted or grew to sampleable size with the last decade. The desire to measure this noticeable change objectively was in fact the impetus for this survey. However, the possibility that the sampling method, which aimed to better distribute random points throughout each landscape than the transect method used by Alvarez (2012), should not be ruled out, as it could have essentially led to an increase in the size of the ‘sample population’ by including some previously unsampled areas. Not knowing which exact landscapes were sampled previously makes this hard to speculate upon, but the possibility remains.
Natives vs. Exotics Another interesting metric is the comparison of the Importance Values of native species and non-native species relative to each other (see Table 3 below). This comparison is not between percentage of total individuals that are native, or percentage basal area that is native, or percentage area coverage by natives, but a relative value that includes all three of these metrics (important to remember when interpreting Importance Values throughout the rest of this paper). Also, while many non-native individuals/species are not deemed invasive, most of them are. While the Ramble maintains the highest relative values in most of the diversity indices listed in the table above, its sample dataset was composed of slightly less total Importance Values from natives relative to the Hallett and North Woods. The large presence of long-ago introduced, ‘legacy’ horticultural exotics such as Phellodendron amurense and Styphnolobium japonicum is likely the reason for this disparity, as the Ramble has otherwise seen the most sustained management oriented towards native species. The Great Hill is also significantly lower in native percentage than the other woodlands, which is notable given that it is not (currently) within the Natural Areas management framework.
North Total N.A. Ramble Woods Hallett Great Hill Native IV 235.24 229.22 243.51 237.73 192.39 Exotic IV 64.76 70.78 56.49 62.27 107.61 % Native 78.41 76.41 81.17 79.24 64.13
Table 3.) Native vs. non-native species for each dataset, in terms of Importance Values. Color coding is intended to help visualize differences between woodlands, with dark green indicating a relatively high value, and yellow relatively low.
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Importance Values This section focuses primarily on the comparison of the Importance Values of species within each woodland, as well as the entire combined dataset for all 3 of the Natural Areas woodlands, to the those from the previous survey. In various ways these values, and by extension the overall forest community, have changed in noticeable ways from the previous study by Regina Alvarez (2012).
Total Woodlands This dataset combines the total sample points from the Ramble, North Woods, and Hallett Sanctuary. The Great Hill, not currently considered a designated Natural Area, is not included in this combined dataset as to better compare to the data from the Alvarez study, as well as allow for better measurement of management progress in the Natural Areas. As mentioned previously, total species tallied rose from 82 to 119 since the last survey, with total genera increasing from 50 to 67. Of the 119 species in this survey, 81 (or 68%) were native, though 78.4% of all Importance Value was native, indicating that most non-native species were found in smaller numbers and sizes on average. The 81 native species is only one less species than the entire species count, including non-native species, of the previous survey. In the last survey, 22 of 82 species were represented by a single individual, while in this survey 32 of 119 species were also only represented by a single individual. In the previous study, Alvarez (2012) found that Prunus serotina (black cherry), a native disturbance-adapted early successional species, was ranked first with an Importance Value (IV) of 67.45, while Quercus rubra (Northern red oak) was second with an IV of 29.99, though the previous study noted that it had relatively low regeneration rates in comparison to other species with high IV ranking. Acer platanoides (Norway maple), an invasive species noted to have heavy influence and increasing spread in the Alvarez study, came in #3, and the similar New York State listed invasive Acer pseudoplatanus (Sycamore maple) (NYDEC) was #9. Quercus palustris (pin oak), Ulmus americana (American elm), Celtis occidentalis (common hackberry) and Viburnum dentatum (arrowwood viburnum) were also all in the top 10, and desirable natives Liriodendron tulipifera (tulip tree), Lindera benzoin (spicebush), Sassafras albidum (Sassafras), and Carya cordiformis (bitternut hickory) were in the top 20. It was noted in that study that the high value of Ulmus americana at #5 was significant, given the heavy presence of Dutch Elm Disease limiting spread, and that Prunus serotina, while never planted at any stage of the park’s history and periodically thinned actively by park managers, was still #1. Two exotic species with potential to exhibit invasive spread, Phellodendron amurense (Amur cork tree) and Styphnolobium japonicum (Chinese scholar tree), were #11 and #12, respectively, and the need for their monitoring for potential management was noted as well. The current study finds notable differences from the previous results (see Table 4 below). P. serotina is still #1 in IV, but its value has dropped significantly, to 34.72 from the previous 67.45. Quercus palustris now holds the #2 spot instead of Q. rubra (which dropped to #3). As in the previous study, it should be noted that while both the relative density and frequency of Q.
35 rubra (and this time Q. palustris as well) are lower that P. serotina, the relative dominance (or basal area) of the oaks was much higher, even with much fewer individuals. Celtis occidentalis and Viburnum dentatum both increased significantly as well, moving from #8 to #5 and #10 to #6, respectively, Ulmus americana remained in the top five, moving up one spot to #4. Another interesting finding is that, while only one vine was found in the previous survey (Parthenocissus tricuspidata), 4 individual vines (2 Ampelopsis brevipedunculata, 1 Celastrus orbiculatus, and 1 Wisteria), all invasive, were found in this one. This might not be statistically significant, but is worth noting, nonetheless. Most native vines throughout the park, including the native Parthenocissus quinquefolia (Virginia creeper) and Toxicodendron radicans (poison ivy), grow directly upon other larger species in order to achieve space within the canopy. If this was the case for a vine in this survey, the vine was not counted, and the tree/shrub it was growing on was measured instead. Only ‘free-standing’ vines, or ones whose stem at and below 1.3 m was not attached to another tree/shrub, were counted.
Invasive Species Changes A noticeable absence from the top 5 of the combined dataset is Acer platanoides, now ranked #10 vs #3 in the previous study. This species is still a large focus of management efforts, but it would appear that those efforts are showing dividends. It was noted in the previous study that A. platanoides was found in all 3 woodland landscapes, and this is still the case for this survey, but in greatly decreased numbers. Even more striking is the movement of Acer pseudoplatanus from #9 to #53, with only 6 total individuals found in the sample compared to 38 in the previous survey. Other successful decreases in invasive species can be found as well. Morus alba, or white mulberry, is a very common volunteer tree in the park. While mature trees are often managed for their wildlife value, when they fall or are removed for safety reasons, they are not replaced, and saplings are often removed. Ranked #6 in IV in the last study, white mulberry is now #27 after continued but gradual removal. While Phellodendron went relatively unchanged (#11 to #12), likely due to the presence of many large specimens in the sample (for which there are no plans for removal due to their aesthetic value and legacy position in high- profile areas), Styphnolobium japonicum decreased from #12 to #48. The total number of specimens for this species was only 6, vs 22 in the last survey. This is largely due to concerted efforts to limit regeneration of this species, while also allowing mature specimens to continue in the landscape. Ailanthus altissima, the tree-of-heaven, once occupied #16 but is now #45, going from 18 individuals to only 4, two of them mature and being monitored for removal, but with no plans to do so as of now. Another interesting finding is the total non-existence of some invasive species in this survey that were found in the previous survey. Rhodotypos scandens, or jetbead, is an introduced ornamental understory shrub that has shown great propensity to spread throughout all three woodlands, and was ranked #20, with 19 individuals in the previous survey. This species is now absent from this survey, and an independent observer could attest that near complete exclusion of this species from the woodlands has been achieved, while it is still used as an ornamental
36 elsewhere in the park. Frangula alnus (glossy buckthorn), previously #30 with 14 individuals found (and common in the Hallett especially), is also absent from the data, as is Rosa multiflora, which was found 4 times in the previous survey. However, there are a few areas where non-native species have begun to show increasing numbers, and future monitoring and/or management may become a larger priority. Malus spp. (which here describes a species complex of ornamental crabapples including Tea crabapple, Malus hupehensis, and Siebold’s crabapple, Malus toringo) increased its rank from # 18 to #8. Prunus spp., (which for this survey covers a species complex of Asian ornamental cherries such as Prunus serrulata) also increased its IV rank from #43 to #25.
Native Species Changes Besides the increases in ranking of Quercus palustris, Celtis occidentalis, Viburnum dentatum and Ulmus americana, mentioned previously, a few other native species saw notable increases. Acer rubrum, or red maple, was previous ranked only #23 in IV, despite it being a common and adaptable locally occurring tree with one of the widest ranges in eastern North America. Now, this ranking has increased to #7, with the Ramble accounting for the most stems. This is a species, along with Acer saccharinum (silver maple) and Acer saccharum (sugar maple), which park managers would rather see in place of Acer platanoides and Acer pseudoplatanus, at least in the Natural Areas. Sassafras albidum and Nyssa sylvatica (black tupelo) are also natives that were surprising in their increases. Sassafras was once a more secondary species, ranked #15, but in this survey ranked #9, with an over 50% increase in the number of stems surveyed as well as an increase in mean DBH, even as its presence within the lower size range increased as well. Nyssa sylvatica was one of the bigger surprises, as it was actually not tallied at all in the original dataset, whereas 33 individuals (all in the Ramble) were found in this survey. It was specifically noted in the Alvarez study that this was a species present in the Ramble, though by chance did not make it into the sample, but the large difference is still unexpected. There are a number of notable mature Nyssa sylvatica in the park, most of them in the Ramble, but it was observed that many of the counted stems in this survey appeared to be young (likely less than a decade old) vegetative sprouts or saplings within relatively close proximity to these larger specimens. It is possible that efforts to reduce trampling and reduce competition from invasive species such as Japanese knotweed have led to a significant sapling increase, thus leading to the large difference in the presence of Nyssa sylvatica in the woodlands. This decrease in trampling and competing species, combined with other restoration efforts could also help explain both the relative and absolute increase in understory presence of Celtis, Viburnum, Acer rubrum, Sassafras and even Hamamelis virginiana in all size classes, as invasive species like Acer platanoides, Morus alba, Frangula alnus, Rosa multiflora and Rhodotypos scandens, all competitive in the understory, are removed from the landscape. Finally, worth mentioning is the presence of Ptelea trifoliata (hop tree, or wafer-ash), even as only a few individuals were captured within the survey (none were in the previous
37 survey sample). It is unknown whether this species has experienced concrete changes in its numbers within the park since the last survey, but this species is notable as a state recognized endangered species (NYNHP), perhaps the only naturally occurring woody species with this designation in the park. It is not believed to have been planted and was noted to exist on the park’s grounds in the original survey of the park’s flora (Rawolle and Pilat, 1857). This species is remarkably found in a fairly stable population within one of the areas of the North Woods that remains the most trampled and open to the public, as well as in close proximity to large remaining stands of Japanese knotweed, which compete with it for resources on rocky dry slopes in open woods. However, evidence of its reproduction in adjacent areas is also apparent (however minor), so further monitoring of these populations (as well as a few disjunct individuals in the Hallett Sanctuary) is recommended. This area is also notable for large mature specimens of Quercus alba (white oak), perhaps naturally occurring, which it should be noted are not reproducing well as this species is barely found in lower size classes, hardly any of which were not planted.
By Genus The previous survey also noted changes by family and genus ranking, though this study only provides tables and analysis based on genus, foregoing analysis based on family (see Table 6 below). The major point of note when looking at IV rankings based on genus is that Quercus has actually passed Prunus for the top spot among all genera in the total Natural Areas dataset. This is even despite a slight increase found in the Prunus spp. complex of ornamental Asian cherries. Another genus worthy of note is Fraxinus, which stayed at the ranking of #7 through both surveys. This summer marked the first full year since the Emerald ash borer was found in the park. The Conservancy has undertaken a regimen of treating a large number of mature ash trees every few years, so it will be interesting to note the potential decline in this genus in future similar surveys, or perhaps the treatment will have been relatively successful in a way measurable by size class. As of now, there is healthy recruitment and representation of seemingly healthy Fraxinus specimens in all size classes. Acer is another genus worth of note here. Despite the large decline in IV from what were previously the most numerous maple species, the invasive Norway and Sycamore maples, the overall genus IV ranking only moved from #3 to #4. This is likely due to the concurrent increase in Acer rubrum, as well as the slight increase in Acer saccharum from #26 overall to #23 overall. Though the increase in Acer saccharum in IV rank is small, it should be noted that the total number of individuals found in this survey was 23, in comparison to only 14 in the last survey.
Lower Quartile Range Finally, one important value that the Alvarez study analyzed was the share of the sample that comprised the lowest DBH quarter of individuals, or the subset of smaller specimens, called the lower quartile range (LQR) (See Table 7 below). This metric is important, as it can provide a proxy for evaluating both the understory forest layer, as well as provide insights as to which
38 woody species appear to be reproducing more successfully, thus potentially impacting future canopy composition. For this combined dataset, this was every stem between 2.3 cm and 1 cm (the minimum size). The first thing we notice here is that Acer platanoides, ranked first of all species in the LQR of the previous survey, is now ranked #10. This is a significant decrease, since the IV rank in the lowest DBH range now mirrors the total IV rank of #10, which would suggest that the regeneration of this species is being kept in check in line with the staged selective removal (or topping/girdling to create habitat trees in low risk areas) of mature specimens. Acer pseudoplatanus also decreased from #7 in this category to #31. Aralia elata (Japanese angelica tree) is another species specifically mentioned in the Alvarez study for monitoring in future surveys. It is a pernicious invasive species which has been targeted heavily for removal, and it also decreased in this group from #11 to #22 in IV ranking. Where native species Celtis occidentalis and Ulmus americana were once ranked #3 and #5 in the LQR, they are now #1 and #2, respectively, with Prunus serotina now down one spot to #3. Quercus rubra, noted in the previous survey for the disparity in its high overall IV rank of #2 but relatively low rank of #22 in the LQR, showed an increase to #8 in the LQR for the combined Natural Areas dataset, which show’s improvement toward the regeneration that Alvarez had noted would be desirable. Other natives showing notable increases in LQR IV ranking were Carya cordiformis (#18 to #6), Sassafras albidum (#8 to #4), and Nyssa sylvatica (previously not found, now #11). It is likely that each of these species has seen increases in their regeneration due to factors mentioned in the ‘Native Species Changes’ section above. Yet another possibility is the great reduction in Japanese knotweed and multiflora rose, which competed for light in the understory. Acer rubrum, noted above as having seen an increase in overall IV, also saw a large increase here, going from not being present at all in the previous survey’s lower quartile to being ranked #13. The LQR range also saw an increase in the emerging potential invasives identified for the whole dataset, Malus spp. and Prunus spp. These saw increases from #14 to #8 and #32 to #18, respectively. This indicates that a significant reason for these species’ increase in overall IV rank is due to their successful reproduction, which seems to have increased in the last decade specifically. The rise of both of these species complexes in the woodlands has become increasingly notable, and each are actively thinned in areas where other restoration efforts are being prioritized concurrently.
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Figure 5.) Areas within Central Park sampled in this study, delineated by specific woodland. All woodlands except the Great Hill are included in the Natural Areas, the total dataset of which is analyzed above.
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Table 4.) Ecological dominance in terms of Importance Value for total Natural Areas dataset (The Ramble, North Woods, and Hallett Sanctuary).
Relative Relative Relative Importance Rank Species Name Density Dominance Frequency Value 1 Prunus serotina 13.31 9.13 12.28 34.72 2 Quercus palustris 3.72 18.56 4.21 26.48 3 Quercus rubra 3.31 11.16 3.51 17.98 4 Ulmus americana 7.64 2.73 7.19 17.56 5 Celtis occidentalis 6.49 1.95 5.35 13.79 6 Viburnum dentatum 5.27 0.26 3.77 9.30 7 Acer rubrum 2.64 3.99 2.54 9.17 8 Malus spp. 4.05 0.44 4.12 8.62 9 Sassafras albidum 3.99 1.32 3.07 8.38 10 Acer platanoides 2.64 1.75 2.81 7.19 11 Carya cordiformis 2.50 1.77 2.72 6.99 12 Phellodendron amurense 0.74 5.10 0.88 6.72 13 Nyssa sylvatica 2.23 2.27 1.49 5.99 14 Robinia pseudoacacia 1.55 2.73 1.67 5.95 15 Fraxinus americana 2.43 0.27 2.81 5.51 16 Fraxinus pennsylvanica 1.82 1.28 2.02 5.12 17 Tilia americana 1.08 3.05 0.96 5.09 18 Lindera benzoin 2.30 0.19 2.46 4.95 19 Quercus velutina 0.74 3.16 0.96 4.87 20 Carpinus caroliniana 1.15 2.21 1.32 4.68 21 Liriodendron tulipifera 1.42 1.37 1.67 4.45 22 Platanus acerifolia 0.27 3.32 0.35 3.94 23 Acer saccharum 1.55 0.68 1.58 3.81 24 Hamamelis virginiana 1.69 0.44 1.67 3.80 25 Prunus spp. 1.28 0.49 1.49 3.27 26 Quercus phellos 0.34 2.48 0.26 3.08 27 Morus alba 0.88 1.13 1.05 3.06 28 Ulmus spp. 0.74 1.42 0.88 3.04 29 Photinia villosa 1.42 0.11 1.49 3.02 30 Viburnum prunifolium 1.28 0.07 1.49 2.84 31 Platanus occidentalis 0.14 2.46 0.18 2.77 32 Crataegus spp. 1.22 0.22 1.32 2.76 33 Fagus grandifolia 0.27 1.37 0.35 1.99 34 Amelanchier spp. 0.81 0.33 0.79 1.93 35 Fagus sylvatica 0.14 1.56 0.18 1.87 36 Liquidambar styraciflua 0.47 0.72 0.53 1.72 37 Gingko biloba 0.20 1.17 0.26 1.64 38 Quercus alba 0.27 0.88 0.35 1.50 39 Pinus strobus 0.27 0.83 0.35 1.46
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40 Gymnocladus dioica 0.27 0.83 0.35 1.45 41 Betula lenta 0.54 0.20 0.70 1.45 42 Sambucus canadensis 0.68 0.06 0.70 1.43 43 Ulmus procera 0.68 0.14 0.61 1.43 44 Rubus allegheniensis 0.74 0.00 0.61 1.36 45 Ailanthus altissima 0.27 0.71 0.35 1.33 46 Pinus nigra 0.20 0.77 0.26 1.24 47 Philadelphus coronarius 0.54 0.01 0.61 1.16 48 Styphnolobium japonicum 0.41 0.20 0.53 1.14 49 Lonicera maackii 0.47 0.23 0.35 1.06 50 Quercus montana 0.47 0.06 0.53 1.06 51 Quercus bicolor 0.07 0.89 0.09 1.05 52 Rhus typhina 0.47 0.04 0.53 1.04 53 Acer pseudoplatanus 0.41 0.03 0.53 0.96 54 Aralia elata 0.47 0.01 0.44 0.92 55 Juglans nigra 0.27 0.22 0.35 0.84 56 Ulmus glabra 0.34 0.03 0.44 0.80 57 Cornus mas 0.34 0.09 0.35 0.78 58 Ilex verticillata 0.34 0.00 0.35 0.69 59 Prunus virginiana 0.34 0.00 0.35 0.69 60 Ilex opaca 0.27 0.04 0.35 0.66 61 Viburnum dilatatum 0.27 0.02 0.35 0.64 62 Rhododendron maximum 0.27 0.01 0.35 0.63 63 Rhus glabra 0.34 0.01 0.26 0.61 64 Cercis canadensis 0.20 0.05 0.26 0.52 65 Acer negundo 0.27 0.06 0.18 0.51 66 Quercus cerris 0.14 0.17 0.18 0.48 67 Cornus amomum 0.20 0.00 0.26 0.47 68 Rhododendron spp. 0.20 0.00 0.26 0.47 69 Ilex crenata 0.14 0.13 0.18 0.44 70 Corylus americana 0.20 0.05 0.18 0.43 71 Cornus racemosa 0.20 0.01 0.18 0.39 72 Broussonetia papyrifera 0.20 0.01 0.18 0.39 73 Viburnum lentago 0.20 0.01 0.18 0.39 74 Tetradium daniellii 0.14 0.07 0.18 0.38 75 Fraxinus excelsior 0.20 0.00 0.18 0.38 76 Forsythia spp. 0.20 0.00 0.18 0.38 77 Aesculus parviflora 0.20 0.00 0.18 0.38 78 Aesculus glabra 0.14 0.03 0.18 0.34 79 Quercus coccinea 0.14 0.02 0.18 0.33 80 Cladrastis kentukea 0.14 0.02 0.18 0.33 81 Amelanchier canadensis 0.14 0.01 0.18 0.32 82 Magnolia virginiana 0.14 0.01 0.18 0.32 83 Unknown spp. 0.14 0.01 0.18 0.32 84 Cornus sericea 0.14 0.00 0.18 0.31 42
Ampelopsis 85 brevipedunculata 0.14 0.00 0.18 0.31 86 Ptelea trifoliata 0.14 0.00 0.18 0.31 87 Ligustrum sinense 0.14 0.00 0.18 0.31 88 Carya ovata 0.07 0.08 0.09 0.23 89 Diospyros virginiana 0.14 0.00 0.09 0.22 90 Aesculus flava 0.07 0.06 0.09 0.21 91 Crataegus crus-galli 0.07 0.04 0.09 0.19 92 Salix nigra 0.07 0.04 0.09 0.19 93 Quercus imbricaria 0.07 0.03 0.09 0.18 94 Betula pendula 0.07 0.02 0.09 0.17 95 Acer palmatum 0.07 0.02 0.09 0.17 96 Catalpa speciosa 0.07 0.02 0.09 0.17 97 Viburnum spp. 0.07 0.01 0.09 0.17 98 Populus deltoides 0.07 0.01 0.09 0.16 99 Cornus florida 0.07 0.01 0.09 0.16 100 Tsuga canadensis 0.07 0.01 0.09 0.16 101 Ulmus pumila 0.07 0.00 0.09 0.16 102 Juniperus virginiana 0.07 0.00 0.09 0.16 103 Gleditsia triacanthos 0.07 0.00 0.09 0.16 104 Rhus aromatica 0.07 0.00 0.09 0.16 105 Calycanthus floridus 0.07 0.00 0.09 0.16 106 Populus grandidentata 0.07 0.00 0.09 0.16 107 Viburnum acerifolium 0.07 0.00 0.09 0.16 108 Wisteria floribunda 0.07 0.00 0.09 0.16 109 Betula papyrifera 0.07 0.00 0.09 0.16 110 Cephalanthus occidentalis 0.07 0.00 0.09 0.16 111 Clethra alnifolia 0.07 0.00 0.09 0.16 112 Pyrus calleryana 0.07 0.00 0.09 0.16 113 Celastrus orbiculatus 0.07 0.00 0.09 0.16 114 Crataegus phaenopyrum 0.07 0.00 0.09 0.16 115 Rhus copallinum 0.07 0.00 0.09 0.16 116 Parthenocissus quinquefolia 0.07 0.00 0.09 0.16 117 Populus tremuloides 0.07 0.00 0.09 0.16 118 Cornus drummondii 0.07 0.00 0.09 0.16 119 Rubus laciniatus 0.07 0.00 0.09 0.16 Total Total Total Total 100 100 100 300
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Table 5.) Descriptive statistics for total Natural Areas dataset, species listed alphabetically.
