Assessing the Introduction and Age of the Acer Platanoides (Norway Maple) Invasion Within Wilket Creek Ravine in Toronto, Ontario

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Assessing the Introduction and Age of the Acer platanoides (Norway Maple) invasion within Wilket Creek ravine in Toronto, Ontario.

Madison Postma

A capstone project submitted in conformity with the requirements for the degree of Master of Forest Conservation

Daniels Faculty of Architecture, Landscape and Design University of Toronto

©Copyright by Madison Postma 2020

Abstract: After over a century of disturbance the property that encompasses the Toronto Botanical Garden and the Wilket Creek ravine in Toronto, Ontario has fallen victim to the invasive Norway maple (Acer platanoides). The objectives of this study were to improve the overall knowledge of Norway maple invasions within the Wilket Creek ravine, to determine when and where Norway maples were introduced in the study area, and to improve the overall understanding of Norway maple age dynamics within the property. The results show that Norway maple was introduced into the Wilket Creek ravine in the 1940s and 50% of the sampled Norway maple within the study were established between 1980s and 2000s (18 and 40 years old). The results also show that Norway maple regeneration is present in almost all wooded areas. To control Norway maple, it is recommended to implement an intensive management plan that includes mechanical and chemical control methods, strict invasive species policy, and the development of public education and outreach programs to halt the regeneration and growth of the invasive tree species.

Acknowledgements I would like to thank my internal supervisor, Danijela Puric-Mladenovic for her undying guidance and support throughout the entire process of this graduate capstone project. I would also like to thank my external supervisor, Katherine Baird for assistance in collecting a large portion of the data needed to complete this project, as well as providing additional VSP monitoring data. To Tony Ung, Dr. Jay Malcolm, and Paul Piascik for their constant support and expertise throughout the entire core sampling process. To Adam Tweedle and Krishna Selvakumar who assisted with core sampling. To Project Learning Tree Canada, the Daniels Faculty, and the University of Toronto Work Study Program for funding this project. And finally, to the 2020 Master of Forest Conservation class at the University of Toronto for their support throughout this entire program.

Table of Contents

Introduction & Background…………………………………………………………………….8 Objectives………………………………………………………………………………………13 Methods………………………………………………………………………………………...13 Results………………………………………………………………………………………….16 Discussion………………………………………………………………………………………20 Conclusion……………………………………………………………………………………...23 Recommendations…………………………………………………………………………….23 References……………………………………………………………………………………..26 Appendices……………………………………………………………………………………..30

List of Tables Table 1: Summary of Norway maple abundance in shrub and ground layer of study area based on data provided by TBG (Baird, 2020b)…………………………………….17 Table 2: Summary statistics showing minimum, mean, and maximum values from collected data…………………………………………………………………………………19

List of Figures Figure 1: Map of Rupert Edward's property in 1947. The red circle marks the project study area……………………………………………………………….……………………….9 Figure 2: Map of Rupert Edward's property in 1956. The red circle marks the project study area. The image shows the increase in development near Edward’s property from 1947 to 1956 …………………………………………………………………………………….9 Figure 3: Map of Study Area. Map Author: Madison Postma…………………………….14 Figure 4: Histogram showing the number of Norway maple in each DBH class based on data provided by TBG (Baird, 2020a)………………………………………..…….…….….17 Figure 5: Distribution of Norway maples within the study area, based on data provided by TBG (Baird, 2020a). Map Author: Madison Postma…………...……..……….……….18 Figure 6: Histogram showing the number of sampled Norway Maple within each age range…………………………………………………………………………………………….19 Figure 7: Linear regression analysis for the tree age and diameter at 1.05m (y= 3.026112 + 0.019360x R² =0.5569)………………….…………………………………20

List of Appendices Appendix 1: Statistical Regression Results (Summary)…………….…………………..…30 Appendix 2: Archival Aerial Photographs of Study Site (City of Toronto, n.d.).……..31-33 Appendix 3: Tools & Materials………………………………………………………………..34