Absolute Standard % Density Mean DBH Deviation Min, Max Median Sampling Species Name Abundance (trees/ha) (cm) (cm) (cm) (cm) Units Acer negundo 4 5.35 7.90 5.85 1.6, 16.8 6.60 0.54 Acer palmatum 1 1.34 10.70 0.00 10.7, 10.7 10.70 0.27 Acer platanoides 39 52.14 10.08 13.69 1.1, 49.5 3.70 8.65 Acer pseudoplatanus 6 8.02 4.13 3.96 1.1, 12.7 2.55 1.62 Acer rubrum 39 52.14 13.17 22.05 1.1, 95.9 4.60 7.84 Acer saccharum 23 30.75 8.42 10.95 1.4, 54.8 4.40 4.86 Aesculus flava 1 1.34 19.40 0.00 19.4, 19.4 19.40 0.27 Aesculus glabra 2 2.67 10.00 2.90 7.1, 12.9 10.00 0.54 Aesculus parviflora 3 4.01 1.43 0.61 1, 2.3 1.00 0.54 Ailanthus altissima 4 5.35 25.33 22.49 1.7, 50.6 24.50 1.08 Amelanchier canadensis 2 2.67 5.80 0.80 5, 6.6 5.80 0.54 Amelanchier spp. 12 16.04 9.77 9.07 1, 30.3 6.70 2.43 Ampelopsis brevipedunculata 2 2.67 2.75 0.05 2.7, 2.8 2.75 0.54 Aralia elata 7 9.36 2.44 0.95 1.2, 4.2 2.30 1.35 Betula lenta 8 10.70 9.05 9.09 1, 31.1 6.30 2.16 Betula papyrifera 1 1.34 3.00 0.00 3, 3 3.00 0.27 Betula pendula 1 1.34 11.20 0.00 11.2, 11.2 11.20 0.27 Broussonetia papyrifera 3 4.01 3.70 1.20 2.4, 5.3 3.40 0.54 Calycanthus floridus 1 1.34 3.60 0.00 3.6, 3.6 3.60 0.27 Carpinus caroliniana 17 22.73 22.05 18.78 1.3, 57.7 16.80 4.05 Carya cordiformis 37 49.47 8.01 15.64 1.1, 77.9 2.50 8.38 Carya ovata 1 1.34 22.20 0.00 22.2, 22.2 22.20 0.27 Catalpa speciosa 1 1.34 10.10 0.00 10.1, 10.1 10.10 0.27 Celastrus orbiculatus 1 1.34 1.90 0.00 1.9, 1.9 1.90 0.27 Celtis occidentalis 96 128.35 6.36 9.60 1.1, 72 3.30 16.49 Cephalanthus occidentalis 1 1.34 2.40 0.00 2.4, 2.4 2.40 0.27 Cercis canadensis 3 4.01 9.33 4.45 5.7, 15.6 6.70 0.81 Cladrastis kentukea 2 2.67 6.90 1.80 5.1, 8.7 6.90 0.54 Clethra alnifolia 1 1.34 2.10 0.00 2.1, 2.1 2.10 0.27 Cornus amomum 3 4.01 2.83 1.28 1.6, 4.6 2.30 0.81 Cornus drummondii 1 1.34 1.30 0.00 1.3, 1.3 1.30 0.27 Cornus florida 1 1.34 6.00 0.00 6, 6 6.00 0.27 Cornus mas 5 6.68 9.76 4.28 3.1, 15.3 10.60 1.08 Cornus racemosa 3 4.01 4.40 0.71 3.9, 5.4 3.90 0.54 Cornus sericea 2 2.67 2.95 0.15 2.8, 3.1 2.95 0.54 Corylus americana 3 4.01 9.10 5.35 2.4, 15.5 9.40 0.54 Crataegus crus-galli 1 1.34 15.20 0.00 15.2, 15.2 15.20 0.27 Crataegus phaenopyrum 1 1.34 1.80 0.00 1.8, 1.8 1.80 0.27
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Crataegus spp. 18 24.07 6.72 5.91 1.2, 22.5 4.95 4.05 Diospyros virginiana 2 2.67 2.25 0.35 1.9, 2.6 2.25 0.27 Fagus grandifolia 4 5.35 35.78 30.51 1.5, 75.5 33.05 1.08 Fagus sylvatica 2 2.67 70.75 4.55 66.2, 75.3 70.75 0.54 Forsythia spp. 3 4.01 1.90 0.22 1.7, 2.2 1.80 0.54 Fraxinus americana 36 48.13 4.38 5.39 1.1, 27.9 2.40 8.65 Fraxinus excelsior 3 4.01 2.00 0.24 1.7, 2.3 2.00 0.54 Fraxinus pennsylvanica 27 36.10 11.33 13.31 1.1, 41.3 4.30 6.22 Gingko biloba 3 4.01 40.33 29.80 3.9, 76.9 40.20 0.81 Gleditsia triacanthos 1 1.34 3.80 0.00 3.8, 3.8 3.80 0.27 Gymnocladus dioica 4 5.35 23.88 27.75 1.5, 69.6 12.20 1.08 Hamamelis virginiana 25 33.42 8.29 6.77 1.1, 31 5.90 5.14 Ilex crenata 2 2.67 15.80 12.50 3.3, 28.3 15.80 0.54 Ilex opaca 4 5.35 7.45 3.77 4.3, 13.8 5.85 1.08 Ilex verticillata 5 6.68 2.16 0.47 1.4, 2.7 2.10 1.08 Juglans nigra 4 5.35 15.05 11.18 1.9, 31.1 13.60 1.08 Juniperus virginiana 1 1.34 4.00 0.00 4, 4 4.00 0.27 Ligustrum sinense 2 2.67 1.10 0.00 1.1, 1.1 1.10 0.54 Lindera benzoin 34 45.46 5.19 3.08 1.2, 11.8 4.70 7.57 Liquidambar styraciflua 7 9.36 14.96 20.93 1.2, 60.7 3.10 1.62 Liriodendron tulipifera 21 28.08 15.60 13.30 1.4, 60.9 12.70 5.14 Lonicera maackii 7 9.36 10.73 9.96 1.5, 27.7 6.10 1.08 Magnolia virginiana 2 2.67 3.75 2.45 1.3, 6.2 3.75 0.54 Malus spp. 60 80.22 5.48 4.20 1, 26.3 4.90 12.70 Morus alba 13 17.38 18.24 15.16 1.8, 45.4 9.60 3.24 Nyssa sylvatica 33 44.12 9.55 18.78 1.4, 93.8 4.20 4.59 Parthenocissus quinquefolia 1 1.34 1.40 0.00 1.4, 1.4 1.40 0.27 Phellodendron amurense 11 14.71 38.52 38.79 1.6, 117.1 12.20 2.70 Philadelphus coronarius 8 10.70 2.39 1.11 1.1, 4.6 2.05 1.89 Photinia villosa 21 28.08 4.76 3.24 1.2, 13.7 3.90 4.59 Pinus nigra 3 4.01 38.43 13.55 19.3, 48.8 47.20 0.81 Pinus strobus 4 5.35 25.25 26.58 6.8, 71 11.60 1.08 Platanus acerifolia 4 5.35 72.38 10.72 61.1, 87.5 70.45 1.08 Platanus occidentalis 2 2.67 79.60 39.90 39.7, 119.5 79.60 0.54 Populus deltoides 1 1.34 6.30 0.00 6.3, 6.3 6.30 0.27 Populus grandidentata 1 1.34 3.50 0.00 3.5, 3.5 3.50 0.27 Populus tremuloides 1 1.34 1.40 0.00 1.4, 1.4 1.40 0.27 Prunus serotina 197 263.38 13.28 11.05 1, 55.5 10.60 37.84 Prunus spp. 19 25.40 8.45 9.78 1.1, 37 3.20 4.59 Prunus virginiana 5 6.68 1.40 0.24 1.1, 1.8 1.30 1.08 Ptelea trifoliata 2 2.67 2.00 0.40 1.6, 2.4 2.00 0.54 Pyrus calleryana 1 1.34 2.00 0.00 2, 2 2.00 0.27 Quercus alba 4 5.35 21.08 31.29 1, 75.2 4.05 1.08 Quercus bicolor 1 1.34 75.90 0.00 75.9, 75.9 75.90 0.27
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Quercus cerris 2 2.67 19.80 12.80 7, 32.6 19.80 0.54 Quercus coccinea 2 2.67 8.75 0.85 7.9, 9.6 8.75 0.54 Quercus imbricaria 1 1.34 13.60 0.00 13.6, 13.6 13.60 0.27 Quercus montana 7 9.36 4.83 5.39 1.1, 16.3 1.90 1.62 Quercus palustris 55 73.53 28.82 36.68 1.1, 123.5 9.70 12.97 Quercus phellos 5 6.68 35.50 43.97 5.6, 123 16.70 0.81 Quercus rubra 49 65.51 23.55 30.23 1, 116.5 6.70 10.81 Quercus velutina 11 14.71 26.72 33.72 1.5, 89.4 9.30 2.97 Rhododendron maximum 4 5.35 3.83 0.78 2.7, 4.8 3.90 1.08 Rhododendron spp. 3 4.01 1.60 0.16 1.4, 1.8 1.60 0.81 Rhus aromatica 1 1.34 3.80 0.00 3.8, 3.8 3.80 0.27 Rhus copallinum 1 1.34 1.50 0.00 1.5, 1.5 1.50 0.27 Rhus glabra 5 6.68 2.32 1.35 1, 4.8 2.00 0.81 Rhus typhina 7 9.36 5.37 2.57 1, 9.9 5.50 1.62 Robinia pseudoacacia 23 30.75 21.19 17.78 1.6, 57.4 13.90 5.14 Rubus allegheniensis 11 14.71 1.46 0.50 1.1, 2.9 1.30 1.89 Rubus laciniatus 1 1.34 1.00 0.00 1, 1 1.00 0.27 Salix nigra 1 1.34 15.20 0.00 15.2, 15.2 15.20 0.27 Sambucus canadensis 10 13.37 4.72 3.69 1.3, 14 4.20 2.16 Sassafras albidum 59 78.88 7.73 9.19 1, 46.2 4.30 9.46 Styphnolobium japonicum 6 8.02 10.18 10.75 2.1, 32.5 5.25 1.62 Tetradium daniellii 2 2.67 15.30 0.30 15, 15.6 15.30 0.54 Tilia americana 16 21.39 20.24 28.60 1.1, 88.7 7.00 2.97 Tsuga canadensis 1 1.34 5.80 0.00 5.8, 5.8 5.80 0.27 Ulmus americana 113 151.08 8.20 9.41 1, 75.5 4.80 22.16 Ulmus glabra 5 6.68 4.78 3.13 1.3, 10.5 4.30 1.35 Ulmus procera 10 13.37 6.87 6.50 1.6, 25.3 5.45 1.89 Ulmus pumila 1 1.34 4.70 0.00 4.7, 4.7 4.70 0.27 Ulmus spp. 11 14.71 18.62 22.04 1.7, 75.3 9.60 2.70 Unknown spp. 2 2.67 3.95 0.95 3, 4.9 3.95 0.54 Viburnum acerifolium 1 1.34 3.30 0.00 3.3, 3.3 3.30 0.27 Viburnum dentatum 78 104.28 4.07 2.20 1.2, 11.1 3.45 11.62 Viburnum dilatatum 4 5.35 5.18 2.41 1.7, 7.8 5.60 1.08 Viburnum lentago 3 4.01 3.40 1.82 1.4, 5.8 3.00 0.54 Viburnum prunifolium 19 25.40 4.21 2.44 1.2, 9 4.00 4.59 Viburnum spp. 1 1.34 9.40 0.00 9.4, 9.4 9.40 0.27 Wisteria floribunda 1 1.34 3.10 0.00 3.1, 3.1 3.10 0.27 Total Total Total (w/CI) Total Total Total 1480 1978.69 11.32 17.53 1, 123.5 5.00 (10.43, 12.22)
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Table 6.) Ecological dominance in terms of Importance Value grouped by Genus for total Natural Areas dataset (The Ramble, North Woods, and Hallett Sanctuary).
Relative Relative Relative Importance Rank Genus Density Dominance Frequency Value 1 Quercus 9.26 37.41 10.35 57.02 2 Prunus 14.93 9.62 14.12 38.67 3 Ulmus 9.46 4.32 9.21 22.99 4 Acer 7.57 6.53 7.72 21.81 5 Celtis 6.49 1.95 5.35 13.79 6 Viburnum 7.16 0.37 5.96 13.50 7 Fraxinus 4.46 1.55 5.00 11.01 8 Malus 4.05 0.44 4.12 8.62 9 Sassafras 3.99 1.32 3.07 8.38 10 Carya 2.57 1.85 2.81 7.22 11 Phellodendron 0.74 5.10 0.88 6.72 12 Platanus 0.41 5.78 0.53 6.71 13 Nyssa 2.23 2.27 1.49 5.99 14 Robinia 1.55 2.73 1.67 5.95 15 Tilia 1.08 3.05 0.96 5.09 16 Lindera 2.30 0.19 2.46 4.95 17 Carpinus 1.15 2.21 1.32 4.68 18 Liriodendron 1.42 1.37 1.67 4.45 19 Fagus 0.41 2.93 0.53 3.86 20 Hamamelis 1.69 0.44 1.67 3.80 21 Crataegus 1.35 0.26 1.49 3.10 22 Morus 0.88 1.13 1.05 3.06 23 Photinia 1.42 0.11 1.49 3.02 24 Pinus 0.47 1.61 0.61 2.69 25 Cornus 1.01 0.11 1.14 2.26 26 Amelanchier 0.95 0.34 0.96 2.25 27 Rhus 0.95 0.05 0.96 1.96 28 Ilex 0.74 0.17 0.88 1.79 29 Betula 0.68 0.22 0.88 1.78 30 Liquidambar 0.47 0.72 0.53 1.72 31 Gingko 0.20 1.17 0.26 1.64 32 Rubus 0.81 0.00 0.70 1.52 33 Gymnocladus 0.27 0.83 0.35 1.45 34 Sambucus 0.68 0.06 0.70 1.43 35 Ailanthus 0.27 0.71 0.35 1.33 36 Philadelphus 0.54 0.01 0.61 1.16 37 Styphnolobium 0.41 0.20 0.53 1.14 38 Rhododendron 0.47 0.01 0.61 1.10 39 Lonicera 0.47 0.23 0.35 1.06
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40 Aesculus 0.41 0.09 0.44 0.94 41 Aralia 0.47 0.01 0.44 0.92 42 Juglans 0.27 0.22 0.35 0.84 43 Cercis 0.20 0.05 0.26 0.52 44 Populus 0.20 0.01 0.26 0.47 45 Corylus 0.20 0.05 0.18 0.43 46 Broussonetia 0.20 0.01 0.18 0.39 47 Tetradium 0.14 0.07 0.18 0.38 48 Forsythia 0.20 0.00 0.18 0.38 49 Cladrastis 0.14 0.02 0.18 0.33 50 Magnolia 0.14 0.01 0.18 0.32 51 Unknown 0.14 0.01 0.18 0.32 52 Ampelopsis 0.14 0.00 0.18 0.31 53 Ptelea 0.14 0.00 0.18 0.31 54 Ligustrum 0.14 0.00 0.18 0.31 55 Diospyros 0.14 0.00 0.09 0.22 56 Salix 0.07 0.04 0.09 0.19 57 Catalpa 0.07 0.02 0.09 0.17 58 Tsuga 0.07 0.01 0.09 0.16 59 Juniperus 0.07 0.00 0.09 0.16 60 Gleditsia 0.07 0.00 0.09 0.16 61 Calycanthus 0.07 0.00 0.09 0.16 62 Wisteria 0.07 0.00 0.09 0.16 63 Cephalanthus 0.07 0.00 0.09 0.16 64 Clethra 0.07 0.00 0.09 0.16 65 Pyrus 0.07 0.00 0.09 0.16 66 Celastrus 0.07 0.00 0.09 0.16 67 Parthenocissus 0.07 0.00 0.09 0.16 Total Total Total Total 100 100 100 300
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Table 7.) Ecological dominance in terms of Importance Value for lower DBH quartile range (LQR) of total Natural Areas dataset (The Ramble, North Woods, and Hallett Sanctuary). In this case, the lowest quartile included all stems below 2.3 cm DBH.
Relative Relative Relative Importance Rank Species Name Density Dominance Frequency Value 1 Celtis occidentalis 9.41 9.10 8.87 27.38 2 Ulmus americana 6.99 6.83 6.42 20.25 3 Prunus serotina 5.38 5.51 5.20 16.09 4 Sassafras albidum 5.38 5.21 4.59 15.18 5 Viburnum dentatum 4.84 4.22 4.89 13.96 6 Carya cordiformis 4.57 4.99 4.28 13.84 7 Fraxinus americana 4.03 4.15 4.59 12.77 8 Quercus rubra 3.76 3.88 3.67 11.31 9 Malus spp. 3.76 3.52 3.98 11.26 10 Acer platanoides 3.49 3.49 3.67 10.65 11 Nyssa sylvatica 2.96 3.39 1.83 8.18 12 Fraxinus pennsylvanica 2.42 2.50 2.75 7.67 13 Acer rubrum 2.42 2.39 2.75 7.56 14 Lindera benzoin 2.15 2.91 2.45 7.50 15 Quercus palustris 2.15 2.68 2.45 7.28 16 Rubus allegheniensis 2.69 1.81 2.14 6.63 17 Photinia villosa 1.61 2.10 1.22 4.93 18 Prunus spp. 1.61 1.38 1.83 4.83 19 Viburnum prunifolium 1.61 1.05 1.83 4.50 20 Philadelphus coronarius 1.34 1.43 1.53 4.31 21 Tilia americana 1.34 1.41 1.53 4.28 22 Aralia elata 1.08 1.30 1.22 3.60 23 Prunus virginiana 1.34 1.02 1.22 3.58 24 Crataegus spp. 1.08 0.97 1.22 3.27 25 Sambucus canadensis 1.08 0.96 1.22 3.26 26 Quercus montana 1.08 0.89 0.92 2.88 27 Ilex verticillata 0.81 1.05 0.92 2.77 28 Liriodendron tulipifera 0.81 0.99 0.92 2.71 29 Fraxinus excelsior 0.81 1.23 0.61 2.65 30 Acer saccharum 0.81 0.85 0.92 2.58 31 Acer pseudoplatanus 0.81 0.82 0.92 2.54 32 Forsythia spp. 0.81 1.11 0.61 2.52 33 Rhododendron spp. 0.81 0.78 0.92 2.51 34 Amelanchier spp. 0.81 0.65 0.92 2.37 35 Lonicera maackii 0.81 0.92 0.61 2.34 36 Aesculus parviflora 0.81 0.74 0.61 2.15 37 Rhus glabra 0.81 0.67 0.61 2.09 38 Styphnolobium japonicum 0.54 0.89 0.61 2.04 39 Cornus amomum 0.54 0.79 0.61 1.94 40 Quercus velutina 0.54 0.63 0.61 1.78
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41 Carpinus caroliniana 0.54 0.62 0.61 1.76 42 Gymnocladus dioica 0.54 0.55 0.61 1.70 43 Hamamelis virginiana 0.54 0.38 0.61 1.53 44 Liquidambar styraciflua 0.54 0.29 0.61 1.44 45 Ligustrum sinense 0.54 0.24 0.61 1.39 46 Clethra alnifolia 0.27 0.44 0.31 1.02 47 Pyrus calleryana 0.27 0.40 0.31 0.98 48 Celastrus orbiculatus 0.27 0.36 0.31 0.94 49 Diospyros virginiana 0.27 0.36 0.31 0.94 50 Juglans nigra 0.27 0.36 0.31 0.94 51 Crataegus phaenopyrum 0.27 0.33 0.31 0.90 52 Morus alba 0.27 0.33 0.31 0.90 53 Ailanthus altissima 0.27 0.29 0.31 0.87 54 Ulmus spp. 0.27 0.29 0.31 0.87 55 Viburnum dilatatum 0.27 0.29 0.31 0.87 56 Acer negundo 0.27 0.26 0.31 0.83 57 Phellodendron amurense 0.27 0.26 0.31 0.83 58 Ptelea trifoliata 0.27 0.26 0.31 0.83 59 Robinia pseudoacacia 0.27 0.26 0.31 0.83 60 Ulmus procera 0.27 0.26 0.31 0.83 61 Fagus grandifolia 0.27 0.23 0.31 0.80 62 Rhus copallinum 0.27 0.23 0.31 0.80 63 Parthenocissus quinquefolia 0.27 0.20 0.31 0.77 64 Populus tremuloides 0.27 0.20 0.31 0.77 65 Viburnum lentago 0.27 0.20 0.31 0.77 66 Cornus drummondii 0.27 0.17 0.31 0.75 67 Magnolia virginiana 0.27 0.17 0.31 0.75 68 Ulmus glabra 0.27 0.17 0.31 0.75 69 Betula lenta 0.27 0.10 0.31 0.68 70 Quercus alba 0.27 0.10 0.31 0.68 71 Rhus typhina 0.27 0.10 0.31 0.68 72 Rubus laciniatus 0.27 0.10 0.31 0.68 Total Total Total Total 100 100 100 300
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The Ramble As noted previously, the Ramble dataset in this survey tallied 660 individuals vs. in the previous study, where 444 were tallied. Total species numbers increased from 50 to 84, and genera from 36 to 54, which are proportionately larger increases than the increase in sample size, so we can reasonably speculate that species counts would have increased even if the sample size had stayed the same. Of the 84 species, 60 (or 71%) were native, though 76.4% of Importance Value was from native species, suggesting that non-native species are generally found in smaller numbers and sizes that native species. The main reason for increasing the overall sample size (from 111 points to 165) was to better mirror the number of sample points from the North Woods, as the two woodlands are quite similar in overall size. The previous study by Alvarez (2012) noted a number of takeaways for the Ramble. Prunus serotina held the top rank in IV, though the lead was less pronounced than for other woodlands and the combined dataset. It was also noted that, among the top 20 or so ranked species, the overall IV’s ‘fell off’ less steeply in the Ramble than in other woodlands, indicating more even distribution among the top species. Quercus rubra ranked the lowest in that study in comparison to the total dataset (#20 in the Ramble, compared to #2 overall) with Acer platanoides ranked #2 and Quercus palustris #3. It was noted that the high rank of Acer platanoides was largely due to high density and frequency, while the oaks had much higher basal area/relative dominance values. Celtis had a strong showing at #4, while non-natives Phellodendron amurense and Styphnolobium japonicum took the #5 and #6 spots. Alvarez notes that Phellodendron has ‘not shown invasive tendencies’, even though it has been shown to do so in other areas around New York City, such as the New York Botanical Garden’s Thain Family Forest (Glaeser and Kincaid, 2005, Morgan and Borysiewicz, 2012). However, she notes that Styphnolobium japonicum has shown excellent recruitment, which other managers in the park have also observed, and deems it a species ‘of concern’. Also, worth noting is that Morus alba, a ‘weedy’ species treated as an invasive but with larger specimens largely left alone, was ranked #7 in IV.
Invasive Species Changes The largest noticeable change in the Ramble from the last survey (see Table 8 below) was the decline in Acer platanoides. Previously ranked #2, this species has fallen all the way down to #39, with only 5 individuals found in the sample. And while Phellodendron amurense remained largely unchanged in the last decade (it went from #5 to #6), largely due to many mature specimens remaining in the Ramble and being captured in the sample, Styphnolobium japonicum saw a major decrease, going from and IV ranking of #6 down to #44. Morus alba, previously #7, decreased to #22 and Ailanthus altissima, previously #9, also decreased to #29. In line with the total data set, however, there were some trends where certain non-native species not previously on the radar of managers saw noticeable increases. These were Malus spp. going from #14 to #10, and Prunus spp., moving from #24 to #15, trends which likely helped drive the overall increases seen in these species in the total dataset. Finally, I would like to
51 address the increase in Platanus x acerifolia (hybridus) from IV rank #22 to #11. London plane trees were planted heavily throughout the Ramble during the Robert Moses era of park rejuvenation efforts, and most of these are all similar in age and size and have remained fixtures in the landscape. Though they rank #11 here, this is almost completely due to their high relative dominance/basal area. Only 4 total specimens were captured in the sample, but each one was mature, which drove up the relative IV. As mentioned in the Data Analysis section, values taken from species with relatively low numbers of individuals should be put into perspective, though in this case the relatively high amount of London planes in the Ramble might justify the rank of #11. Most London planes are in lawn areas or near pathways, which might lead one to believe that they would be sampled less given the design of this survey. However, many lawn areas have slowly shrunken over the years as efforts to expand adjacent woodlands has increased. In the process, many London planes have now also been absorbed deeper into wooded landscapes, thus being more likely to be sampled. This phenomenon was observed to be the case for all of the London plane trees in the sample.
Native Species Changes An important change for native species composition between this and the previous survey in the Ramble was that Prunus serotina no longer holds the #1 ranking, that distinction now belonging to the previous #3, Quercus palustris. As expected, this is due largely to the difference in basal area, as P. serotina still holds the top spots in the categories of density and abundance. In both of these categories Celtis occidentalis is a close second to P. serotina, increasing to #3 in overall IV from its previous #4 ranking. Interestingly, Quercus rubra, which was noted as having a relatively low IV ranking in the Ramble previously (#20) also increased to a ranking of #9. One dramatic change among native species in the Ramble was the precipitous rise in Acer rubrum, which rose from #39 and only 2 individuals to #4, with 27 individuals in the sample. This is almost a mirror image of the decline of Acer platanoides, which went from #2 to #39. While the Conservancy does plant some red maples to augment landscapes where invasives have been removed, experience in the field suggests that many of these individuals are also the result of natural recruitment. Another dramatic change was seen in Ulmus americana, which rose from #17 to #5 in the Ramble. This species is common in all size classes as well. The most surprising change was already noted in the results of the total dataset, which is the rise of Nyssa sylvatica, which went from being unrepresented in the last survey to seeing 33 individuals in this one. Again, this is potentially due in large part to decreases in disturbance/compaction of the root zones of larger specimens, allowing for sharp increases in recruitment both vegetative and otherwise. The same goes for Sassafras albidum, which saw most of its increases in the Ramble, increasing its IV rank there from #12 to #8. Alvarez noted that in her experience and others (Dirr, 2009), Sassafras shows less tolerance as a pioneer species in disturbed areas, which would also support the idea that its increase is due to decrease in trampling. Carya cordiformis is also a success, moving from only 3 specimens and a ranking of #29 to 15 specimens and a ranking of #13.
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A few more noteworthy changes among native species were Carpinus caroliniana (#42 to #12) and Hamamelis virginiana (#40 to #17). The increase in Carpinus, however, seems less due to an increase in planting than for Hamamelis. Each species is a desirable native understory species with high wildlife value and species associations (Tallamy, 2007), and I would like to add that their further increase through planting is a current priority for Natural Areas staff, with planting planned for the near future.
By Genus There are a few more insights that can be gained by looking at the differences in composition when grouped by genus (see Table 10 below). First, the widening lead of Quercus over Prunus is more pronounced than when just looking at Quercus palustris or rubra vs Prunus serotina alone. Second, Fraxinus increased overall from #27 to #12, which outpaces the overall increase for the combined dataset. Finally, in tandem with the increase in Ulmus americana but also including other elm species, the Ulmus genus increased greatly from #8 to #3. This is important to note, because even in the event that there could have been differences in identification between elms in each dataset (there are numerous occurrences in which specimens appear to be hybrids, recorded as Ulmus spp., and as such their classification in the field can be tricky) grouping them by genus still results in a noticeable increase.
Lower Quartile Range Lastly, a particularly important observation is to look at the lowest quartile of individuals based on DBH (see Table 11 below). In the case of the Ramble, this was every stem between 1 cm and 2.4 cm. The biggest takeaway is perhaps that Acer platanoides went from being ranked #1 in this category to #17. Alvarez (2012) noted that its top ranking in the Ramble’s LQR was observed ‘even after many years of removal by park managers’ and that ‘future studies will be needed to see if this management will eventually bring down this number, or if it is a futile effort’. It is reported with no little satisfaction that this effort appears very much not to have been futile, particularly in the Ramble. In contrast, Carya cordiformis, the most common hickory in the woodlands, greatly increased in the LQR, going from #18 to #5. Despite this species rarely if ever being planted, Carya cordiformis has seen a large increase in sapling production (personal observation), with some areas in the Ramble resembling thickets of young hickories. In certain areas where a meadow or herbaceous edge is desired, this species requires annual sapling removal, though its extensive taproot makes it difficult to remove completely. While it might be a thorn in the side of meadow gardeners, it is encouraging to see that it is performing so well. Quercus palustris and Quercus rubra both did relatively well in this range, but curiously dropped slightly from the last survey (#3 to #6, and #9 to #14, respectively). However, their slight decrease is made up by other native species, with Sassafras albidum going from #4 to #2, and Nyssa sylvatica taking the #3 spot after having not been sampled at all previously, which is remarkable. It should be noted that Natural Areas staff have even considered thinning Sassafras
53 in some situations, in order to give light to desirable planted specimens. A similar phenomenon has happened to Gymnocladus dioicus, which went from #5 in the LQR to #29. This is likely due to efforts in the recent past by different gardeners (who treated it as nearly an invasive species given its recruitment success) thinning it proactively in the understory, even though this is not necessarily desirable. Indeed, it was enough to help take the overall IV rank of Gymnocladus from #10 to #29 overall.
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Figure 6.) Sample unit (point) locations within the Ramble, in Central Park, New York City, with 165 total points.
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Table 8.) Ecological dominance in terms of Importance Value for Ramble dataset.
Relative Relative Relative Importance Rank Species Name Density Dominance Frequency Value
1 Quercus palustris 4.85 21.57 5.61 32.03 2 Prunus serotina 10.76 6.98 9.86 27.60 3 Celtis occidentalis 8.33 2.77 6.77 17.87 4 Acer rubrum 4.09 6.86 3.87 14.82 5 Ulmus americana 5.45 2.43 5.22 13.11 6 Phellodendron amurense 1.67 9.14 1.93 12.74 7 Nyssa sylvatica 5.00 4.07 3.29 12.36 8 Sassafras albidum 5.91 1.95 4.45 12.30 9 Quercus rubra 1.82 5.12 2.13 9.07 10 Malus spp. 3.33 0.39 3.87 7.60 11 Platanus acerifolia 0.61 5.95 0.77 7.33 12 Carpinus caroliniana 1.97 2.82 2.13 6.92 13 Carya cordiformis 2.27 1.77 2.51 6.55 14 Viburnum dentatum 3.64 0.18 2.51 6.33 15 Prunus spp. 2.27 0.85 2.90 6.03 16 Quercus phellos 0.76 4.44 0.58 5.78 17 Hamamelis virginiana 2.12 0.48 2.32 4.93 18 Viburnum prunifolium 2.12 0.11 2.51 4.74 19 Fraxinus americana 2.12 0.08 2.32 4.52 20 Crataegus spp. 1.97 0.31 2.13 4.41 21 Robinia pseudoacacia 0.76 2.49 0.97 4.21 22 Morus alba 1.36 1.12 1.55 4.03 23 Liriodendron tulipifera 1.21 1.21 1.16 3.58 24 Amelanchier spp. 1.36 0.58 1.35 3.30 25 Gingko biloba 0.45 2.10 0.58 3.13 26 Lindera benzoin 1.36 0.15 1.55 3.06 27 Ulmus spp. 1.06 0.57 1.35 2.99 28 Gymnocladus dioica 0.61 1.49 0.77 2.87 29 Ailanthus altissima 0.61 1.27 0.77 2.65 30 Quercus velutina 0.30 1.82 0.39 2.51 31 Fraxinus pennsylvanica 1.06 0.03 1.35 2.44 32 Ulmus procera 1.21 0.06 1.16 2.43 33 Sambucus canadensis 1.06 0.09 1.16 2.31 34 Acer saccharum 0.76 0.86 0.58 2.20 35 Pinus nigra 0.30 1.28 0.39 1.97 36 Quercus bicolor 0.15 1.60 0.19 1.95 37 Fagus sylvatica 0.15 1.58 0.19 1.92 38 Pinus strobus 0.15 1.40 0.19 1.75 39 Acer platanoides 0.76 0.01 0.97 1.74 40 Ilex verticillata 0.76 0.01 0.77 1.54
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41 Ulmus glabra 0.61 0.04 0.77 1.42 42 Viburnum dilatatum 0.61 0.04 0.77 1.42 43 Lonicera maackii 0.61 0.40 0.39 1.40 44 Styphnolobium japonicum 0.45 0.35 0.58 1.39 45 Liquidambar styraciflua 0.45 0.25 0.58 1.29 46 Cercis canadensis 0.45 0.09 0.58 1.12 47 Rhus typhina 0.45 0.04 0.58 1.07 48 Ilex opaca 0.45 0.02 0.58 1.06 49 Tilia americana 0.61 0.06 0.39 1.05 50 Philadelphus coronarius 0.45 0.01 0.58 1.04 51 Cornus mas 0.45 0.09 0.39 0.93 52 Ilex crenata 0.30 0.23 0.39 0.92 53 Cornus racemosa 0.45 0.02 0.39 0.86 54 Broussonetia papyrifera 0.45 0.01 0.39 0.85 55 Viburnum lentago 0.45 0.01 0.39 0.85 56 Rubus allegheniensis 0.45 0.00 0.39 0.85 57 Forsythia spp. 0.45 0.00 0.39 0.84 58 Aesculus parviflora 0.45 0.00 0.39 0.84 59 Cladrastis kentukea 0.30 0.03 0.39 0.72 60 Amelanchier canadensis 0.30 0.02 0.39 0.71 61 Magnolia virginiana 0.30 0.01 0.39 0.70 62 Cornus amomum 0.30 0.01 0.39 0.70 63 Cornus sericea 0.30 0.00 0.39 0.69 64 Diospyros virginiana 0.30 0.00 0.19 0.50 65 Crataegus crus-galli 0.15 0.06 0.19 0.41 66 Salix nigra 0.15 0.06 0.19 0.41 67 Acer palmatum 0.15 0.03 0.19 0.38 68 Catalpa speciosa 0.15 0.03 0.19 0.37 69 Quercus coccinea 0.15 0.03 0.19 0.37 70 Corylus americana 0.15 0.02 0.19 0.37 71 Unknown unknown 0.15 0.01 0.19 0.35 72 Ulmus pumila 0.15 0.01 0.19 0.35 73 Juniperus virginiana 0.15 0.00 0.19 0.35 74 Gleditsia triacanthos 0.15 0.00 0.19 0.35 75 Rhus aromatica 0.15 0.00 0.19 0.35 76 Calycanthus floridus 0.15 0.00 0.19 0.35 77 Betula lenta 0.15 0.00 0.19 0.35 78 Rhododendron maximum 0.15 0.00 0.19 0.35 79 Clethra alnifolia 0.15 0.00 0.19 0.35 80 Crataegus phaenopyrum 0.15 0.00 0.19 0.35 81 Acer negundo 0.15 0.00 0.19 0.35 82 Quercus montana 0.15 0.00 0.19 0.35 83 Rhus copallinum 0.15 0.00 0.19 0.35
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84 Populus tremuloides 0.15 0.00 0.19 0.35 Total 100 100 100 300
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Table 9.) Descriptive Statistics for Ramble dataset listed alphabetically.