Introduction & Background: The History of the Toronto Botanical Garden and Edwards Gardens The land that occupies the Toronto Botanical Garden and Wilket Creek ravine was once a part of the 202 ha (500 acres) property purchased by Scottish immigrant and prosperous wool and lumber producer Alexander Milne in 1827 (Goldenburg, 2020). In 1832 Milne moved his wool and sawmill east to the Don River as the Wilket Creek (then known as the Milne Creek) was unable to provide a steady supply of water to power his three-story mill (Toronto and Regional Conservation Authority, 2018). Although Milne left the property, the land stayed within the Milne family for over 100 years (Toronto Botanical Garden, 2020). In 1944 Rupert Edwards purchased the overgrown property and with a twelve-person crew cleared the land and completely transformed it by adding elaborate gardens, ponds, a 9-hole golf course, and one of the largest rockeries in Canada (Toronto Botanical Garden, 2020; Goldenburg, 2020). The Wilket ravine slopes were “planted with thousands of bulbs, shrubs, and trees” and Edward’s “dammed the creek and constructed an electricity-generating waterwheel to irrigate his gardens” (The Cultural Landscape Foundation, n.d.). Ten years later as the urban development started to inch toward the property (Figure 1 and Figure 2) Edwards decided to sell it. However, he wanted to ensure that it was preserved as a public park. He sold his private country garden oasis to the City of Toronto and in 1956 Edwards Gardens was opened to the public (Toronto Botanical Garden, 2020). The Garden Club of Toronto, which occupied in the original Milne farmhouse, established the Toronto Civic Garden Centre to provide horticultural support to the public. In 2006 the Toronto Civic Garden Centre was transformed into the Toronto Botanical Garden (hereby known as TBG). TBG opened a series of themed public gardens over four acres, allowing visitors to “enjoy and engage the splendor of nature while learning practical applications for their own gardens” (Toronto Botanical Garden, 2020). In early 2019 TBG, partnered with the City of Toronto, began phase 1 of the “Edwards Gardens and Toronto Botanical Garden Master Plan and Management Plan”, a project involving the expansion of the botanical garden from four to 35 acres (Toronto Botanical Garden, 2020). The growth of suburbia around the TBG began the unsolicited expansion of invasive plants and trees from surrounding urban areas into the Wilket Creek ravine, including Norway maple (Acer platanoides).

Figure 1: Map of Rupert Edward's property in 1947. The red Figure 2: Map of Rupert Edward's property in 1956. The red circle marks the project study area. circle marks the project study area. The image shows the increase in development near Edward’s property from 1947 to 1956. The Natural History of the Norway maple The Norway maple, a large deciduous tree, is native across Europe. Its natural range is from southern Scandinavia to Northern Italy and further from Eastern Europe to Asia Minor (Nowak & Rowntree, 1990). In the Balkan Peninsula, Norway maples have been known to live up to 200 years, but in its more common European ranges the lifespan varies between 100 and 150 years. Along with its use as a street tree throughout Europe, Norway maples are also harvested and used for veneer wood as well as “speciality items such as tool handles, gunstocks and violins” (Nowak & Rowntree, 1990). Due to its vigorous growth rate, size and shape, and its ability to withstand different environments and urban pressures, the Norway maple was introduced to England in 1683. Soon after, in 1756, John Bartam introduced the species to Philadelphia, USA (Nowak & Rowntree, 1990). The Norway maple was soon considered one of the rarest and “finest” maples, and thereby deemed “one of the finest ornamental trees” in North America (Nowak & Rowntree, 1990). Soon, many high society Americans, including George Washington, were asking for Norway maple seedlings. The introduced maple species was considered suitable for streets and avenues by 1833 and from there onward became the most popular urban tree species (Nowak &