Absolute Standard % Species Name Abundance Density Mean Deviation Min, Max Median Sampling (trees/ha) DBH (cm) (cm) (cm) (cm) Units Acer negundo 1 2.93 1.60 0.00 1.6, 1.6 1.6 0.61 Acer palmatum 1 2.93 10.70 0.00 10.7, 10.7 10.7 0.61 Acer platanoides 5 14.63 2.74 1.48 1.1, 5.4 2.2 3.03 Acer rubrum 27 79.01 15.79 25.80 1.1, 95.9 4.6 12.12 Acer saccharum 5 14.63 15.00 19.91 3.7, 54.8 5.7 1.82 Aesculus parviflora 3 8.78 1.43 0.61 1, 2.3 1 1.21 Ailanthus altissima 4 11.70 25.33 22.49 1.7, 50.6 24.5 2.42 Amelanchier canadensis 2 5.85 5.80 0.80 5, 6.6 5.8 1.21 Amelanchier spp. 9 26.34 12.04 9.31 2, 30.3 7.5 4.24 Betula lenta 1 2.93 2.80 0.00 2.8, 2.8 2.8 0.61 Broussonetia papyrifera 3 8.78 3.70 1.20 2.4, 5.3 3.4 1.21 Calycanthus floridus 1 2.93 3.60 0.00 3.6, 3.6 3.6 0.61 Carpinus caroliniana 13 38.04 20.95 18.47 1.3, 49.3 12.4 6.67 Carya cordiformis 15 43.89 8.22 18.87 1.1, 77.9 2.1 7.88 Catalpa speciosa 1 2.93 10.10 0.00 10.1, 10.1 10.1 0.61 Celtis occidentalis 55 160.94 7.39 11.24 1.1, 72 3.8 21.21 Cercis canadensis 3 8.78 9.33 4.45 5.7, 15.6 6.7 1.82 Cladrastis kentukea 2 5.85 6.90 1.80 5.1, 8.7 6.9 1.21 Clethra alnifolia 1 2.93 2.10 0.00 2.1, 2.1 2.1 0.61 Cornus amomum 2 5.85 3.45 1.15 2.3, 4.6 3.45 1.21 Cornus mas 3 8.78 10.13 2.31 7.1, 12.7 10.6 1.21 Cornus racemosa 3 8.78 4.40 0.71 3.9, 5.4 3.9 1.21 Cornus sericea 2 5.85 2.95 0.15 2.8, 3.1 2.95 1.21 Corylus americana 1 2.93 9.40 0.00 9.4, 9.4 9.4 0.61 Crataegus crus-galli 1 2.93 15.20 0.00 15.2, 15.2 15.2 0.61 Crataegus phaenopyrum 1 2.93 1.80 0.00 1.8, 1.8 1.8 0.61 Crataegus spp. 13 38.04 6.43 6.71 1.2, 22.5 4.4 6.67 Diospyros virginiana 2 5.85 2.25 0.35 1.9, 2.6 2.25 0.61 Fagus sylvatica 1 2.93 75.30 0.00 75.3, 75.3 75.3 0.61 Forsythia spp. 3 8.78 1.90 0.22 1.7, 2.2 1.8 1.21 Fraxinus americana 14 40.97 3.74 2.32 1.1, 9.4 3.25 7.27 Fraxinus pennsylvanica 7 20.48 3.11 1.97 1.6, 7.2 2.1 4.24 Gingko biloba 3 8.78 40.33 29.80 3.9, 76.9 40.2 1.82 Gleditsia triacanthos 1 2.93 3.80 0.00 3.8, 3.8 3.8 0.61 Gymnocladus dioica 4 11.70 23.88 27.75 1.5, 69.6 12.2 2.42 Hamamelis virginiana 14 40.97 7.94 7.83 1.1, 31 4.7 7.27 Ilex crenata 2 5.85 15.80 12.50 3.3, 28.3 15.8 1.21 Ilex opaca 3 8.78 5.33 1.01 4.3, 6.7 5 1.82
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Ilex verticillata 5 14.63 2.16 0.47 1.4, 2.7 2.1 2.42 Juniperus virginiana 1 2.93 4.00 0.00 4, 4 4 0.61 Lindera benzoin 9 26.34 6.73 3.64 1.8, 11.8 7.3 4.85 Liquidambar styraciflua 3 8.78 11.40 13.10 1.2, 29.9 3.1 1.82 Liriodendron tulipifera 8 23.41 15.03 17.83 1.4, 60.9 10.15 3.64 Lonicera maackii 4 11.70 16.45 9.68 2, 27.7 18.05 1.21 Magnolia virginiana 2 5.85 3.75 2.45 1.3, 6.2 3.75 1.21 Malus spp. 22 64.38 5.94 5.41 1.1, 26.3 5.15 12.12 Morus alba 9 26.34 15.11 14.81 1.8, 45.4 7.2 4.85 Nyssa sylvatica 33 96.56 9.55 18.78 1.4, 93.8 4.2 10.30 Phellodendron amurense 11 32.19 38.52 38.79 1.6, 117.1 12.2 6.06 Philadelphus coronarius 3 8.78 2.77 1.30 1.7, 4.6 2 1.82 Pinus nigra 2 5.85 48.00 0.80 47.2, 48.8 48 1.21 Pinus strobus 1 2.93 71.00 0.00 71, 71 71 0.61 Platanus acerifolia 4 11.70 72.38 10.72 61.1, 87.5 70.45 2.42 Populus tremuloides 1 2.93 1.40 0.00 1.4, 1.4 1.4 0.61 Prunus serotina 71 207.76 14.83 11.57 1, 55.5 11.8 30.91 Prunus spp. 15 43.89 9.60 10.63 1.1, 37 6 9.09 Quercus bicolor 1 2.93 75.90 0.00 75.9, 75.9 75.9 0.61 Quercus coccinea 1 2.93 9.60 0.00 9.6, 9.6 9.6 0.61 Quercus montana 1 2.93 1.60 0.00 1.6, 1.6 1.6 0.61 Quercus palustris 32 93.64 27.62 40.79 1.3, 123.5 4.9 17.58 Quercus phellos 5 14.63 35.50 43.97 5.6, 123 16.7 1.82 Quercus rubra 12 35.11 18.78 34.40 1, 99.5 3.65 6.67 Quercus velutina 2 5.85 41.75 39.05 2.7, 80.8 41.75 1.21 Rhododendron maximum 1 2.93 2.70 0.00 2.7, 2.7 2.7 0.61 Rhus aromatica 1 2.93 3.80 0.00 3.8, 3.8 3.8 0.61 Rhus copallinum 1 2.93 1.50 0.00 1.5, 1.5 1.5 0.61 Rhus typhina 3 8.78 5.73 3.66 1, 9.9 6.3 1.82 Robinia pseudoacacia 5 14.63 36.76 20.94 8.5, 57.4 46.2 3.03 Rubus allegheniensis 3 8.78 1.97 0.70 1.2, 2.9 1.8 1.21 Salix nigra 1 2.93 15.20 0.00 15.2, 15.2 15.2 0.61 Sambucus canadensis 7 20.48 5.51 3.93 1.5, 14 4.9 3.64 Sassafras albidum 39 114.12 8.32 10.50 1.1, 46.2 3.7 13.94 Styphnolobium japonicum 3 8.78 16.87 11.74 4.2, 32.5 13.9 1.82 Tilia americana 4 11.70 6.08 3.92 1.1, 11.7 5.75 1.21 Ulmus americana 36 105.34 9.08 12.67 1.1, 75.5 4.85 16.36 Ulmus glabra 4 11.70 4.73 3.50 1.3, 10.5 3.55 2.42 Ulmus procera 8 23.41 4.71 2.35 1.6, 9.2 4.75 3.64 Ulmus pumila 1 2.93 4.70 0.00 4.7, 4.7 4.7 0.61
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Ulmus spp. 7 20.48 10.77 13.38 1.7, 42.7 5.5 4.24 Unknown spp. 1 2.93 4.90 0.00 4.9, 4.9 4.9 0.61 Viburnum dentatum 24 70.23 4.84 1.97 1.5, 8 5.4 7.88 Viburnum dilatatum 4 11.70 5.18 2.41 1.7, 7.8 5.6 2.42 Viburnum lentago 3 8.78 3.40 1.82 1.4, 5.8 3 1.21 Viburnum prunifolium 14 40.97 4.71 2.40 1.3, 9 4.85 7.88 Total 660 1931.28 12.29 1, 123.5 5.1 (11.77, 13.81)
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Table 10.) Ecological dominance in terms of Importance Value grouped by Genus for the Ramble dataset.
Relative Relative Relative Importance Rank Genus Density Dominance Frequency Value 1 Quercus 8.18 34.58 9.28 52.05 2 Prunus 13.03 7.83 12.77 33.63 3 Ulmus 8.48 3.11 8.70 20.30 4 Acer 5.91 7.77 5.80 19.49 5 Celtis 8.33 2.77 6.77 17.87 6 Viburnum 6.82 0.34 6.19 13.35 7 Phellodendron 1.67 9.14 1.93 12.74 8 Nyssa 5.00 4.07 3.29 12.36 9 Sassafras 5.91 1.95 4.45 12.30 10 Malus 3.33 0.39 3.87 7.60 11 Platanus 0.61 5.95 0.77 7.33 12 Fraxinus 3.18 0.10 3.68 6.96 13 Carpinus 1.97 2.82 2.13 6.92 14 Carya 2.27 1.77 2.51 6.55 15 Crataegus 2.27 0.38 2.51 5.16 16 Hamamelis 2.12 0.48 2.32 4.93 17 Robinia 0.76 2.49 0.97 4.21 18 Morus 1.36 1.12 1.55 4.03 19 Amelanchier 1.67 0.60 1.74 4.01 20 Pinus 0.45 2.68 0.58 3.72 21 Liriodendron 1.21 1.21 1.16 3.58 22 Ilex 1.52 0.26 1.74 3.51 23 Cornus 1.52 0.12 1.55 3.18 24 Gingko 0.45 2.10 0.58 3.13 25 Lindera 1.36 0.15 1.55 3.06 26 Gymnocladus 0.61 1.49 0.77 2.87 27 Ailanthus 0.61 1.27 0.77 2.65 28 Sambucus 1.06 0.09 1.16 2.31 29 Fagus 0.15 1.58 0.19 1.92 30 Rhus 0.76 0.04 0.97 1.77 31 Lonicera 0.61 0.40 0.39 1.40 32 Styphnolobium 0.45 0.35 0.58 1.39 33 Liquidambar 0.45 0.25 0.58 1.29 34 Cercis 0.45 0.09 0.58 1.12 35 Tilia 0.61 0.06 0.39 1.05 36 Philadelphus 0.45 0.01 0.58 1.04 37 Broussonetia 0.45 0.01 0.39 0.85 38 Rubus 0.45 0.00 0.39 0.85 39 Forsythia 0.45 0.00 0.39 0.84 40 Aesculus 0.45 0.00 0.39 0.84 41 Cladrastis 0.30 0.03 0.39 0.72
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42 Magnolia 0.30 0.01 0.39 0.70 43 Diospyros 0.30 0.00 0.19 0.50 44 Salix 0.15 0.06 0.19 0.41 45 Catalpa 0.15 0.03 0.19 0.37 46 Corylus 0.15 0.02 0.19 0.37 47 Unknown 0.15 0.01 0.19 0.35 48 Juniperus 0.15 0.00 0.19 0.35 49 Gleditsia 0.15 0.00 0.19 0.35 50 Calycanthus 0.15 0.00 0.19 0.35 51 Betula 0.15 0.00 0.19 0.35 52 Rhododendron 0.15 0.00 0.19 0.35 53 Clethra 0.15 0.00 0.19 0.35 54 Populus 0.15 0.00 0.19 0.35 100 100 100 300
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Table 11.) Ecological dominance in terms of Importance Value for lower DBH quartile range (LQR) of the Ramble dataset. In this case, the lowest quartile included all stems below 2.4 cm DBH.
Relative Relative Relative Importance Rank Species Name Density Dominance Frequency Value 1 Celtis occidentalis 10.53 10.79 9.74 31.06 2 Sassafras albidum 8.19 7.18 7.14 22.51 3 Nyssa sylvatica 7.02 7.85 4.55 19.41 4 Acer rubrum 5.26 4.90 5.84 16.01 5 Carya cordiformis 5.26 4.65 4.55 14.46 6 Quercus palustris 4.09 5.05 4.55 13.69 7 Prunus serotina 4.09 4.62 4.55 13.26 8 Fraxinus americana 3.51 4.09 3.90 11.50 9 Prunus spp. 3.51 3.37 3.90 10.78 10 Malus spp. 3.51 2.74 3.90 10.14 11 Fraxinus pennsylvanica 2.92 3.84 3.25 10.02 12 Ulmus americana 3.51 2.97 3.25 9.72 13 Crataegus spp. 2.34 1.92 2.60 6.85 14 Quercus rubra 2.34 1.70 2.60 6.64 15 Lindera benzoin 1.75 2.49 1.95 6.19 16 Ilex verticillata 1.75 2.07 1.95 5.77 17 Acer platanoides 1.75 1.93 1.95 5.63 18 Forsythia spp. 1.75 2.19 1.30 5.24 19 Viburnum prunifolium 1.75 1.06 1.95 4.77 20 Aesculus parviflora 1.75 1.45 1.30 4.51 21 Amelanchier spp. 1.17 1.95 1.30 4.41 22 Viburnum dentatum 1.75 1.35 1.30 4.40 23 Ulmus procera 1.17 1.66 1.30 4.13 24 Philadelphus coronarius 1.17 1.37 1.30 3.84 25 Carpinus caroliniana 1.17 1.22 1.30 3.68 26 Sambucus canadensis 1.17 1.17 1.30 3.64 27 Gymnocladus dioica 1.17 1.09 1.30 3.56 28 Rubus allegheniensis 1.17 0.93 1.30 3.40 29 Broussonetia papyrifera 0.58 1.15 0.65 2.38 30 Cornus amomum 0.58 1.05 0.65 2.29 31 Clethra alnifolia 0.58 0.88 0.65 2.11 32 Lonicera maackii 0.58 0.80 0.65 2.03 33 Diospyros virginiana 0.58 0.72 0.65 1.95 34 Crataegus phaenopyrum 0.58 0.65 0.65 1.88 35 Morus alba 0.58 0.65 0.65 1.88 36 Ailanthus altissima 0.58 0.58 0.65 1.81 37 Ulmus spp. 0.58 0.58 0.65 1.81 38 Viburnum dilatatum 0.58 0.58 0.65 1.81 39 Acer negundo 0.58 0.51 0.65 1.74 40 Phellodendron amurense 0.58 0.51 0.65 1.74
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41 Quercus montana 0.58 0.51 0.65 1.74 42 Rhus copallinum 0.58 0.45 0.65 1.68 43 Liriodendron tulipifera 0.58 0.39 0.65 1.62 44 Populus tremuloides 0.58 0.39 0.65 1.62 45 Viburnum lentago 0.58 0.39 0.65 1.62 46 Magnolia virginiana 0.58 0.34 0.65 1.57 47 Ulmus glabra 0.58 0.34 0.65 1.57 48 Liquidambar styraciflua 0.58 0.29 0.65 1.52 49 Hamamelis virginiana 0.58 0.24 0.65 1.48 50 Tilia americana 0.58 0.24 0.65 1.48 51 Rhus typhina 0.58 0.20 0.65 1.43 Total Total Total Total 100 100 100 300
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The North Woods The North Woods is the landscape with the most ‘interior’ area, as noted earlier, and of the 3 Natural Areas woodlands was the one least altered during the creation of the park. It has also long been considered the most ‘natural’ area of the park, and it has proportionately fewer pathways, much less streetlamps and no park benches in comparison to the Ramble, which has many of each. This survey of the North Woods mirrored the previous survey very closely, with 652 individuals recorded vs 660 previously. Despite this slightly lower sample size, the species count rose from 57 to 74 and the genera increased from 36 to 44. Of the 74 species, 53 (or 72%) were native (almost the same as in the Ramble), though 81.2% of Importance Value was from native species, which, like the Ramble, suggests that, in general, individuals of non-native species are less numerous and smaller in size than natives. Also, the ‘drop off’ in Importance Values among species was observed to be much more gradual than in the previous survey. For example, 21 species held importance values over 5, vs 14 species with values over 5 in the last survey. This could indicate decreasing dominance by only a few species in favor of more species showing a more even distribution of ecological dominance. In the previous study, it was noted that the North Woods had the closest ‘competitor’ to Prunus serotina, in Quercus rubra, though P. serotina was still ranked #1. This is in contrast to the Ramble, where Q. rubra was relatively lower in IV. However, she also notes that in the North Woods, observed lack of regeneration of oaks could have implications for the woodland in the future, despite the presence of many mature specimens. The relatively high amount of invasive woody plants (similar to the other woodlands) was also noted, as well the relatively higher presence of Aralia elata (a tough invasive) and the genus Carya, as compared to the woodlands as a whole.
Invasive Species Changes For this survey, the biggest difference between the North Woods and the two other Natural Areas woodlands (see Table 12 below) is the relatively higher amounts of the two invasive Acer species. The North Woods was officially brought under the Woodlands (eventually renamed Natural Areas) umbrella relatively recently, in 2016. Much invasive species removal, along with planting, soil stabilization and other restoration efforts, has taken place since then, and the data suggests that dividends are already beginning to show. Acer platanoides, previously ranked #3 in IV behind black cherry and red oak, has now dropped to the #5 position. This drop is less dramatic than in other woodlands, but when the descriptive statistics are compared it appears to be more significant than at first glance. The abundance dropped from 61 to 28, the percentage of sampling units (points) in which it was found went from 28.5% to 12.9% and the mean DBH dropped from 16.85 cm to 12.68 cm, among other concrete decreases. Each of these translates to a roughly 50% decrease from the previous absolute values of density, frequency, and basal area. The other invasive maple, Acer pseudoplatanus, fared worse, going from IV rank #4 to #28, with the abundance in the sample dropping from 38 to 6. Both of these species require ongoing work to keep these numbers low, but the approach has been steady and incremental,
66 with the goal being to minimize disturbance through removal over time. While by far the largest number of mature Acer platanoides and Acer pseudoplatanus still exist in the North Woods, each year a few are selected to remove for use in the park’s rustic trail system as check-steps, water- bars, and other uses such as adding to a barren slope for erosion control. The Conservancy will top or girdle some specimens in remote areas to add value as habitat trees. Many are left purposely in the overstory until young Acer saccharum and Acer rubrum planted in the understory are able to mature, with the plan being to remove the invasive mature maples when the younger native maples are better poised to claim canopy dominance. This staged approach is intended to avoid creating large sunny canopy gaps that invasive species (like porcelain-berry vine) could take advantage of, instead strategically replacing one canopy with another. While the invasive Acer reductions are notable, there were other invasive species that saw changes as well. Notably, Morus alba dropped from #12 to #31, while Aralia elata, mentioned previously as a species for which to actively track its spread and/or remove, decreased from #22 to #35. However, this species has arisen in many new locations in the last few years, and management continues to be a concern, especially given the difficulty of manual removal. On the other hand, one species group that has seen a notable increase is Malus spp., which rose from #16 to #7. This species is increasingly common in the understory across wide areas of the North Woods, and active management has begun to thin smaller specimens while the removal of larger specimens proceeds much more slowly. They are favorite trees in the spring for their blooms, and their fruit provides a late-season food source for birds, so the removal of larger specimens is not undertaken without planned replacement by either nearby recruitment of fruit-bearing shrubs, or direct planting of fruit-bearing native shrubs (such as Ilex, Cornus, Lindera, Crataegus, etc.). There were also a number of invasive species completely absent from this dataset that were present in the previous dataset. These were Ailanthus altissima, Rhodotypos scandens, Frangula alnus, and Euonymus alatus. The tree-of-heaven and jetbead were once ranked #26 and #19, respectively, while the glossy buckthorn and winged euonymus were found only once each previously. While a small amount of each of these species still remains, the effort within the past few years to completely remove them has been concerted, so the fact that the survey did not record any of them is rewarding even if the low numbers in the original dataset might suggest that not finding them this time around is within the margin of error.
Native Species Changes Within the subset of native species, numerous notable changes are apparent. Prunus serotina retains its spot at #1 and Quercus rubra is still #2, but this margin is even slimmer than before, with less than a single point between them (35.52 vs 34.64). Another important canopy tree in the North Woods that has seen increases is Tilia americana. While only two large specimens were found in the previous survey, a mixture of both mature specimens and saplings were found in this survey’s 12 specimens, with its total rank climbing from #29 to #6 due largely to increases in both abundance and spread/frequency. This was also a species noted to have existed in the park in the pre-construction survey (Rawolle and Pilat, 1857), though it was not
67 widespread at the time. The previous study noted the relatively smaller presence of Celtis occidentalis in the North Woods compared to the Ramble, but this survey shows an impressive increase in this species, with it moving from #23 to #9 in IV rank, and total abundance in the sample climbing to 27 from the previous survey which only counted 8 specimens. Acer saccharum was noted to have seen a slight rise in IV ranking for the combined dataset, and that was largely due to increases in the North Woods, where it rose from a previous #39 rank to #19 in this survey (going from 2 individuals to 15). This is largely a result of concerted effort to plant sugar maples in the understory, some of which came from the Million Trees NYC program. Many of these trees were chosen to eventually replace mature Norway and Sycamore maples, as noted above. Whether future climate related stresses will prove hospitable to this species (NYC’s climate puts it at the edge of the range of sugar maple) remains to be seen, though it has been observed reproducing in the park (personal experience, as well as Atha et al., 2020, Alvarez, 2012). Carya cordiformis, by far the most common hickory in the park, was noted in the previous study as being more numerous in the North Woods than in other areas. This continues in this survey, where it even increased its IV ranking from #13 to #8. There were also a few species that we found in this survey which, while relatively small in abundance within the sample, are notable given that many of them are the result of recent planting/reintroduction to the North Woods. These are Betula lenta, Cornus florida, and various Populus and Aesculus species. Each of the species was either not found in the Alvarez survey, or was only found sparingly and increased its representation in this survey. Each species was also noted to have occurred frequently in the pre-construction survey (Rawolle and Pilat, 1857) but are currently almost absent excepting a small but increasing number of planted saplings. Having planted enough saplings over the years to have some of them show up in this survey is indicative that efforts to repopulate these species are working. Finally, one difference in native species representation in the North Woods between this survey and the last is the relative drop in Liquidambar styraciflua. It dropped in IV ranking from #14 to #25, with 4 individuals found in comparison to 9 in the last survey. It is estimated that this species is relatively more numerous in the North Woods than in other landscapes, with the most impressive mature specimens in the park being found in the North Woods. This species also seems to be reproducing relatively readily. However, its concentration in relatively few landscapes might have led to its slight underrepresentation in this sample.