Rowntree, 1990). By 1861, the demand for Norway maples had crossed the country and Norway maples began to grow in California tree nurseries (Nowak & Rowntree, 1990). Norway maple increased in popularity after World War II when the native white elm (Ulmus americana) population was killed off by Dutch elm disease (Ophiostoma ulmi) (Nowak & Rowntree, 1990). Norway maple’s ability to grow quickly and provide ample shade made the species one of the replacements for the dying elm population. Unfortunately, the invasive properties of Norway maple were not observed or of concern at the time. Norway maple can self-establish in native forests and can outcompete native trees, and therefore is considered a harmful and invasive species throughout urban areas and woodlots in North America (Webb, Pendergast & Dwyer, 2001). The Ecology & Biology of the Norway Maple The Norway maple can survive in a variety of temperatures and climates; however, it thrives where the mean annual temperature is approximately 12°C, comparative to the annual temperature of 8.6°C in Toronto, Ontario (Munger, 2003: Nowak & Rowntree, 1990; Climate Data, n.d.). While Norway maples are considered a resilient species to urban stressors, their growth rate is low when subjected to excessive heat, cold, evapotranspiration or high soil pH. Norway maple growth is optimal in environments with a lot of precipitation or in areas with fresh soils (Munger, 2003: Nowak & Rowntree, 1990). Deep, moist, and fertile soils (i.e. loamy soil) with sufficient moisture and a pH of 5.5-6.5 is optimal for Norway maple’s high growth rate. The Norway maple’s average height ranges from 18-22m with average crown spread of 15m at the age of maturity in a closed canopy (approximately 30-45 years) (Munger, 2003: Nowak & Rowntree, 1990). The Norway maple develops round clusters of small, green flowers that are approximately 1cm across, and rely on pollination insects. One can identify a Norway maple by the size of its leaves and the milky white sap produced when a leaf stalk is broken. Norway maples leaves are opposite on the branches and each leaf is palmately lobed, ranging from 8-16cm long and 10-18cm wide (Munger, 2003). Norway maple bloom between April and early June, earlier than most native maples in North America (Munger, 2003). The Norway maple is also known to produce many seeds that are widely dispersed by wind due to their “winged” shape (Munger, 2003). Its seeds can be carried over 100m from the source tree (Bertin, Manner, Larrow, Cantwell & Berstene, 2005; Munger, 2003). Norway maple fruit- paired samaras (or keys) are grown in clusters at the tip of branches and are considerably larger than North American native maple keys (ex. 65% larger than sugar maple), ranging from 3- 5cm in length (Meiners, 2005; Webb & Kaunzinger, 1993). Seedlings and young Norway maples are considered highly shade tolerant and can grow in many different soil types. Besides, rapid regeneration allows the Norway maple to outcompete other species in the understory and reach canopy openings

(Webster, Nelson & Wangen, 2004). Norway maples are also known to hold on to their leaves longer than most native species in North America. Norway maple cultivars The Norway maple has over 100 cultivars. Many of the Norway maple cultivars were bred in Germany, France, and Belgium and imported to North America (Nowak & Rowntree, 1990). Norway maple cultivars are just as resilient as the typical species and are often used for their distinctive colours and ability to create large areas of shade (Roussy, Kevan, Dale, & Thomas, 2008). Cultivars of Norway maple differ in some phenotype and/or functional characteristics which often determine their suitability for urban areas (Conklin & Sellmer, 2009a). For example, Acer platanoides “Crimson King” is by far the most popular due to its deep marron coloured foliage (Roussy, Kevan, Dale, & Thomas, 2008), while Acer platanoides “Columnare” develops narrow, columnar tree canopy. For instance, a study was conducted by J. Conklin and J. Sellmer (2009b) to see the differences in flower and seed production throughout six Norway maple cultivars. The study found that the cultivars Acer platanoides “Columnare” and Acer platanoides “Emerald Queen” produced more seeds than Norway maple cultivars “Crimson King”, “Globosum”, “Faasen’s Black”, and “Rubrum” (Conklin & Sellmer, 2009b). The study suggests that the cultivars that produce the largest number of seeds would be more problematic in non-native landscapes, whereas those with lower seed yield were considered less invasive alternatives (Conklin & Sellmer, 2009b). The Role of Norway maple in Urban Forests: Issues & Impacts Municipalities and landowners are drawn to Norway maple for many reasons; its resilience to pests and urban stresses (i.e. pollution, road salt, and compact soil), its ability to provide dense shade, its rapid growth rates, and its overall aesthetics (Lapointe & Brisson, 2015; Roussy, Kevan, Dale, & Thomas, 2008). For these reasons, Norway maples and Norway maple cultivars are still commonly sold in tree nurseries and evidently continue to expand beyond sidewalks and private gardens into native forests and woodlots (Bertin, et al. 2005; Lapointe & Brisson, 2015; Kloeppel & Abrams, 1995). However, Norway maple is not a perfect urban tree species, as it develops structural problems such as girdling roots and contorted branching. Girdling roots that emerge toward the surface of the soil and cut into the tree’s trunk. This is an issue because it restricts movement of water and nutrients and puts pressure on the wood and trunk, which eventually leads to the tree’s death (Fraedrich, n.d.; Munger, 2003). It is not uncommon to observe girdling roots on older and larger Norway maples due to compact soils (Munger, 2003) and/or due to tree nursery practices (trees kept in containers for too long). Other issues that can often be observed on larger and older Norway maples are that branches start to contort and become esthetically displeasing to many landowners (Munger, 2003; Roussy, Kevan, Dale, & Thomas, 2008). Contorted

11 branching and girdled roots often mean larger Norway maples are more susceptible to storm and ice damage, resulting in fallen trees, maintenance costs, and property damage (Roussy, Kevan, Dale & Thomas, 2008). Despite its evident invasive characteristics, it was not until the late 1970s that the Norway maple was observed as a potential invader of North American woodlands. Despite this, it was not until the 1990s that intensive research on the Norway maple and its adverse impacts on urban woodlots and forests would be conducted (Bertin, et al. 2005). The largest issue caused by Norway maples is its seed dispersal from trees planted across urban areas into urban woodlots and forests, due to their high shade tolerance and quick growth rate. Due to their high shade tolerance and quick growth rate, Norway maple seedlings and saplings quickly outcompete native tree species in woodlots. For example, compared to the native sugar maple (Acer saccharum), the Norway maple is known to hold it’s leaves for approximately 12 days longer, and utilizes light, water, and nutrients more efficiently (Kloeppel & Abrams, 1995; Bertin, et al., 2005). It has been also shown that native tree seedlings often struggle with regeneration in the presence of Norway maple due to the invasive species’ desne shade, surface roots and lower rates of predation (Cincotta, Adams, & Holzapfel, 2008; Meiners, 2005; Martin, 1999). The deep shade and intense seed dispersal of Norway maples make it difficult for native trees and flora to recover once Norway maple dominates a forest stand (Bertin, et al. 2005; Martin, 1999). A study conducted by S.L. Galbraith-Kent & S.N. Handel (2008) looked at sapling growth of Norway maple and native saplings under various canopy tree species. The study found that native tree species grew significantly less seedlings under a canopy of Norway maple than under a canopy of native trees (Galbraith-Kent & Handel, 2008). It is recommended that Norway maple be eradicated from urban woodlots and native forests before it reaches the canopy, as the invasive maple was found to be detrimental to the health and growth of native seedlings and other flora (Wyckoff & Webb, 1996; Galbraith-Kent & Handel, 2008). However, a study done by Lapointe & Brisson (2011) comparing the growth and survival of tar spot disease on Norway maples show some reassuring information. The study was conducted in Mount Royal, an urban forest in Montreal, Quebec, and compared the growth of Norway maple and sugar maple seedlings and trees before and after the tar spot disease outbreak. The study showed that before the outbreak, the Norway maple had higher growth rates than to the sugar maple (Lapointe & Brisson, 2011). However, once the disease was introduced, the roles were reserved. With the introduction of the disease, Norway maple seedling and tree growth saw a sharp decline, as well as an increase in the mortality rate (Lapointe & Brisson, 2011). The study concluded that the tar spot disease might be used to reduce the invasive Norway maple in natural areas without negatively affecting native maples or other tree species (Lapointe & Brisson, 2011).

Understanding Species Invasions When managing forests that have been altered by invasive tree species, it is important to know when, where, and how the invasive was introduced. Fortunately, with enough data predictions of invasive species age and introduction within natural areas can be made. Understanding the timelines and history of introduction is also useful in knowing when native trees and flora began to be negatively impacted by the invasive species’ presence (Martin, 1996; Webster, Nelson, & Wangen, 2004). Knowing the age of an invasion allows for a more in depth understanding of the species and how it may have been introduced into the natural area; creating effective management strategies (Martin, 1996; Webster, Nelson, & Wangen, 2004). Webster, Nelson and Wangen (2004) collected tree cores from the largest trees on Mackinac Island. Tree ring analysis from the cores was used to understand and recreate the original recruitment canopy over 80 years prior to the study (Webster, Nelson, & Wangen, 2004). This analysis, along with a gap capture method and radial growth patterns were used to determine how long after the first Norway maple was introduced, the invasive species began to take over the forest (Webster, Nelson & Wangen, 2004).