By Genus When ranking by genus (see Table 14 below) the biggest difference is that, while Prunus serotina narrowly edged out Quercus rubra for the top spot, Quercus is far and away the #1 genus over Prunus here (67.43 vs 37.29 relative IVs). Quercus was represented by 8 species here (more than any other woodland including the Ramble, which had 7), one of which was Quercus velutina (black oak), which ranked #10 in IV for this survey. As was mentioned previously, the Alvarez survey recorded no Quercus velutina, and the recent flora of the park does not list it as reproducing either (Atha et al., 2020). In this case, if this survey misidentified certain specimens
68 of Quercus rubra as Quercus velutina, it would follow that Quercus rubra would instead be declared the leader in IV when ranking by species. However, this study asserts that there are indeed Quercus velutina in the woodlands, given close inspection of various specimens throughout the North Woods which exhibit traits (buds, bark, leaves and acorns) which seem to clearly distinguish them from Quercus rubra. Admittedly, it would seem that Quercus velutina is largely underrepresented in the understory, with some of the small ones found in this survey likely to have been planted. Nevertheless, this is a genus which is clearly most dominant in the North Woods, and its continued regeneration and reintroduction of previously found species (12 were found in the pre-park survey) will be a source of further interest and management attention.
Lower Quartile Range This is the size class where the most obvious changes, and perhaps the best management ‘takeaways’, can be seen in this woodland (see Table 15 below). In the case of the North Woods, the lowest diameter quartile consisted of all specimens between 1 cm and 2.3 cm DBH. The largest change was the increase in Ulmus americana from #5 in the previous survey to #1, but the difference in other species in the lower size class is also interesting. Celtis occidentalis, noted as having a more muted presence in the North Woods in the last survey, has increased its LQR IV rank from #12 to #2, in line with its increases noticed in all other woodlands. This is a compaction and disturbance tolerant species similar to Prunus serotina, yet P. serotina decreased from rank #2 to #4 in this quartile. One possible explanation could be that while P. serotina is a pioneer species that is relatively dependent on light and competes for canopy dominance, C. occidentalis is much more tolerant of shade and can proliferate in the mid-story when competing species are removed. Carya cordiformis seems to have seen much of its overall IV gains coming from this size class as well, with increases in observed recruitment moving it from #17 previously to #6. This species also seems to be relatively shade-tolerant in comparison to P. serotina. The biggest question about the LQR of the North Woods from the previous survey was whether oak species could increase recruitment to match their high overall IV ranking, which was largely dependent on large mature specimens. For Quercus rubra this was a large improvement, as it went from #28 in this category previously to #5 out of smaller sized specimens now. However, other oak species (Q. alba, Q. velutina) remained relatively the same or absent in this category. Continued planting and landscape protection efforts aim to change this, but it has also been observed that oak seedlings have been regenerating much more readily in landscapes where Japanese knotweed has been removed. Expanding these efforts to areas where Japanese knotweed is still a dominant presence in the understory should be a continued management priority. Two more trends in native species within the LQR are also noteworthy. First, it should be pointed out that Fraxinus americana (white ash) is ranked #3, whereas it was ranked #8 previously. Additionally, the previous survey recognized all ashes as Fraxinus americana, where this survey recognized Fraxinus pennsylvanica (green ash) and Fraxinus excelsior (European
69 ash) as well. As such, this difference is even more dramatic if we combine the two native Fraxinus species, which boosts it to #2 in the lower size class even without inclusion of F. excelsior. This is pointed out because of the growing presence of Emerald ash borer in the park, first discovered only last year. If such a high proportion of saplings in the North Woods (with their recruitment seemingly accelerating) are destined for potential functional extinction, this will warrant monitoring and future study as to which other species will take advantage of an expanding niche if/when ashes are wiped out. Perhaps the periodic treatment against EAB for a subset of mature specimens in the park will allow for continued recruitment, but that remains to be seen. Finally, included here is a short roundup of the successful decreases in invasive species in the North Woods’ lower size class. Acer platanoides and Acer pseudoplatanus saw greater proportional decreases here than in the North Woods sample as a whole, decreasing from #1 to #7 and #4 to #17, respectively. This is due largely to these species drawing more ongoing management attention. Aralia elata, another growing species of concern as noted in the Alvarez study, encouragingly decreased here as well, from #7 to #14. However, as has been the case throughout the woodlands, Malus spp. saw an increase from #13 to #9 in this size class, with the increased spread and thinning of this species requiring further attention, and future studies will show if this species is here to stay, or can be replaced by native species that provide similar ecological functions while potentially fostering more ecological connections with native wildlife.
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Figure 7.) Sample unit locations within the North Woods in Central Park, New York City, with 165 points, 163 of which were viable.
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Table 12.) Ecological dominance in terms of Importance Value for the North Woods dataset.
Relative Relative Relative Importance Rank Species Name Species Density Frequency Value Dominance 1 Prunus serotina 14.72 7.66 13.13 35.52 2 Quercus rubra 5.67 23.10 5.86 34.64 3 Ulmus americana 11.35 3.72 10.51 25.58 4 Quercus palustris 2.91 10.61 3.23 16.76 5 Acer platanoides 4.29 4.79 4.24 13.32 6 Tilia americana 1.84 8.39 1.82 12.05 7 Malus spp. 4.91 0.58 4.85 10.33 8 Carya cordiformis 3.37 2.19 3.64 9.20 9 Celtis occidentalis 4.14 0.74 3.64 8.52 10 Quercus velutina 1.07 5.96 1.41 8.45 11 Viburnum dentatum 4.75 0.28 3.23 8.26 12 Platanus occidentalis 0.31 6.84 0.40 7.56 13 Lindera benzoin 3.53 0.30 3.64 7.47 14 Fraxinus americana 2.91 0.62 3.43 6.97 15 Photinia villosa 3.07 0.30 3.23 6.60 16 Liriodendron tulipifera 1.84 1.76 2.42 6.03 17 Robinia pseudoacacia 1.84 2.30 1.82 5.96 18 Sassafras albidum 2.91 0.65 2.22 5.78 19 Acer saccharum 2.30 0.44 2.63 5.36 20 Fagus grandifolia 0.61 3.82 0.81 5.24 21 Fraxinus pennsylvanica 1.69 1.32 2.02 5.02 22 Quercus alba 0.61 2.46 0.81 3.88 23 Hamamelis virginiana 1.69 0.49 1.41 3.59 24 Ulmus spp. 0.31 2.46 0.40 3.17 25 Liquidambar styraciflua 0.61 1.61 0.61 2.83 26 Acer rubrum 1.23 0.40 1.01 2.64 27 Fagus sylvatica 0.15 1.89 0.20 2.25 28 Acer pseudoplatanus 0.92 0.08 1.21 2.22 29 Juglans nigra 0.61 0.61 0.81 2.03 30 Betula lenta 0.77 0.15 1.01 1.92 31 Morus alba 0.46 0.83 0.61 1.89 32 Quercus montana 0.77 0.12 0.81 1.69 33 Philadelphus coronarius 0.77 0.01 0.81 1.59 34 Carpinus caroliniana 0.46 0.34 0.61 1.41 35 Aralia elata 0.77 0.02 0.61 1.39 36 Viburnum prunifolium 0.61 0.01 0.61 1.23 37 Pinus strobus 0.46 0.14 0.61 1.21 38 Quercus cerris 0.31 0.48 0.40 1.19 39 Prunus virginiana 0.46 0.00 0.61 1.07 40 Amelanchier spp. 0.46 0.02 0.40 0.88
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41 Lonicera maakii 0.46 0.02 0.40 0.88 42 Fraxinus excelsior 0.46 0.01 0.40 0.87 43 Acer negundo 0.46 0.17 0.20 0.83 44 Aesculus glabra 0.31 0.09 0.40 0.80 45 Ulmus procera 0.31 0.29 0.20 0.80 46 Crataegus spp. 0.31 0.07 0.40 0.78 47 Ampelopsis brevipedunculata 0.31 0.01 0.40 0.72 48 Ptelea trifoliata 0.31 0.00 0.40 0.71 49 Ligustrum sinense 0.31 0.00 0.40 0.71 50 Prunus spp. 0.46 0.04 0.20 0.70 51 Corylus americana 0.31 0.11 0.20 0.61 52 Carya ovata 0.15 0.21 0.20 0.57 53 Aesculus flava 0.15 0.16 0.20 0.52 54 Cornus mas 0.15 0.10 0.20 0.46 55 Quercus imbricaria 0.15 0.08 0.20 0.44 56 Viburnum spp. 0.15 0.04 0.20 0.39 57 Quercus coccinea 0.15 0.03 0.20 0.38 58 Populus deltoides 0.15 0.02 0.20 0.37 59 Cornus florida 0.15 0.02 0.20 0.37 60 Tsuga canadensis 0.15 0.01 0.20 0.37 61 Ulmus glabra 0.15 0.01 0.20 0.37 62 Populus grandidentata 0.15 0.01 0.20 0.36 63 Viburnum acerifolium 0.15 0.00 0.20 0.36 64 Rhus typhina 0.15 0.00 0.20 0.36 65 Betula papyrifera 0.15 0.00 0.20 0.36 66 Cephalanthus occidentalis 0.15 0.00 0.20 0.36 67 Pyrus calleryana 0.15 0.00 0.20 0.36 68 Celastrus orbiculatus 0.15 0.00 0.20 0.36 69 Cornus amomum 0.15 0.00 0.20 0.36 70 Parthenocissus quinquefolia 0.15 0.00 0.20 0.36 71 Rubus allegheniensis 0.15 0.00 0.20 0.36 72 Sambucus canadensis 0.15 0.00 0.20 0.36 73 Cornus drummondii 0.15 0.00 0.20 0.36 74 Rubus laciniatus 0.15 0.00 0.20 0.36 Total Total Total Total 100 100 100 300
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Table 13.) Descriptive Statistics for North Woods dataset, listed alphabetically.
Absolute Standard % Mean Min, Max Median Species Name Abundance Density Deviation Sampling DBH (cm) (cm) (cm) (trees/ha) (cm) Units Acer negundo 3 9.30 10.00 5.29 3.9, 16.8 9.3 0.61 Acer platanoides 28 86.77 12.68 15.34 1.2, 49.5 4.1 12.88 Acer pseudoplatanus 6 18.59 4.13 3.96 1.1, 12.7 2.55 3.68 Acer rubrum 8 24.79 8.56 6.49 2.2, 21.8 7 3.07 Acer saccharum 15 46.48 6.45 5.08 1.4, 20.2 4.1 7.98 Aesculus flava 1 3.10 19.40 0.00 19.4, 19.4 19.4 0.61 Aesculus glabra 2 6.20 10.00 2.90 7.1, 12.9 10 1.23 Amelanchier spp. 3 9.30 2.93 2.59 1, 6.6 1.2 1.23 Ampelopsis brevipedunculata 2 6.20 2.75 0.05 2.7, 2.8 2.75 1.23 Aralia elata 5 15.49 2.52 0.90 1.7, 4.2 2.3 1.84 Betula lenta 5 15.49 7.50 3.43 3.1, 13.3 7.3 3.07 Betula papyrifera 1 3.10 3.00 0.00 3, 3 3 0.61 Carpinus caroliniana 3 9.30 14.93 6.42 6.3, 21.7 16.8 1.84 Carya cordiformis 22 68.17 7.87 12.99 1.4, 50.1 2.65 11.04 Carya ovata 1 3.10 22.20 0.00 22.2, 22.2 22.2 0.61 Celastrus orbiculatus 1 3.10 1.90 0.00 1.9, 1.9 1.9 0.61 Celtis occidentalis 27 83.67 4.19 6.80 1.1, 37.6 1.9 11.04 Cephalanthus occidentalis 1 3.10 2.40 0.00 2.4, 2.4 2.4 0.61 Cornus amomum 1 3.10 1.60 0.00 1.6, 1.6 1.6 0.61 Cornus drummondii 1 3.10 1.30 0.00 1.3, 1.3 1.3 0.61 Cornus florida 1 3.10 6.00 0.00 6, 6 6 0.61 Cornus mas 1 3.10 15.30 0.00 15.3, 15.3 15.3 0.61 Corylus americana 2 6.20 8.95 6.55 2.4, 15.5 8.95 0.61 Crataegus spp. 2 6.20 9.00 2.00 7, 11 9 1.23 Fagus grandifolia 4 12.40 35.78 30.51 1.5, 75.5 33.05 2.45 Fagus sylvatica 1 3.10 66.20 0.00 66.2, 66.2 66.2 0.61 Fraxinus americana 19 58.88 5.07 7.05 1.2, 27.9 2.1 10.43 Fraxinus excelsior 3 9.30 2.00 0.24 1.7, 2.3 2 1.23 Fraxinus pennsylvanica 11 34.09 11.24 12.29 1.3, 41.3 4.3 6.13 Hamamelis virginiana 11 34.09 8.75 5.08 1.6, 17.8 8.1 4.29 Juglans nigra 4 12.40 15.05 11.18 1.9, 31.1 13.6 2.45 Ligustrum sinense 2 6.20 1.10 0.00 1.1, 1.1 1.1 1.23 Lindera benzoin 23 71.27 4.84 2.64 1.4, 10.4 4.7 11.04 Liquidambar styraciflua 4 12.40 17.63 24.92 1.2, 60.7 4.3 1.84 Liriodendron tulipifera 12 37.19 15.64 9.79 1.6, 33.1 18.15 7.36 Lonicera maackii 3 9.30 3.10 2.12 1.5, 6.1 1.7 1.23 Malus spp. 32 99.16 5.46 3.45 1, 16 5.25 14.72 Morus alba 3 9.30 21.40 13.49 5.4, 38.4 20.4 1.84 Parthenocissus quinquefolia 1 3.10 1.40 0.00 1.4, 1.4 1.4 0.61 Philadelphus coronarius 5 15.49 2.16 0.90 1.1, 3.5 2.1 2.45 Photinia villosa 20 61.98 4.93 3.23 1.2, 13.7 3.95 9.82 Pinus strobus 3 9.30 10.00 3.46 6.8, 14.8 8.4 1.84 Platanus occidentalis 2 6.20 79.60 39.90 39.7, 119.5 79.6 1.23 Populus deltoides 1 3.10 6.30 0.00 6.3, 6.3 6.3 0.61 Populus grandidentata 1 3.10 3.50 0.00 3.5, 3.5 3.5 0.61
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Prunus serotina 96 297.49 10.63 8.47 1.1, 52.8 8.6 39.88 Prunus spp. 3 9.30 4.47 3.18 1.6, 8.9 2.9 0.61 Prunus virginiana 3 9.30 1.47 0.24 1.3, 1.8 1.3 1.84 Ptelea trifoliata 2 6.20 2.00 0.40 1.6, 2.4 2 1.23 Pyrus calleryana 1 3.10 2.00 0.00 2, 2 2 0.61 Quercus alba 4 12.40 21.08 31.29 1, 75.2 4.05 2.45 Quercus cerris 2 6.20 19.80 12.80 7, 32.6 19.8 1.23 Quercus coccinea 1 3.10 7.90 0.00 7.9, 7.9 7.9 0.61 Quercus imbricaria 1 3.10 13.60 0.00 13.6, 13.6 13.6 0.61 Quercus montana 5 15.49 4.58 5.88 1.1, 16.3 1.9 2.45 Quercus palustris 19 58.88 26.33 24.50 1.1, 90.4 21.5 9.82 Quercus rubra 37 114.66 25.09 28.58 1.1, 116.5 14.6 17.79 Quercus velutina 7 21.69 29.46 33.25 2, 89.4 14.1 4.29 Rhus typhina 1 3.10 3.20 0.00 3.2, 3.2 3.2 0.61 Robinia pseudoacacia 12 37.19 15.57 14.23 1.6, 48.7 10.5 5.52 Rubus laciniatus 1 3.10 1.00 0.00 1, 1 1 0.61 Rubus allegheniensis 1 3.10 1.40 0.00 1.4, 1.4 1.4 0.61 Sambucus canadensis 1 3.10 1.40 0.00 1.4, 1.4 1.4 0.61 Sassafras albidum 19 58.88 6.87 5.63 1.1, 24.1 6.6 6.75 Tilia americana 12 37.19 24.96 31.57 1.2, 88.7 7.95 5.52 Tsuga canadensis 1 3.10 5.80 0.00 5.8, 5.8 5.8 0.61 Ulmus americana 74 229.31 7.82 7.45 1, 33.8 4.8 31.90 Ulmus glabra 1 3.10 5.00 0.00 5, 5 5 0.61 Ulmus procera 2 6.20 15.50 9.80 5.7, 25.3 15.5 0.61 Ulmus spp. 2 6.20 40.20 35.10 5.1, 75.3 40.2 1.23 Viburnum acerifolium 1 3.10 3.30 0.00 3.3, 3.3 3.3 0.61 Viburnum dentatum 31 96.06 3.80 2.47 1.2, 11.1 3.1 9.82 Viburnum prunifolium 4 12.40 1.95 1.13 1.2, 3.9 1.35 1.84 Viburnum spp. 1 3.10 9.40 0.00 9.4, 9.4 9.4 0.61 Total Total Total Total Total 652 2020.42 10.63 1, 119.5 5.1 (9.43, 11.83)
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Table 14.) Ecological dominance in terms of Importance Value grouped by Genus for the North Woods dataset.
Relative Relative Relative Importance Rank Genus Density Dominance Frequency Value
1 Quercus 11.66 42.84 12.93 67.43 2 Prunus 15.64 7.70 13.94 37.29 3 Ulmus 12.12 6.48 11.31 29.91 4 Acer 9.20 5.87 9.29 24.37 5 Fraxinus 5.06 1.94 5.86 12.86 6 Tilia 1.84 8.39 1.82 12.05 7 Malus 4.91 0.58 4.85 10.33 8 Viburnum 5.67 0.33 4.24 10.24 9 Carya 3.53 2.40 3.84 9.77 10 Celtis 4.14 0.74 3.64 8.52 11 Platanus 0.31 6.84 0.40 7.56 12 Fagus 0.77 5.71 1.01 7.49 13 Lindera 3.53 0.30 3.64 7.47 14 Photinia 3.07 0.30 3.23 6.60 15 Liriodendron 1.84 1.76 2.42 6.03 16 Robinia 1.84 2.30 1.82 5.96 17 Sassafras 2.91 0.65 2.22 5.78 18 Hamamelis 1.69 0.49 1.41 3.59 19 Liquidambar 0.61 1.61 0.61 2.83 20 Betula 0.92 0.15 1.21 2.28 21 Juglans 0.61 0.61 0.81 2.03 22 Morus 0.46 0.83 0.61 1.89 23 Philadelphus 0.77 0.01 0.81 1.59 24 Cornus 0.61 0.12 0.81 1.54 25 Carpinus 0.46 0.34 0.61 1.41 26 Aralia 0.77 0.02 0.61 1.39 27 Aesculus 0.46 0.26 0.61 1.32 28 Pinus 0.46 0.14 0.61 1.21 29 Amelanchier 0.46 0.02 0.40 0.88 30 Lonicera 0.46 0.02 0.40 0.88 31 Crataegus 0.31 0.07 0.40 0.78 32 Populus 0.31 0.02 0.40 0.73 33 Ampelopsis 0.31 0.01 0.40 0.72 34 Ptelea 0.31 0.00 0.40 0.71 35 Rubus 0.31 0.00 0.40 0.71 36 Ligustrum 0.31 0.00 0.40 0.71 37 Corylus 0.31 0.11 0.20 0.61 38 Tsuga 0.15 0.01 0.20 0.37 39 Rhus 0.15 0.00 0.20 0.36 40 Cephalanthus 0.15 0.00 0.20 0.36
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41 Pyrus 0.15 0.00 0.20 0.36 42 Celastrus 0.15 0.00 0.20 0.36 43 Parthenocissus 0.15 0.00 0.20 0.36 44 Sambucus 0.15 0.00 0.20 0.36 Total Total Total Total 100 100 100 300
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Table 15.) Ecological dominance in terms of Importance Value for lower DBH quartile range (LQR) of the North Woods dataset. In this case, the lowest quartile included all stems below 2.3 cm DBH.