Objectives The overarching objective for this project is to better understand the history of Norway maple invasion within the Wilket Creek Ravine, Toronto, Ontario as well as its distribution and abundance within the study area. My specific objectives for this project are as follows: 1. To improve the overall knowledge of Norway maple invasions within the Wilket Creek ravine in Toronto using abundance data from the ground, shrub, sub- canopy, and canopy layers of the forest.

2. To determine the age of the invasion, and investigate when, where, and how Norway maples were introduced in Wilket Creek ravine.

3. To improve our understanding of the post-invasion dynamics of Norway maples within Wilket Creek ravine.

Methods Study Area The study area, which encompasses the TBG, Edwards Gardens, and the Wilket Creek ravine portion in Toronto, Ontario (see Figure 1) is 15.5 hectares. The forested portion covers 4.1ha (or 25.9%) and the Wilket Creek ravine floodplain covers 2.2ha (or

14.1%) (Toronto Botanical Garden, 2018). The area generally has clay loam soil type, with some sandier soil near the creek (Toronto Botanical Garden, 2018). The natural areas within the study area range from hardwood to mixed wood stands dominated by native species such as sugar maple (Acer saccharum), white cedar (Thuja occidentalis), black cherry (Prunus serotina), American beech (Fagus grandifolia), eastern hemlock (Tsuga canadensis), and red oak (Quercus rubra) (Toronto Botanical Garden, 2018). Wilket Creek ravine is a tributary of the Don River watershed, and a portion of the study area encompassing Wilket Creek Forest is designated an Environmentally Significant Area (North-South Environmental Inc, 2002). Beyond the site’s natural area are “ornamental gardens, cultural plantations, manicured lawns, and urban cover” (Toronto Botanical Garden, 2018). There are also numerous paved and woodchip paths throughout the property.

Figure 3: Map of Study Area. Map Author: Madison Postma Data Collection Norway maples within the TBG property and Wilket Creek ravine were mapped in the summer of 2020 by Katherine Baird, ecologist at TBG (Baird, 2020a). The TBG dataset which included locations of all Norway maples above 5cm in diameter at breast height (DBH), was used to select 44 trees for this study and for which to collect more detailed data (Baird, 2020a). Sample size of 44 trees was chosen based on the size of

14 the study site and methodology of the Webster, Nelson, & Wangen (2005) study. Ten trees from each DBH class (10-19.9 cm, 20.29.9 cm, 30-39.9 cm, 40-49.9 cm, ≥50 cm) were randomly sampled, with exception of 40-49.9 cm and ≥50 cm as there were less than 10 trees (8 trees between 40-49.9cm and 3 trees ≥50 cm) within those diameter classes in the study area (Baird, 2020a; Webster, Nelson, & Wangen, 2005). Three trees with a diameter at breast height (DBH) of 5-9.9cm were also sampled to retrieve a representation of younger sub-canopy trees. Four measurements were taken at each of the 44 sampled trees: DBH, diameter at tree core sample height (1.05m), total tree height, canopy base height, canopy closure (%), and percent canopy dieback. DBH was measured at 1.30m from the base of each tree. Both tree height and canopy base heights were measured using a rangefinder and clinometer (Webster, Nelson, & Wangen, 2005). Canopy closure was assessed by looking up into the canopy 1m from the base of the tree and determining the percentage of canopy present (Dahir & Lorimer, 1996). Collection of canopy dieback data was based the Neighbourwoods© tree inventory protocol (Kenney & Puric- Mladenovic, 1995). Dieback was determined based on a scale of 0 to 3, 0 indicating no sign of dieback and 3 indicating severe dieback over 75% of tree canopy (Kenney & Puric-Mladenovic, 1995; Lapointe & Brisson, 2011). All measurements, including any additional observations, were recorded in an ArcGIS Collector application that was downloaded onto field tablets. Two cores were taken from each tree (one on each side of the trunk) 20cm or greater in diameter to ensure all rings were captured. Trees with a DBH under 20cm had one core sampled as the increment borer was able to go through the tree and therefore retrieve both sides of the pith (Grissino-Mayer, 2003). Cores were collected using an increment borer 1.05m from the base of the tree and the diameter was measured. Although a study conducted by Webster, Nelson, & Wangen (2005) collected cores 30cm from the base of the tree, it was decided for this study to collect at 1.05m to ergonomically collect a whole core sample (Webster, Nelson, & Wangen, 2005; Grissino-Mayer, 2003). Each extracted core was placed into a labelled plastic straw. The labelling included the tree i.d., core identifier (either A or B) and date (Webster, Nelson, & Wangen, 2005; Grissino-Mayer, 2003). A total of 75 cores for 44 trees were taken. Data Analysis All tree cores were mounted onto precut wooden blocks using wood glue and labeled again. After the core samples dried for 38 hours, they were sanded ⅓ down using a palm sander (Webb & Kaunzinger, 1993). For each core total core length was measured (in cm) as well as the length from the tree center to the end of the core. Once scanned onto the computer CooReader software was used to measure and digitize the annual rings of each core (Webster, Nelson, & Wangen, 2005; Webb & Kaunzinger, 1993; Yamaguchi, 1990).

The age of each tree was determined via the number of rings that were present on each core. For trees with two core samples, the average age between samples A and B was used. For the few Norway maple cores that were unable to reach the center of the tree the age was estimated using an equation by Norton, Palmer, & Ogden (1987): age = (r-p)/(d/n) + N

“Age was estimated by dividing the length of the missing geometric radius by an estimate of the mean ring width of the innermost 20 rings and adding the number of rings counted to the core. R is the geometric radius, p the partial core length, d the length of the last n (20) rings, and N the total number of rings present in the partial core. All measurements were made in mm” (Norton, Palmer, Ogden, 1987). Using the statistical software R, Shapiro-Wilks tests were used to test normality of tree age and DBH at 1.05m data. Once the tree age data was transformed, a linear regression statistical test was performed to show the relationship between the two data sets. The GIS software ArcGIS was used to map the Norway maple diameter and age distribution within the study site. Maps created in ArcGIS and graphs created in Microsoft Excel were also used to visualize Norway maple abundance within the shrub, sub-canopy, and canopy layer of 38 of VSP plots (Baird, 2020b). The abundance data was collected July to October 2020 from 38 VSP plots (37 400m² plots and one 100m² located in a narrow forest patch) and provided by Katherine Baird, ecologist at the Toronto Botanical Gardens (Baird, 2020b).

Results Abundance Data The Norway maple data from 38 VSP fixed area plots (Baird, 2020b) (37 plots 400m² and one 100m²) was collected and summarized to determine abundance in the shrub and ground layers of the study area (Table 1). Within the shrub layer (0.5-3m in height), Norway maple had an average absolute cover of 0.4% with a summed cover across all plots of 15%. Within the ground layer (<0.5m in height), Norway maple had an average cover of 0.12% with a summed cover across all plots of 4.7%.

Table 1: Summary of Norway maple abundance in shrub and ground layer of study area based on data provided by TBG (Baird, 2020b). Shrub Layer Ground Cover (0.5-2m) (<0.5m) Average absolute 0.41 0.12 cover per plot (%) Range of absolute Max: 4.50 Max: 1.50 cover per plot (%) Min: 0.00 Min: 0.00 Absolute cover 15.45 4.73 summed across all plots (%) Frequency across 23/38 (60%) 31/38 (81%) plots

Mature Norway Maple within the Study Site A total of 172 Norway maple over 5cm DBH were located within the study site (Baird, 2020a). Within the study site 48% of Norway maple have a DBH between 5 and 15cm (Figure 4). The second most common diameter size class was 15.1-25cm at roughly 27% (Figure 4). The rest of the trees decreased in numbers has diameter size increased, having only one tree with a diameter of over 55cm (Figure 4).

Figure 4: Histogram showing the number of Norway maple in each DBH class based on data provided by TBG (Baird, 2020a).