Relative Relative Relative Importance Rank Species Name Density Dominance Frequency Value
1 Ulmus americana 11.90 11.52 10.96 34.38 2 Celtis occidentalis 8.33 6.77 7.53 22.63 3 Fraxinus americana 5.95 6.73 6.85 19.54 4 Prunus serotina 6.55 7.01 5.48 19.04 5 Quercus rubra 5.95 6.51 5.48 17.95 6 Carya cordiformis 4.76 5.70 4.79 15.26 7 Acer platanoides 4.76 4.70 4.79 14.25 8 Viburnum dentatum 4.76 3.85 5.48 14.09 9 Malus spp. 4.17 3.90 4.11 12.18 10 Photinia villosa 2.98 4.16 2.05 9.19 11 Lindera benzoin 2.38 3.24 2.74 8.36 12 Sassafras albidum 2.98 3.19 2.05 8.23 13 Tilia americana 2.38 2.78 2.74 7.90 14 Aralia elata 1.79 2.49 2.05 6.33 15 Fraxinus excelsior 1.79 2.65 1.37 5.81 16 Acer saccharum 1.79 1.84 2.05 5.68 17 Acer pseudoplatanus 1.79 1.76 2.05 5.60 18 Philadelphus coronarius 1.79 1.59 2.05 5.43 19 Fraxinus pennsylvanica 1.79 1.49 2.05 5.33 20 Prunus virginiana 1.79 1.44 2.05 5.28 21 Viburnum prunifolium 1.79 1.11 2.05 4.95 22 Quercus montana 1.79 1.36 1.37 4.52 23 Liriodendron tulipifera 1.19 1.71 1.37 4.27 24 Amelanchier spp. 1.19 0.53 1.37 3.09 25 Ligustrum sinense 1.19 0.53 1.37 3.09 26 Lonicera maakii 1.19 1.12 0.68 2.99 27 Acer rubrum 0.60 1.05 0.68 2.33 28 Pyrus calleryana 0.60 0.87 0.68 2.15 29 Quercus velutina 0.60 0.87 0.68 2.15 30 Celastrus orbiculatus 0.60 0.79 0.68 2.07 31 Juglans nigra 0.60 0.79 0.68 2.07 32 Cornus amomum 0.60 0.56 0.68 1.84 33 Hamamelis virginiana 0.60 0.56 0.68 1.84 34 Prunus spp. 0.60 0.56 0.68 1.84 35 Ptelea trifoliata 0.60 0.56 0.68 1.84 36 Robinia pseudoacacia 0.60 0.56 0.68 1.84 37 Fagus grandifolia 0.60 0.49 0.68 1.77 38 Parthenocissus quinquefolia 0.60 0.43 0.68 1.71 39 Rubus allegheniensis 0.60 0.43 0.68 1.71
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40 Sambucus canadensis 0.60 0.43 0.68 1.71 41 Cornus drummondii 0.60 0.37 0.68 1.65 42 Liquidambar styraciflua 0.60 0.31 0.68 1.59 43 Quercus palustris 0.60 0.26 0.68 1.54 44 Quercus alba 0.60 0.22 0.68 1.50 45 Rubus laciniatus 0.60 0.22 0.68 1.50 Total Total Total Total 100 100 100 300
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The Hallett Sanctuary Hallett is the smallest of the Natural Areas woodlands (only the Dene Native Meadow is slightly smaller among natural areas). It has the interesting distinction of being one of the few ‘untouched’ parts of the park, having been left to grow wild with little to no maintenance for over 70 years, save the addition of various exotic shrubs meant to provide scenic backdrop when viewed from across the Pond (which largely surrounds the Hallett). In this 70-year period, it changed from a sparse hill composed of fill soil and rock outcrops to one heavily covering in early-successional species, namely Prunus serotina, Acer platanoides, Wisteria spp., Ailanthus altissima, and Rhodotypos scandens. Since 2001 it has seen increased efforts to remove invasive species and add trails, with these effort culminating in its opening to the public in 2016. Regina Alvarez, through nearly two decades with Central Park Conservancy, charted the changes and progress in this landscape in detail (Alvarez, 2012). Her final survey of the Hallett was a part of her larger woodland-wide survey, and while it made note of changes that had already been made to that point, she made a few conclusions. Namely, the Hallett in the previous survey was still overwhelmingly composed of Prunus serotina, and had ongoing high levels of Frangula alnus and Rhodotypos scandens. However, species such as Wisteria spp., Acer platanoides, and Ailanthus altissima had already seen large declines at that point. This survey visited 42 points for a total of 168 individuals sampled, the same as the previous survey. The species richness increased from 27 to 41, and the genera increased from 24 to 29. This increase was more than expected, given that much of the restoration efforts in the past few years have focused more heavily on the Ramble and North Woods than the Hallett. Of the 41 species, 26 (or 63%) were native. This percentage is slightly lower than the Ramble and North Woods, but like those other woodlands, the Importance Value percentage held by natives was higher, at 79.2%. This is the largest disparity of all woodlands between native species richness percentage and native species Importance Value percentage, suggesting that while there are many non-native species in the Hallett, most of them either have few individuals, are small in size, or both. The steady, long-term restoration-style management has continued to see removal of invasive species and augmentation with native species. A striking overall observation is that, while rich in total species, only 5 species had a sampled abundance over 7, and only 3 species had more than 10 individuals (Prunus serotina, Viburnum dentatum, and Celtis occidentalis) out of 168 total tallied individuals (see Table 17 below). Finally, it is important to note that during the last survey there were no formal pathways in the Hallett, while a relatively new network of trails is now defined. This impacted the random point placement, due to buffering 5 meters from these new paths before placing random points.
Invasive Species Changes Efforts to decrease the presence of invasive species in Hallett have been consistent over the years, and the data from this survey shows that this has continued since the last survey (see Table 16 below). Frangula alnus, ranked #5 in the previous survey, was not found at all in this survey, and the small size of the Hallett has made it easy to ensure via a visual survey that nearly
80 all of these have been removed. Rhodotypos scandens, once #10, is now also no longer found in the Hallett due to consistent removal of mature specimens and seedlings that arise periodically from the seed bank. Robinia pseudoacacia, a near-native that is nevertheless often considered invasive in the New York City region, has also decreased from #3 to #6. This is largely due to the death of some mature specimens, though smaller DBH saplings and root sprouts still remain. Interestingly, one of the species that has seen relatively large management focus, Acer platanoides, actually increased its rank from #13 to #7. However, the number of individuals (6) remained the same for each dataset. The total increase in IV seems largely due to an increased value of relative frequency, as total basal area was relatively similar, and density was the same. As was noted in Alvarez (2012), all (9) mature specimens of Acer platanoides were removed from the Hallett, along with two Acer saccharum, after an infestation of Asian Longhorned Beetle was found on the sugar maples in the Hallett in 2001 (this was the only sighting in the park). However, saplings continue to appear. Many of the individuals located during this survey are slated for removal in the near future, in conjunction with native sapling plantings. Another non-native species found to be increasing was Malus spp., which went from 3 individuals in the previous survey to 6 in this one. This difference is slight and surely could be due simply to the margin of error in sampling, given the small abundances. However, observation indicates that, like in the rest of the woodlands, Malus hupehensis and/or Malus toringo are also increasing in the Hallett.
Native Species Changes The largest changes of a native species in the Hallett between surveys was one that did not even change rank: Prunus serotina. While still occupying the #1 spot, its IV values went from 102.3 to 66.61. The canopy in the Hallett is in a state of flux in which most black cherries, while still noticeably the most numerous of tall canopy trees, are beginning to be overtaken by other canopy species, like oaks. Another large difference was the increase of Celtis occidentalis from #12 to #5. It was the 3rd most abundant and 3rd most frequent species, even as its relative dominance only ranked 9th, suggesting this is an up-and-coming species in the Hallett as in other woodlands. The most prominent tree in the Hallett Sanctuary is a large, mature Celtis occidentalis located at the top of the primary overlook and surrounded by an ornate wooden circular bench. A large difference was also seen in Viburnum dentatum, which increased from #6 to #3, nearly doubling its overall IV from 13.05 to 25.34. This species is very common on the eastern slope of the Hallett, and one of the major trails is informally referred to as the Viburnum Trail. A few other interesting changes can be seen in some of the species that were found in relatively low abundance but were not found at all previously. Eight individuals of native Rhus were found, all either planted or regenerated from specimens planted in the past few years. Rubus allegheniensis was also found 7 times in the sample, which is more than in the rest of the woodlands combined. The Hallett has quickly become dominated in the understory by thick
81 stands of native shrubs like Rubus, Rhus, Viburnum and Rhododendron, and was probably the most difficult landscape of the 4 woodlands to traverse during the survey because of this.
By Genus The main distinction between genera to note (see Table 18 below) is Fraxinus, since this study has essentially split what was originally thought to have all been Fraxinus americana into both F. americana and F. pennsylvanica. Fraxinus pennsylvanica was actually found to be more numerous that F. americana, though the largest ash in the Hallett (not captured in this survey) is a F. americana, as was noted in the previous study (Alvarez, 2012). In the previous study, F. americana occupied the #2 ranking, with a total IV of 21.76. For this study, if both Fraxinus are again grouped together, the ranking would be #4. However, this is not a representation of decline, as the relative IV value actually increased to 23.82 in this survey. Thus, it seems that ashes represent a similar position within the Hallett as they did previously. The other genus to draw attention to here is Quercus. While Quercus palustris occupied the #2 position in IV ranking, this was likely due to a few large diameter specimens (as was largely the case in the last survey), since only 4 individuals were sampled, some of which represented the largest trees in the Hallett. However, even though only 4 specimens were captured in the sample, it is observed that this species is reproducing well in the Hallett, with prominent natural recruits now occupying the midstory. Also, even though only a few other oak species made it into the survey, planted specimens of Q. velutina and Q. montana, both of which were captured in the sample in small numbers, have also begun to occupy prominent positions in the midstory. It is possible that oaks will replace Prunus in overall dominance in the coming decades, but future studies would be needed to confirm this.
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Figure 8.) Sample unit locations, within the Hallett Sanctuary in Central Park, New York City, with 42 total points.
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Table 16.) Ecological dominance in terms of Importance Value for the Hallett Sanctuary dataset.
Relative Relative Relative Importance Rank Species Name Density Dominance Frequency Value 1 Prunus serotina 17.86 30.00 18.75 66.61 2 Quercus palustris 2.38 32.73 2.34 37.46 3 Viburnum dentatum 13.69 0.71 10.94 25.34 4 Fraxinus pennsylvanica 5.36 9.58 4.69 19.62 5 Celtis occidentalis 8.33 1.73 6.25 16.31 6 Robinia pseudoacacia 3.57 6.22 3.91 13.70 7 Acer platanoides 3.57 0.25 4.69 8.51 8 Carpinus caroliniana 0.60 6.25 0.78 7.63 9 Rubus allegheniensis 4.17 0.02 3.13 7.31 10 Malus spp. 3.57 0.20 2.34 6.11 11 Acer rubrum 2.38 0.20 3.13 5.70 12 Rhus glabra 2.98 0.07 2.34 5.39 13 Ulmus spp. 1.19 2.62 0.78 4.59 14 Ulmus americana 1.79 0.46 2.34 4.59 15 Betula lenta 1.19 1.82 1.56 4.57 16 Rhododendron maximum 1.79 0.10 2.34 4.23 Styphnolobium 17 japonicum 1.79 0.09 2.34 4.22 18 Fraxinus americana 1.79 0.06 2.34 4.19 19 Rhododendron spp. 1.79 0.01 2.34 4.14 20 Morus alba 0.60 2.56 0.78 3.93 21 Acer saccharum 1.79 0.51 1.56 3.86 22 Tetradium daniellii 1.19 0.88 1.56 3.63 23 Crataegus spp. 1.79 0.28 1.56 3.63 24 Rhus typhina 1.79 0.19 1.56 3.53 25 Aralia elata 1.19 0.02 1.56 2.78 26 Lindera benzoin 1.19 0.02 1.56 2.78 27 Quercus velutina 1.19 0.02 1.56 2.77 28 Liriodendron tulipifera 0.60 0.72 0.78 2.10 29 Pinus nigra 0.60 0.70 0.78 2.08 30 Sambucus canadensis 1.19 0.07 0.78 2.04 31 Prunus virginiana 1.19 0.01 0.78 1.98 32 Ilex opaca 0.60 0.36 0.78 1.73 33 Betula pendula 0.60 0.24 0.78 1.61 34 Quercus montana 0.60 0.16 0.78 1.54 35 Viburnum prunifolium 0.60 0.07 0.78 1.45 36 Prunus spp. 0.60 0.02 0.78 1.40 37 Cornus mas 0.60 0.02 0.78 1.39 38 Wisteria floribunda 0.60 0.02 0.78 1.39 39 Unkown spp. 0.60 0.02 0.78 1.39 40 Photinia villosa 0.60 0.00 0.78 1.38
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41 Sassafras albidum 0.60 0.00 0.78 1.38 Total 100.00 100.00 100.00 300.00
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Table 17.) Descriptive statistics for Hallett Sanctuary dataset, ordered alphabetically.
% Abs Density Mean DBH Standard Sampling Species Name Abundance (trees/ha) (cm) Deviation (cm) Min, Max (cm) Units Acer platanoides 6 71.76 4.05 2.45 1.3, 7.8 14.29 Acer rubrum 4 47.84 4.68 2.05 2.5, 7.7 9.52 Acer saccharum 3 35.88 7.30 6.10 2.4, 15.9 4.76 Aralia elata 2 23.92 2.25 1.05 1.2, 3.3 4.76 Betula lenta 2 23.92 16.05 15.05 1, 31.1 4.76 Betula pendula 1 11.96 11.20 0.00 11.2, 11.2 2.38 Carpinus caroliniana 1 11.96 57.70 0.00 57.7, 57.7 2.38 Celtis occidentalis 14 167.44 6.19 5.24 1.3, 18.9 19.05 Cornus mas 1 11.96 3.10 0.00 3.1, 3.1 2.38 Crataegus spp. 3 35.88 6.47 2.83 2.5, 8.9 4.76 Fraxinus americana 3 35.88 3.03 1.52 1.1, 4.8 7.14 Fraxinus pennsylvanica 9 107.64 17.83 15.77 1.1, 40.3 14.29 Ilex opaca 1 11.96 13.80 0.00 13.8, 13.8 2.38 Lindera benzoin 2 23.92 2.25 1.05 1.2, 3.3 4.76 Liriodendron tulipifera 1 11.96 19.60 0.00 19.6, 19.6 2.38 Malus spp. 6 71.76 3.87 1.65 1.8, 6.7 7.14 Morus alba 1 11.96 19.17 14.53 1.3, 36.9 2.38 Photinia villosa 1 11.96 1.30 0.00 1.3, 1.3 2.38 Pinus nigra 1 11.96 19.30 0.00 19.3, 19.3 2.38 Prunus serotina 30 358.80 18.11 14.31 1.2, 49.3 57.14 Prunus spp. 1 11.96 3.20 0.00 3.2, 3.2 2.38 Prunus virginiana 2 23.92 1.30 0.20 1.1, 1.5 2.38 Quercus montana 1 11.96 9.30 0.00 9.3, 9.3 2.38 Quercus palustris 4 47.84 50.25 42.82 7.4, 97.5 7.14 Quercus velutina 2 23.92 2.10 0.60 1.5, 2.7 4.76 Rhododendron maximum 3 35.88 4.20 0.49 3.6, 4.8 7.14 Rhododendron spp. 3 35.88 1.60 0.16 1.4, 1.8 7.14 Rhus glabra 5 59.80 2.32 1.35 1, 4.8 7.14 Rhus typhina 3 35.88 5.73 0.40 5.4, 6.3 4.76 Robinia pseudoacacia 6 71.76 19.45 13.18 3.3, 42.2 11.90 Rubus allegheniensis 7 83.72 1.26 0.17 1.1, 1.6 9.52 Sambucus canadensis 2 23.92 3.60 2.30 1.3, 5.9 2.38 Sassafras albidum 1 11.96 1.00 0.00 1, 1 2.38 Styphnolobium japonicum 3 35.88 3.50 1.98 2.1, 6.3 7.14 Tetradium daniellii 2 23.92 15.30 0.30 15, 15.6 4.76 Ulmus americana 3 35.88 7.20 5.45 3.2, 14.9 7.14 Ulmus spp. 2 23.92 24.50 9.90 14.6, 34.4 2.38 Unknown spp. 1 11.96 3.00 0.00 3, 3 2.38 Viburnum dentatum 23 275.08 3.63 1.81 1.3, 7.2 33.33 Viburnum prunifolium 1 11.96 6.10 0.00 6.1, 6.1 2.38 Wisteria floribunda 1 11.96 3.10 0.00 3.1, 3.1 2.38 Total (w/ 95% Confidence Total Total intervals) Total Total 10.23 (8.01, 168 2009.289774 12.45) 14.57 1.0, 97.5
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Table 18.) Ecological dominance in terms of Importance Value for Hallett Sanctuary data set, grouped by Genus.