All 172 mature Norway maple within the study site were mapped along with their DBH to show species and size distribution (Figure 5). Norway maple density per hectare is 34.4 trees with a basal area of 1.31m² per hectare.

Figure 5: Distribution of Norway maples within the study area, based on data provided by TBG (Baird, 2020a). Map Author: Madison Postma Summary statistics were run on the collected data from each of the sampled Norway maple to determine the minimum, maximum, and average for each of the field measurements (Table 2). The average tree height for the sampled Norway maple is 16.98m with a minimum of 7.3m and a maximum of 23.6m. The average height to base of canopy for the sampled trees was 4.9m with a minimum of 1.8m and a maximum of 9.2m. The average canopy width for the sampled trees was 9.2m with a minimum of 4.9m and a maximum of 18.8m. The average canopy closure percentage for the sampled trees was 79.1% with the minimum percentage of 45% and the maximum percentage of 95%. The average diameter at breast height for the sampled trees was 28.9cm with a minimum of 6.9cm and a maximum of 57cm. The average diameter at 1.05m for the sampled trees was 29.7cm with a minimum of 7.4cm and maximum of 54.3cm. The average tree age for the sampled Norway maple was 38.7 years with a minimum of 18 years and maximum of 77 years.

Table 2: Summary statistics showing minimum, mean, and maximum values from collected data. Minimum Mean Maximum Tree Height (m) 7.3 16.98 23.6 (SE±0.55) Height to Base of 1.8 4.9 9.2 Canopy (m) (SE±0.26) Canopy Width (m) 4.9 9.2 18.8 (SE±0.54) Canopy Closure (%) 45 79.1 95 (SE±1.97) Diameter at breast 6.9 28.9 57 height (cm) (SE±1.99) Diameter at 1.05m 7.4 29.7 54.3 (cm) (SE±1.99) Tree Age (years) 18 38.7 77 (SE±1.99)

Amongst the 44 sampled trees, 14 Norway maples were between the ages of 39 and 48 years old, making it the most common sampled age, and 12 trees were between the ages of 29 and 38 years old, the second most common sampled age (Figure 6). Ten trees were between the ages of 18 and 28 years old and five trees were between the age of 49 to 58 years old (Figure 6). The least common age ranges amongst the sampled Norway maples were 59 to 68 years old with only one tree and 69 to 78 years old with only two trees (Figure 6).

Figure 6: Histogram showing the number of sampled Norway Maple within each age range.

A regression analysis of the tree age and diameter at 1.05m for the sampled Norway maple shows that there is a positive, and statistically significant, correlation between the two indicators, with an R value of 0.5569 (p <0.00001) (Figure 7). Statistical regression results are included in Appendix 1.

Figure 7: Linear regression analysis for the tree age and diameter at 1.05m (y= 3.026112 + 0.019360x R² =0.5569).

Discussion Rationale The methodology for this project was based on two studies conducted on the stand dynamics of Norway maple. Each of the studies used increment cores from live trees to determine age and invasion trends of Norway maple in comparison to the native species within their research sites. A study conducted by Webster, Nelson & Wangen (2005) used increment cores, tree height, crown shoulder height, DBH, and crown class