Relative Relative Relative Importance Rank Genus Density Dominance Frequency Value 1 Prunus 19.64 30.03 20.31 69.98 2 Quercus 4.17 32.91 4.69 41.77 3 Viburnum 14.29 0.78 11.72 26.79 4 Fraxinus 7.14 9.64 7.03 23.82 5 Acer 7.74 0.96 9.38 18.07 6 Celtis 8.33 1.73 6.25 16.31 7 Robinia 3.57 6.22 3.91 13.70 8 Ulmus 2.98 3.08 3.13 9.18 9 Rhus 4.76 0.25 3.91 8.92 10 Rhododendron 3.57 0.12 4.69 8.37 11 Carpinus 0.60 6.25 0.78 7.63 12 Rubus 4.17 0.02 3.13 7.31 13 Betula 1.79 2.05 2.34 6.18 14 Malus 3.57 0.20 2.34 6.11 15 Styphnolobium 1.79 0.09 2.34 4.22 16 Morus 0.60 2.56 0.78 3.93 17 Tetradium 1.19 0.88 1.56 3.63 18 Crataegus 1.79 0.28 1.56 3.63 19 Aralia 1.19 0.02 1.56 2.78 20 Lindera 1.19 0.02 1.56 2.78 21 Liriodendron 0.60 0.72 0.78 2.10 22 Pinus 0.60 0.70 0.78 2.08 23 Sambucus 1.19 0.07 0.78 2.04 24 Ilex 0.60 0.36 0.78 1.73 25 Cornus 0.60 0.02 0.78 1.39 26 Wisteria 0.60 0.02 0.78 1.39 27 Unknown 0.60 0.02 0.78 1.39 28 Photinia 0.60 0.00 0.78 1.38 29 Sassafras 0.60 0.00 0.78 1.38 Total Total Total Total 100 100 100 300
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The Great Hill: Initial Findings
The Great Hill is most similar to the North Woods, and there is reason to believe that they were managed similarly, or even considered one in the same, in the past (Rogers, 1987). However, in recent decades the North Woods has been considered distinct from the Great Hill, especially given that the area currently comprising the North Woods was designated a ‘Forever Wild’ site (NYC Parks, 2000) in 2000, as well as numerous other such sites around the city, which were identified in an effort to better safeguard natural areas of special value in the city, as well as promote their exploration by New Yorkers (Berger, 2004). As the new Natural Areas section of management has taken shape in the past 5 years, the benefits of this cohesive management unit across disparate landscapes in the park has been recognized. Recently, the idea of ‘absorbing’ much of the woodland landscapes of the Great Hill into the Natural Areas section has gained traction. This makes logical sense, given the close proximity of the Great Hill to the North Woods, as well as the generally forested nature of most of its landscapes which, besides a few adjacent lawn areas and small horticultural beds, have historically been untended except for periodic triage of invasive species such as multiflora rose and porcelain-berry vine (similar to the North Woods during much of its history). Adding these woodlands to the Natural Areas would allow the Conservancy to apply the same workforce and management style to an existing landscape that has received relatively less attention than other woodlands in the park. As was seen in the Diversity section above (Table 2), according to this survey the Great Hill does indeed have a noticeably higher proportion of non-native species, and overall species richness, evenness, Shannon diversity and Simpson diversity values all lag slightly behind the other 3 woodlands (more notably the North Woods, which is relatively close in proximity yet still shows measurable differences in these indices). The species count of 44 and genus count of 30 was only slightly higher than for the Hallett, which counted 41 species and 29 genera despite being much smaller and having less than half the sample size. Of the 44 species, 27 (or 61%), were native, which is similar to the Importance Value percentage of native species (64.1%). In the Great Hill, both the native species richness percentage and the native species Importance Value percentages are the lowest of all 4 woodlands. These indicators are not necessarily values of ecosystem health per se, and this survey only includes woody species over 1 cm DBH instead of the full range of plant species which make up an ecosystem. However, a visual survey of the landscape indicates higher levels of compaction/trampling in the understory (the vast majority of the Great Hill’s landscapes are unfenced, as opposed to nearly half of North Woods landscapes), as well as higher levels of invasive species such as Rosa multiflora, Acer platanoides, Euonymus alatus and others. In many cases, landscapes seem to have been planted heavily with Euonymus alatus and Lonicera maakii, both horticultural species that are now recognized as invasive species and are now either prohibited or regulated in New York State (NYDEC 2014). However, gardeners at the Conservancy have also noted numerous instances of native species that are uncommon in the park, such as Menispermum canadense and Sedum ternatum, as well as various spontaneously
88 occurring ferns and spring wildflowers along the western facing rocky slope, directly adjacent to Central Park West. It was decided to take the opportunity while sampling the other woodlands to include the Great Hill as well, in the process obtaining a baseline set of data for which to measure future trends. We took data from 96 sample units (points), for a total of 384 specimens. This represented 44.89 pts/ha (see Table 1), which was more than the North Woods (30.03pts/ha) but less than the Ramble (50.03 pts/ha). Out of the original 100 points, 3 were deemed non- representative of ‘woodland’ after visiting them, while another point was located on a path. The majority of the landscapes are sloped, rocky, and dominated by mature trees. As these landscapes are not officially deemed woodlands, the tree crew is not often at liberty to leave standing deadwood or snags for wildlife habitat, though the remote nature of these landscapes means trees in more high-trafficked areas receive more attention concerning risk-mitigation tree pruning and removals. Landscape management overall is more sporadic in the Great Hill, as less staff are able to provide invasive species removal, and staff-led plantings are rarely conducted.
Invasive Species When looking at the Great Hill dataset and Importance Value rankings (see Table 19 below), differences in invasive species between the current North Woods survey are obvious, though if you were to compare them with the North Woods survey conducted nearly a decade ago (Alvarez, 2012) they would look quite similar. This is because in the Great Hill Acer platanoides occupies the #3 rank overall (the same as the previous North Woods survey) and Acer pseudoplatanus is ranked #10 (ranked #9 in the previous North Woods survey). It is also notable that Malus spp. which has shown great increases between surveys in the Natural Areas, is more dominant in the Great Hill than in all other woodlands, coming in at #6 overall. Morus alba, at #16, is ranked higher here than in all other woodlands, and Robinia pseudoacacia (#12) is ranked higher here than all landscapes except the Hallett (#6). Finally, the Great Hill is the only woodland in which Rosa multiflora, Rhodotypos scandens, and Euonymus alatus were found in the sample. While relatively low in IV rank, these species were each found at least twice, and many individuals were present near the sample points that were either too short or did not have large enough stems to be counted.
Native Species While there is certainly a higher number of invasive species in the Great Hill, it is also notable how many native species are present. A major and surprising difference between the Great Hill and the other woodlands was that it was the only woodland where Prunus serotina did not make it into the top 2 spots in IV ranking, instead placing #4. This is notable given the relatively compacted understory of the Great Hill, for which Prunus serotina would be thought to be well adapted. Ulmus americana, another compaction-tolerant species, occupies the top spot instead, with a value of 35.56, only slightly ahead of the #2 species Quercus palustris, which had
89 a value of 34.83. Tilia americana (American basswood/linden) is ranked #5 in the Great Hill, though this is almost totally due to the number of large specimens in the sample (only 9 individuals were included). The largest tree sampled all summer, at 125.6 cm DBH, was a twin trunked Tilia americana in the Great Hill. It should be noted that most of this species is located on the northeastern slope of the Great Hill, near the park drive and adjacent to the North Woods. Most of the North Woods’ population of Tilia americana is located relatively near this part of the park drive as well (the trail through that area of the North Woods is called the Linden Trail by staff), and when viewed as a whole it presents as one contiguous population despite the drive that bisects it. Tilia cordata (littleleaf linden), a commonly planted horticultural variety from Europe, surprisingly came in #13 despite only 3 specimens being found out of the total 384-specimen sample. While these large specimens do indeed dominate parts of the landscape, it is important to look closely at the reasons why a species ranks so highly. There are also a few other highlights for native species in the Great Hill. The most notable is perhaps the impressive ranking of Acer saccharum, at #8. This species was found, both in the survey as well as in general observation, to be in much higher abundance in the Great Hill than in any other woodland area, or anywhere else in the park for that matter. Few if any of the specimens were large, with most occupying the midstory, some in large stands. When looking at the density, basal area, and frequency of this species it is apparent that its #5 ranking in relative density is largely responsible for the high showing (25 specimens were sampled), as well as its #8 ranking in relative frequency. It appears that this species has been planted heavily in this area within the past few decades, but it is also apparent that there are many seedlings/saplings that have regenerated naturally. Given that only 4 other species counted more individuals in the Great Hill sample, and most of the specimens are not yet mature, it seems that this species is destined to climb in IV ranking in the future.
By Genus A few insights can be gained when analyzing the relative IV ranking by genus in the Great Hill (see Table 21 below). First, Acer comes in first here. Even though the highest ranked Acer (Acer platanoides) is only ranked #3, the presence of 3 Acer species in the top 10 put its value (53.29) slightly ahead of Ulmus (48.7) and Quercus (44.2). The low ranking of Quercus here is also notable, given that both the nearby North Woods and Ramble saw Quercus take the top spot convincingly. Another notable difference between the Great Hill and the rest of the woodlands is the relatively low ranking of Fraxinus, at only #20 out of 30 genera. This is noticeably lower than the adjacent North Woods, where Fraxinus was ranked #5 out of 44 total genera. Finally, Viburnum was almost nonexistent, with only 2 individuals total and a rank of #21 out of 30, while Viburnum ranked at least #8 in every other woodland, and #6 in the Natural Areas overall.
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Lower Quartile Range The lower size class of the Great Hill (which for this survey was all stems between 1 cm and 2.4 cm) reveals some major differences between it and the other woodlands (see Table 22 below) that might not be apparent just from looking at the overall IV rankings. First, Acer platanoides is ranked #1 here, and Acer pseudoplatanus ranked #4. These are interestingly the exact same rankings for these species as was found in the LQR of the previous North Woods survey nearly a decade ago. Malus spp. is also ranked #2, which is even higher than in the other woodlands, where this species complex has also been shown to have had large increases. This would lead us to believe that the thinning of crabapples in other woodlands has kept it from being ranked even higher in the LQR there, despite the comparatively decreased targeting compared to Acer platanoides. Perhaps the biggest takeaway from the LQR of the Great Hill dataset is that both Tilia and Quercus are both extremely underrepresented. Where genus Quercus is ranked #3 and Tilia #5 overall, there were zero Quercus found in the lower DBH quartile and only one Tilia americana. This is concerning, as it would appear that each of these species are having little success regenerating (very few saplings of these species were seen in the Great Hill, even when looking outside of the sample units). This is in contrast to the North Woods, where just across the park drive Tilia americana and Quercus rubra are found regenerating more commonly and rank #13 and #5 in the LQR, respectively. However, it should be noted that this was not the case in the previous survey of the North Woods, and these values only improved over the last 10 or so years. Fraxinus is also notably low here, with only one specimen found in the lower DBH quartile. This is in stark comparison to the adjacent North Woods, where the combined genus of Fraxinus was ranked #2 in the LQR. There were a few ‘bright spots’ where some native species were represented quite well in the lower DBH quartile. Carya cordiformis ranks #3 here, which is higher than it ranks in any other woodland, where it ranked #6 in the LQR of the combined Natural Areas dataset. Hardy native species Ulmus americana and Celtis occidentalis were ranked #5 and #6, respectively, though these species ranked #2 and #1 in the Natural Areas’ LQR. Finally, Acer saccharum ranked #10 here. While most of the Acer saccharum were relatively small, they were also rather uniform in size. This average size happened to be slightly larger than the LQR cutoff of 2.4 cm. Indeed, the mean DBH for Acer saccharum in the Great Hill was 7.8 cm, with a relatively small standard deviation of 4.47 cm in a sample size n=25.
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The Great Hill: Recommendations
The Great Hill is the landscape that, outside of the Natural Areas, most resembles a ‘woodland’. As show above, there are positive signs of regeneration of certain native trees, while regeneration of dominant canopy trees like Quercus and Tilia is noticeably depressed. However, this landscape is also notable for having perhaps the most impressive population of Acer saccharum in the park, as well as the largest mature stand of Tilia americana, despite its lack of regeneration. It is also striking to compare the similarities of the Great Hill in this survey with the survey of the North Woods a decade ago. The concerns that were originally held for the North Woods (lack of regeneration of dominant native canopy species such as Quercus and Tilia) seem to have been addressed via management in the intervening years. Given this result, it is plausible that the Great Hill could see similar improvements if it were to shift to management as a Natural Area. Invasive shrubs such as Euonymus alatus, Rhodotypos scandens, and Rosa multiflora could be replaced by the underrepresented viburnums, spicebush, or witch-hazel. There is also historical precedent for managing the Great Hill as the other woodlands are managed, with documents showing that one of the few remaining woodlands in Manhattan occupied the site before the park’s construction (See Appendix A.). It was also grouped together as part of the North Woods in early maps from the Central Park Conservancy’s early management plan. (Rogers, 1987) A few goals and recommendations as to this possibility are included below, which largely reflect management goals currently in place across the other Natural Areas woodlands.
1) Integrate the Great Hill into the larger matrix of natural areas in the park by shifting its management regime to that of the North Woods, improving connectivity and effectively expanding the natural areas of the park.
Objective: Shift tree management to include wildlife habitat and conservation of standing/fallen deadwood where appropriate.
Objective: Shift landscape management to include care by Natural Areas Technicians under the overall strategy of the Natural Areas section.
2) Preserve and promote the regeneration of existing native plant populations, keeping in mind the priorities of the local community.
Objective: Promote the spread of hardy native species to compete with invasive species.
Objective: Plant suitable native species from sources such as Greenbelt Native Plant Center when necessary to add diversity, functional redundancy or to compete with aggressive invasive species.
3) Reverse course of invasive species proliferation through more active management.
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Objective: Manually remove ground-layer species in concentrated initial effort, followed by continued maintenance. Focus first on shrubs and vines, and then to invasive saplings.
Objective: Release native understory species through staged removal of invasive canopy tree specimens.
4) By managing the Great Hill under the same framework as the North Woods, minimize the threats posed to the North Woods and other landscapes by reducing invasive propagule sources from the Great Hill, improving the overall resilience of both landscapes.
Objective: Monitor invasive species recruitment in both the North Woods and Great Hill over 5-10 years to assess propagule pressure after restoration is initiated.
5) Bring the landscape to a level in which periodic disturbances (from storm damage, etc.) result in native, rather than invasive species filling the niches/canopy gaps. This has been largely achieved in other park woodlands, particularly the Ramble. Labor input from weeding or planting in these sites should be reduced in the long term.
Objective: Monitor gaps in canopy 1 year after they occur for vegetation structure.
6) Reduce and reverse impacts from excessive off trail use, improving soil quality and rainwater infiltration, and envisioning ways to retain public access without damaging the landscape.
Objective: Install fencing where appropriate.
Objective: Decompact soil through both manual and mechanical means.
Objective: Conserve and manage coarse woody debris and leaf litter to improve humus layer and rainwater infiltration and retention capabilities.
Objective: Retain and formalize a trail or network of trails within the site to maintain access while reducing impact.
7) Involve all management entities within the Conservancy in implementing this restoration plan, as well as ongoing maintenance. Consult with the Natural Areas Conservancy and other agencies as appropriate.
Objective: Share this proposal with all relevant parties within the Central Park Conservancy to incorporate perspectives from all management parties involved.
Objective: Solicit feedback from other relevant agencies as appropriate.
8) Engage the public through community outreach, appropriate signage, volunteer work in the field and interpretive opportunities.
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Figure 9.) Sample unit locations at the Great Hill in Central Park, New York City. A total of 100 points were plotted, 96 of which were viable.
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Table 19.) Ecological dominance in terms of Importance Value for the Great Hill dataset.
Relative Relative Relative Importance Rank Species Name Density Dominance Frequency Value 1 Ulmus americana 11.72 12.49 11.35 35.56 2 Quercus palustris 1.56 31.14 2.13 34.83 3 Acer platanoides 15.10 2.59 13.83 31.53 4 Prunus serotina 11.20 9.34 10.64 31.18 5 Tilia americana 2.34 17.09 2.13 21.56 6 Malus spp. 9.11 1.07 9.57 19.76 7 Carya cordiformis 4.69 3.53 6.03 14.25 8 Acer saccharum 6.51 1.51 3.90 11.92 9 Ulmus spp. 4.69 2.14 4.96 11.80 10 Acer pseudoplatanus 3.91 0.64 4.61 9.15 11 Celtis occidentalis 4.17 0.50 4.26 8.92 12 Robinia pseudoacacia 4.17 0.51 3.55 8.22 13 Tilia cordata 0.78 6.38 0.71 7.87 14 Quercus rubra 1.82 3.08 2.48 7.38 15 Carpinus caroliniana 1.82 3.84 1.42 7.08 16 Morus alba 2.08 0.32 2.84 5.24 17 Photinia villosa 1.56 0.07 1.77 3.41 18 Lindera benzoin 1.30 0.22 1.42 2.94 19 Aesculus parviflora 1.56 0.04 1.06 2.67 20 Crataegus spp. 1.30 0.23 1.06 2.60 21 Catalpa speciosa 0.26 1.43 0.35 2.05 22 Lonicera maackii 1.04 0.12 0.71 1.87 23 Euonymus alatus 0.78 0.22 0.71 1.71 24 Quercus velutina 0.52 0.14 0.71 1.37 25 Philadelphus coronarius 0.52 0.01 0.71 1.24 26 Rosa multiflora 0.52 0.01 0.71 1.24 27 Cornus mas 0.26 0.48 0.35 1.10 28 Liriodendron tulipifera 0.26 0.31 0.35 0.93 29 Rhodotypos scandens 0.52 0.01 0.35 0.88 30 Amelanchier canadensis 0.26 0.11 0.35 0.72 31 Acer rubrum 0.26 0.08 0.35 0.69 32 Ulmus procera 0.26 0.07 0.35 0.68 33 Cornus florida 0.26 0.05 0.35 0.67 34 Ulmus pumila 0.26 0.05 0.35 0.66 35 Hamamelis virginiana 0.26 0.04 0.35 0.66 36 Juniperus virginiana 0.26 0.04 0.35 0.65 37 Fraxinus americana 0.26 0.03 0.35 0.65 38 Viburnum dentatum 0.26 0.03 0.35 0.65 39 Lonicera spp. 0.26 0.02 0.35 0.64
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40 Quercus prinus 0.26 0.01 0.35 0.62 41 Gleditsia triacanthos 0.26 0.01 0.35 0.62 42 Fraxinus pennsylvanica 0.26 0.00 0.35 0.62 43 Viburnum nudum 0.26 0.00 0.35 0.62 Ampelopsis 44 brevipedunculata 0.26 0.00 0.35 0.62 Total 100 100 100 300
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Table 20.) Descriptive statistics for Great Hill dataset, organized alphabetically.
Absolute Standard % Mean Min, Max Species Name Abundance Density Deviation Sampling DBH (cm) (cm) (trees/ha) (cm) units Acer platanoides 58 304.35 4.89 5.99 1, 37.5 40.63 Acer pseudoplatanus 15 78.71 4.44 6.09 1.3, 26.6 13.54 Acer rubrum 1 5.25 10.10 0.00 10.1, 10.1 1.04 Acer saccharum 25 131.18 7.80 4.47 1.5, 17 11.46 Aesculus parviflora 6 31.48 2.68 1.29 1.1, 4.9 3.13 Amelanchier canadensis 1 5.25 12.00 0.00 12, 12 1.04 Ampelopsis brevipedunculata 1 5.25 2.10 0.00 2.1, 2.1 1.04 Carpinus caroliniana 7 36.73 22.84 14.56 2.1, 37.9 4.17 Carya cordiformis 18 94.45 8.73 13.65 1.3, 47.1 17.71 Catalpa speciosa 1 5.25 43.80 0.00 43.8, 43.8 1.04 Celtis occidentalis 16 83.96 5.55 3.28 1.7, 11.1 12.50 Cornus florida 1 5.25 8.40 0.00 8.4, 8.4 1.04 Cornus mas 1 5.25 25.40 0.00 25.4, 25.4 1.04 Crataegus spp. 5 26.24 6.42 4.56 1.6, 14.5 3.13 Euonymus alatus 3 15.74 8.67 4.91 2.4, 14.4 2.08 Fraxinus americana 1 5.25 6.60 0.00 6.6, 6.6 1.04 Fraxinus pennsylvanica 1 5.25 2.40 0.00 2.4, 2.4 1.04 Gleditsia triacanthos 1 5.25 2.60 0.00 2.6, 2.6 1.04 Hamamelis virginiana 1 5.25 7.60 0.00 7.6, 7.6 1.04 Juniperus virginiana 1 5.25 7.20 0.00 7.2, 7.2 1.04 Lindera benzoin 5 26.24 6.50 3.96 2.2, 13.1 4.17 Liriodendron tulipifera 1 5.25 20.40 0.00 20.4, 20.4 1.04 Lonicera maackii 4 20.99 5.15 3.74 2, 11.3 2.08 Lonicera spp. 1 5.25 5.40 0.00 5.4, 5.4 1.04 Malus spp. 35 183.66 4.19 4.82 1, 28.2 28.13 Morus alba 8 41.98 6.53 3.42 1.9, 11.6 8.33 Philadelphus coronarius 2 10.49 2.55 0.95 1.6, 3.5 2.08 Photinia villosa 6 31.48 3.17 2.50 1.1, 7.9 5.21 Prunus serotina 43 225.64 13.55 10.36 1, 44.6 31.25 Quercus palustris 6 31.48 76.75 32.48 11.4, 115.9 6.25 Quercus prinus 1 5.25 2.70 0.00 2.7, 2.7 1.04 Quercus rubra 7 36.73 15.46 18.70 2.6, 59.4 7.29 Quercus velutina 2 10.49 9.15 2.65 6.5, 11.8 2.08 Rhodotypos scandens 2 10.49 2.05 0.25 1.8, 2.3 1.04 Robinia pseudoacacia 16 83.96 5.58 3.35 1.5, 13.9 10.42 Rosa multiflora 2 10.49 1.90 0.10 1.8, 2 2.08 Tilia americana 9 47.23 31.39 39.44 1.7, 125.6 6.25 Tilia cordata 3 15.74 45.07 28.55 5.2, 70.5 2.08 Ulmus americana 45 236.13 11.54 15.43 1.3, 103.4 33.33 Ulmus procera 1 5.25 9.40 0.00 9.4, 9.4 1.04
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Ulmus pumila 1 5.25 7.90 0.00 7.9, 7.9 1.04 Ulmus spp. 18 94.45 7.23 10.35 1, 47.2 14.58 Viburnum dentatum 1 5.25 6.48 0.00 6.48, 6.48 1.04 Viburnum nudum 1 5.25 2.20 0.00 2.2, 2.2 1.04
Total (w/ 95% Confidence Total Total Interval) Total Total 9.94 (8.36, 384 2014.99 11.53) 15.80 1, 125.6
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Table 21.) Ecological dominance in terms of Importance Value grouped by genus, for Great Hill dataset.
Relative Relative Relative Importance Rank Genus Density Dominance Freuency Value
1 Acer 25.78 4.82 22.70 53.29 2 Ulmus 16.93 14.75 17.02 48.70 3 Quercus 4.17 34.36 5.67 44.20 4 Prunus 11.20 9.34 10.64 31.18 5 Tilia 3.13 23.47 2.84 29.43 6 Malus 9.11 1.07 9.57 19.76 7 Carya 4.69 3.53 6.03 14.25 8 Celtis 4.17 0.50 4.26 8.92 9 Robinia 4.17 0.51 3.55 8.22 10 Carpinus 1.82 3.84 1.42 7.08 11 Morus 2.08 0.32 2.84 5.24 12 Photinia 1.56 0.07 1.77 3.41 13 Lindera 1.30 0.22 1.42 2.94 14 Aesculus 1.56 0.04 1.06 2.67 15 Crataegus 1.30 0.23 1.06 2.60 16 Lonicera 1.30 0.14 1.06 2.51 17 Catalpa 0.26 1.43 0.35 2.05 18 Cornus 0.52 0.53 0.71 1.76 19 Euonymus 0.78 0.22 0.71 1.71 20 Fraxinus 0.52 0.04 0.71 1.27 21 Viburnum 0.52 0.03 0.71 1.27 22 Philadelphus 0.52 0.01 0.71 1.24 23 Rosa 0.52 0.01 0.71 1.24 24 Liriodendron 0.26 0.31 0.35 0.93 25 Rhodotypos 0.52 0.01 0.35 0.88 26 Amelanchier 0.26 0.11 0.35 0.72 27 Hamamelis 0.26 0.04 0.35 0.66 28 Juniperus 0.26 0.04 0.35 0.65 29 Gleditsia 0.26 0.01 0.35 0.62 30 Ampelopsis 0.26 0.00 0.35 0.62 Total 100 100 100 300
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Table 22.) Ecological dominance in terms of Importance Value for lower DBH quartile range (LQR) of the Great Hill dataset. In this case, the lowest quartile included all stems below 2.4 cm DBH.
Relative Relative Relative Importance Rank Species Name Density Dominance Frequency Value 1 Acer platanoides 24.74 22.44 24.39 71.57 2 Malus spp. 15.46 13.20 15.85 44.52 3 Carya cordiformis 7.22 6.59 8.54 22.34 4 Acer pseudoplatanus 7.22 7.67 7.32 22.20 5 Ulmus americana 5.15 5.97 4.88 16.00 6 Celtis occidentalis 4.12 5.56 3.66 13.35 7 Prunus serotina 4.12 4.82 3.66 12.60 8 Ulmus spp. 4.12 2.37 4.88 11.38 9 Photinia villosa 4.12 3.09 3.66 10.88 10 Acer saccharum 3.09 2.79 2.44 8.32 11 Aesculus parviflora 3.09 2.56 2.44 8.10 12 Rosa multiflora 2.06 2.41 2.44 6.91 13 Lonicera maackii 2.06 3.09 1.22 6.37 14 Rhodotypos scandens 2.06 2.84 1.22 6.12 15 Euonymus alatus 1.03 1.92 1.22 4.17 16 Fraxinus pennsylvanica 1.03 1.92 1.22 4.17 17 Lindera benzoin 1.03 1.61 1.22 3.86 18 Viburnum nudum 1.03 1.61 1.22 3.86 Ampelopsis 19 brevipedunculata 1.03 1.47 1.22 3.72 20 Carpinus caroliniana 1.03 1.47 1.22 3.72 21 Morus alba 1.03 1.20 1.22 3.45 22 Tilia americana 1.03 0.96 1.22 3.21 23 Crataegus spp. 1.03 0.85 1.22 3.10 24 Philadelphus coronarius 1.03 0.85 1.22 3.10 25 Robinia pseudoacacia 1.03 0.75 1.22 3.00 Total Total Total Total 100 100 100 300
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DISCUSSION
The data obtained in this survey, when compared to the previous survey, strongly indicates that management efforts to decrease invasive species and increase native species diversity have been effective. This can be seen by comparison of diversity indices such as richness, Shannon diversity, Shannon evenness and Simpson D values. These indices are meant to provide a measure of biodiversity, but we also know that ‘biodiversity’ itself is perhaps better understood as a multidimensional construct that not only includes taxonomic diversity, but also functional and genetic diversity (Palmer, 2016, Secretariat of the Convention on Biological Diversity, 2001). It is thus understood that a measure of taxonomic diversity is not a total indicator of ecosystem ‘health’, since genetic and functional diversity are also key components of biodiversity, and losses in these areas, even if coupled with gains in taxonomic diversity, could still mean a loss of biodiversity overall. However, successfully measuring all of the values which could feasibly impact health would be difficult and are outside the scope of this study. Thus, the limits of using these metrics to measure overall health and resilience should be understood, though they are recognized as good indicators of the goals of the Natural Areas team, which is to increase species diversity. This alone is a worthy goal, and diverse landscapes (whether genetic or taxonomic) are often found to contribute to resiliency after disturbance, or to potential climate changes/variability (Oehri et al., 2017, Schippers et al. 2015). It is also debated as to whether increased species richness is correlated with site invasibility, with studies showing both positive and negative correlations between richness and invasibility (Radosevich et al., 2007) depending on scale and location. Stohlgren et al. (1999) indicated that ‘hot spots’ of diversity would also be the most likely to be invaded after human disturbance, due to larger resource availability. It could be that, while Manhattan was once home to a surprising level of both taxonomic and structural diversity (Sanderson, 2013), this level of diversity could have indicated a potential for resource availability that makes it more suitable to invasion. Even with the limitations of not quite being able to define ‘health’ through taxonomic diversity alone, the measured changes in taxonomic biodiversity over the last decade suggest that the composition of the Natural Areas changed along park management’s desired trajectory of more species, and potentially greater ability to respond to natural disturbances. The detailed look at changes in Importance Value rankings allow us to recognize and quantify larger trends in species dominance. It must be reiterated however, that this survey only captured woody plants over a certain size, while much of the restoration efforts in the woodlands focus on herbaceous species as well. Future surveys of this type of vegetation over time would also be helpful in quantifying overall plant diversity, not just woody species. It is important to mention the management methods that brought the Natural Areas to the condition they are in today. In the past decade or so, the Natural Areas have employed sets of weekly volunteers in groups between 2 and 12 members. The Ramble and North Woods each have 2 sets of volunteers that come on 2 different days each week, and the Hallett and Dene
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Slope share 2 sets of volunteers on 2 days each week. These volunteer projects, as well as the work of dedicated staff members (these landscapes see nearly double the number of employees as in years prior) carry out restoration and maintenance efforts on a daily basis. Staff move about either on foot or via a small utility vehicle (such as a golf cart or John Deer Gator), and each day begins with trash removal and landscape/infrastructure inspection by a staff member. Ideally, these tasks require less than half of a day’s attention, and the rest of the day involves either self- led ongoing work like weeding, planting, or preparing for a volunteer project (a large planting, sapling removal, Japanese knotweed removal, garlic mustard removal, multiflora rose removal, etc.). Occasional seasonal team-wide projects are also held, which aim to accomplish more work in a short period of time (for example, re-grade and re-surface rustic pathway or cut back and removal all samplings in a meadow landscape) that are not always best suited for volunteer projects. All work is done manually: invasive saplings, shrubs and larger trees are removed with weed wrenches, shovels, handsaws, and pickaxes, while herbs are either pulled by hand or with shovels and other hand tools. Often weeded material is removed and disposed of, though in many cases the material can be composted or even left on site. As for how plants are obtained, small orders of plants (50-150 gallon pots of trees and shrubs, 10-30 flats of plugs, 3-10 pounds of seed) are ordered each Spring and Fall, with more woody plants prioritized in the Fall and more herbaceous plants prioritized in the Spring. These all come from local nurseries in New York and New Jersey, with plant stock ideally originating from seed collected locally. These numbers might change to reflect changes in ecological stability in the future. Monthly educational meetings are also scheduled to increase knowledge and skills across all staff members. Finally, every summer each woodland (and the Dene) receives at least one college intern to help extend the range of productivity further. A youth group called ROOTS (Restoration of the Outdoors Organized by Teen Students) which met weekly during different seasons between 2001 and 2018 was also instrumental in accomplishing much or the early-stage invasive species removals in the Hallett Sanctuary and North Woods. A key fact to note is that much of each landscape is visited weekly, if not daily, often by the same person. This allows that person to notice incremental changes and create a sense of ownership and responsibility to the landscape, as well as manage adaptively day to day during ongoing restoration projects. The work is done manually and incrementally on small scales, with smaller projects and invasive species sites prioritized first in an early detection, rapid response style. Larger goals, such as removal of invasive canopy trees or large patches of herbaceous species are done strategically and often incrementally, with a plan as to how to maintain progress and not regress. All of these sustained efforts are likely key to the successes measured by this study. Not all land management entities have the resources (staff and volunteers) that Central Park Conservancy has, and thus it might not be expected for these outcomes to translate to all other urban or even rural parks. However, most parks do not see the attention and visitation that Central Park receives, so perhaps parks with similar resources can look to these methods and outcomes as proof that ongoing management efforts can prove successful. When looking back at the original survey by Alvarez (2012), some overarching findings were found, and some of those findings prompted questions that could only be answered by
102 future surveys. Prunus serotina was found as the dominant species, and the genus Prunus was also dominant across the woodlands, though Quercus came close in the North Woods. Now, we see that while Prunus serotina still holds the highest IV, its value is much lower, and now Quercus is the convincing leader among genera instead of Prunus. The previously high Importance Values of Quercus were also slightly undercut by comparatively low rankings within the lower size quartile, with the implication being that the future of oaks, while now quite dominant, might be in question if they were unable to regenerate. This survey shows that oaks have increased measurably within the smaller size class. Similarly, Tilia also showed a marked increase in smaller specimens. In the previous survey, Acer platanoides dominated in this lower size class across the woodlands, and Acer pseudoplatanus was also highly represented in the North Woods. Aralia elata was seen as a species of increasing concern, and it was questioned whether control of Acer platanoides in particular was a futile effort. The data show that, while Acer platanoides still holds a significant presence in the Natural Areas, it has decreased very substantially, while Acer pseudoplatanus has decreased even more so. This is perhaps more encouraging when coupled with the observed increases in Acer rubrum and Acer saccharum, which are poised to quickly replace the invasive maples. Aralia elata, while still an important invasive species to monitor, has also decreased in Importance value. Invasive plants such as Frangula alnus, Rhodotypos scandens and Ailanthus altissima all held significant Importance Values in the previous survey as well, while Rosa multiflora and Euonymus alatus were also still present. The potential continued spread of these species was identified as a priority for management, and all of them were completely nonexistent in the Natural Areas within this survey, except Ailanthus, which has decreased drastically. It should be further noted that Ailanthus is currently considered to be of relatively low concern when compared to other invasive species across the Natural Areas, though its status as a potential host for Spotted Lantern Fly could potentially make it a concern for another reason. Finally, Alverez (2012) noted the high Importance Values of Phellodendron and Styphnolobium as well. While the mature specimens of Phellodendron have gone unremoved, and thus their IV for this survey remained relatively unchanged, saplings and some mature specimens of Styphnolobium have been systematically removed, given their propensity for spreading quickly into other landscapes. This translated to a noticeable decrease in Importance Value for Styphnolobium as well. This survey also provides findings that have other interesting implications. Celtis occidentalis and Ulmus americana, both disturbance and compaction-tolerant native species similar to Prunus serotina, each saw large increases across all woodlands, especially in the lower size class, while Prunus serotina decreased comparatively. It is possible that this is due to an increasingly mature canopy in each woodland, with competition for light limiting the light- sensitive pioneer species P. serotina, while the comparatively shade tolerant C. occidentalis and U. americana can better survive in the mid-story. P. serotina’s dominant presence in the woodlands has been a point of concern since the founding of the Conservancy in 1980, and this survey suggests that the woodlands are in a state of transition away from this dominance. Black cherries are considered a desirable species by Natural Areas staff, more so than in other areas of the park, so their waning dominance is not due to management antagonism so much as the current successional trajectory towards larger canopy species such as Quercus, Ulmus, Tilia and Liriodendron. 103
Finally, other significant implications can be made when analyzing the lower DBH quartile. Fraxinus has seen a significant increase here, which is noteworthy given the eminent spread of Emerald Ash Borer. This is a genus that has reproduced naturally in the park since before the park was constructed and has shown itself to be adaptable to a wide variety of conditions throughout the woodlands, with new species being recognized even recently. Future studies will determine if this species is able to continue to reproduce and maintain its current level of ecological dominance. Sassafras and Nyssa have also impressed with their large increases within the lower size class over the last survey. Given the high number of dense sapling thickets of both of these species, it is apparent that efforts to decrease compaction and trampling around many mature specimens of Sassafras and Nyssa have allowed for increased regeneration, similar to how reduced Japanese knotweed coverage has likely led to a rise in Quercus regeneration in the North Woods. Carya cordiformis is another species that has seen a large increase in the understory, and this was reflected in the survey. It saw large increases both in overall IV and within the smaller size class. Given the potential threats to other species by Emerald Ash Borer, Dutch Elm Disease, Asian Longhorned Beetle, Oak Wilt, and Beech Bark Disease, the drastic increase in a native long-lived hickory species is welcome. While many positive changes have been shown since the last survey, a few points of concern do arise when looking at the increase in certain species. Malus spp. increased dramatically across all landscapes, and Prunus spp. increased noticeably in the Ramble as well. Each of these horticultural species have ecological and aesthetic value, as do all species, native or otherwise. However, future management will need to decide, even if on a case-by-case basis, whether to treat these species as invasives in the Natural Areas. This should be done, as with all removals, with a recognition of their inherent value, only removing them if the required disturbance does not disrupt a sensitive landscape and if the values they provide can be replaced by other native species. A staged removal of a species causes less unnecessary disturbance, which in turn avoids exacerbating the spread of invasive species in landscapes that have less native species in the seed bank.
RECOMMENDATIONS
This survey’s impact is due in large part to the fact that another survey had already been completed nearly a decade ago, allowing for measurement of change. As such, it is recommended that periodic surveys of this type be included in future management plans for the park. A need for more monitoring of other species besides woody plants, which would ideally also be surveyed periodically, is also recognized. Large tree inventory data taken in the 1980s revolutionized the park’s system of tree care and arboriculture, making it more efficient and effective. Similar detailed inventories of the landscapes in the Natural Areas could also make future work more efficient and effective.
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Also, in addition to the recommendations specific to the Great Hill earlier, perhaps it is worth evaluating which areas of the park might warrant management as a Natural Area, or at least could benefit from more targeted removal of invasive species and promotion of hardy natives. The Natural Areas do not exist in a vacuum, and the control of invasive species can be become more difficult when these species are not similarly managed through the park. Not all non-native species that are removed in the Natural warrant removal outside them, but the development of coordinated, standard treatments of specific invasive plants across all management sections could improve control of these species overall. One specific recommendation is to focus on the remaining parts of the Natural Areas that still see heavy trampling, compaction, erosion, and spread of Japanese knotweed and other herbaceous invasive species. Concerted efforts in the northern landscapes of the North Woods, where small populations of Quercus alba, Carya tomentosa, Carya ovata and the state-rare Ptelea trifoliata reside could help apply the same levels of success to these species as has been observed in other areas of the Natural Areas. It is also recommended that use of historic park surveys to guide future species reintroductions be continued and expanded. Many of these species are not terribly sensitive or limited by potential climate changes and are still found in relative abundance outside of the city. As such, their reintroduction and success could be an attainable goal that would further restore an assemblage of species that have historical proof of success. Of course, the research from this study is only one component of what could be much more research shedding light on the ecological and cultural values of the Natural Areas in Central Park. The comparison of woody species data, however different in its form, from previous park surveys like the ones from the early days of the park (Rawolle and Pilat, 1857) and Conservancy (Rogers, 1987) to this dataset could also be quite valuable. It is also recommended that data already being collected from sources such as iNaturalist (a crowd-sourced community naturalist app that allows members of the public to record and verify species occurrences) and the New York City chapter of the Audubon Society be studied and applied directly by park management. Partnership with local academic institutions, either directly or through a research residency program at the Conservancy’s Institute of Urban Parks, should be explored, as well as partnerships with local volunteer/subject-matter experts to organize and maintain projects such as annual species counts or ongoing long-term phenology surveys. The planned upcoming comprehensive park user survey, similar to the one from 2011, should include specific questions related to the Natural Areas in order to better ascertain how the usage/appreciation of these landscapes has changed over time, as well as to better manage for the values of park users in the present and future. A dedicated invasive species management plan/protocol should be created for the park as a whole, as well as a targeted 30-year Natural Areas Management Plan, drafted by and for the Natural Areas team with input as necessary from other teams such as Tree Care, Turf Care, Park Maintenance, Interpretation and Programs, Planning, Design and Construction, Park Planning and the Institute of Urban Parks. Finally, studies such as this one should be available to the public via communications such as the Conservancy’s website.
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CONCLUSIONS
The work in the Natural Areas to remove invasive species and increase native diversity is a long process, and it can be difficult for any one staff member or volunteer to easily recognize this change. This study hopefully shows that their efforts have been a success and can give those who were involved a sort of concrete realization of the changes that have been made so far. Diversity has increased, the share of invasive species of concern has decreased, and native canopy species are beginning to show higher representation in the understory, which are all positive indicators for the future of the woodlands. Planting has likely been an important component of this, especially by increasing both taxonomic and genetic diversity. Planting native plants or seeds also helps displace invasive species that would otherwise be much harder to eradicate. The presence of many planted species in the survey, even in low amounts, also helped increase overall species richness values, and their Importance Values will likely increase over time. However, many of the large changes that took place since the last survey were not dependent on planting at all. Large canopy species already present in the overstory indicated noticeably higher regeneration than in the previous survey. This increased natural regeneration will likely have impacts that last well into the future. These surveys were taken less than a decade apart, and yet large changes have already been made apparent. There will likely come a time in the not-so-distant future when planting of woody plants will become a much less important component of Natural Areas management, as the goals of creating a system that reacts to disturbance without the subsequent increase in invasive species are reached. As mentioned in the Recommendations, continued augmentation through planting and re-introduction of native species is still an important and proven means of success in Natural Areas management that should not be abandoned. However, it is perhaps proportionally more important now to focus on stabilization of understory and herbaceous plant populations, both through planting and removal of invasive species. Even given the improvements that have been found, the nature of Central Park as a heavily designed and visited landscape in the heart of Manhattan means that the work to maintain it, even the Natural Areas, will never finish. Ongoing stewardship from knowledgeable staff and constant monitoring will be required to respond to stochastic shifts and challenges, as well as to adapt to evolving values of park users.
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Appendix A. Historic maps showing the Great Hill site.
A portion of the Randel Farm Map of 1819, with topography shown and the Manhattan grid laid out, but yet to have been built. The Great Hill site is shown here, below 8th Avenue and above what would be 7th Avenue, in the middle of the picture. Link: http://thegreatestgrid.mcny.org/greatest-grid/randel-map-gallery/159
The same site, but on the newer Colton Map of 1836. This shows the site, eventually to become the Great Hill, to have been one the few heavily wooded tracts remaining in the site of Central Park.
Link: https://www.davidrumsey.com/luna/servlet/detail/RUMSEY~8~1~3017~90020003:Topographic al-Map-Of-The-City-and-C
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Appendix B. Historic Park Maps, showing the park before and after construction.
Map of The Central Park, 1873 (after park’s completion). Link: https://digitalcollections.nypl.org/items/4ee14540-3569-0134-fa82-00505686a51c
Map Showing the Original Topography of the Site of the Central Park, 1859 (before construction of park). Follow link in references list for high-definition file of the map. Link: https://digitalcollections.nypl.org/items/933b3030-0f7a-0132-d21e-58d385a7b928
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