20 to recreate canopy recruitment of Norway maple in the dominant crown class and “investigate gap capture rates” for Norway maple and native species on a forested island in Lake Huron (Webster, Nelson, & Wangen, 2005). The second study, which was conducted by Webb & Kaunzinger (1993) used increment cores of native oak, beech, maple trees and Norway maple to assess biological invasions of invasive species within Drew University Forest Preserve in New Jersey, USA. In addition to the increment cores, Webb & Kaunzinger counted and identified surrounding woody saplings to examine the target tree species’ reproductive patterns. As the research questions of the two Norway maple stand dynamic studies are similar to those raised in this study it is appropriate that the data collection and analysis methods used in this project would, to a degree, mimic those executed within those projects. Those studies had a considerable influence on determining which types of measurements should be conducted on each sample tree and how many trees to sample per diameter class. However, unlike the studies conducted by Webster, Nelson, & Wangen and Webb & Kaunzinger, intensive native tree data was not collected as the focus of this study is on Norway maple age and overall distribution throughout the Toronto Botanical Gardens and the surrounding property. Overall, with the guidance of the three Norway maple stand dynamic studies, our project successfully met its goals. Abundance The abundance data show that Norway maple seedlings are present in almost all areas of the forest. Although the ground cover data showed that the seedlings only cover an average 0.12% of a 400m² plot, it is significant to note that they were present in 31 of the 38 plots (82%). The results also show that Norway maple were present in the shrub layer of 23 of the 38 plots (60%), with an average cover of 0.4%. These results are important in determining regeneration within the study area because they show just how far Norway maple can spread away from mature trees. These results support the findings of a similar study conducted by Martin (1999) in which the study compared the understory growth and regeneration patterns of Norway maple and sugar maple. Although this project did not investigate and compare other tree species regeneration data, it is likely that similar regeneration patterns and understory consequences are occurring or will occur with the continued regeneration and expansion of Norway maple within the study area (Martin, 1999; Wyckoff & Webb,1996). Tree Age & Establishment of Invasion The results show that Norway maple density per hectare is 34.4 trees with a basal area of 1.31m² per hectare. The distribution of Norway maple DBH across the study site (Figure 5) shows several high-density pockets, especially in the south-west corner of the property where the largest Norway maples are located. However, we also see Norway maple with a diameter of 5cm to 10cm seem to be distributed all over the area, with dense pockets on the eastern and southern portion of the property. This

21 observation fits well with our shrub layer and ground cover data as we can see that Norway maple seedlings and saplings are not necessarily always located near the mature trees growing within the subcanopy and canopy of the forest.

The results from our core samples showed that the oldest Norway maple within our study site was about 77 years old, meaning that the start of the invasion began in the 1940’s. This correlates well with the history of sub-urban development and Norway maple introduction and use within North America. As soldiers came home from the Great War many wanted to settle down outside of major cities, creating an increased expansion of suburban housing development outside of Toronto (Smith, 2012). Norway maple was favoured as an ornamental tree and was commonly planted in the yards of private landowners in the 1940’s and 1950’s (Nowak & Rowntree, 1990). Although there are no original garden plans, it can be speculated that Rupert Edwards also would have planted Norway maple within his gardens and especially along his golf course because of the large, shaded area that their canopies provide (Toronto Botanical Garden, 2020). While there is no definite pattern of Norway maple distribution on the property, based on historical aerial images (City of Toronto,n.d.) it is assumed that intentional planting by Rupert Edwards and Norway maple seeds from encroaching subdivisions began the invasion into the study area (Appendix 2-aerial photographs) (City of Toronto, n.d.). The results of this study determined that most of our sampled trees were approximately 40 years old (Figure 6) and thus established in the 1980s. The linear regression showed a statistical significance between height and diameter at 1.05m which allows us to roughly infer the other Norway maple ages in the study area (Figure 7). As the majority of Norway maple within the study site have a diameter of 5cm-25cm it can be inferred that their age range is approximately 18 to 30 years old, showing that the Norway maple self-establishment was intensive in the 1980s and 1990s (Figure 4). Although diameter is not necessarily the best predictor of age (Gibbs, 1963), based on the results of this study and the other studies which use similar methods, it is believed that we can still predict the age of the Norway maple invasion within the study area (Webb & Kaunzinger, 1993; Webster, Nelson & Wangen, 2005; Martin, 1999). There are multiple explanations for the boom of Norway maple within the study site in the 1980s and 1990s. By the 1980s the Norway maples planted to replace elm trees effected by the Dutch elm disease were at full maturity. Therefore, there was an abundance of Norway maple seeds escaping from backyard gardens and streets into natural areas (Nowak & Rowntree, 1990). The increase in suburbia surrounding the study area and public access to Wilket Creek would have also played a large role in the increased Norway maple invasion. This is also in addition to the original “invaders” (those planted in the 1940s; 77 years old) reaching full maturity and regenerating within the surrounding natural areas (Nowak & Rowntree, 1990; Webb & Kaunzinger, 1993). The findings of this project support the conclusions of similar studies such as those conducted by Martin (1999), Wyckoff & Webb (1996), and Webb & Kaunzinger (1993). In a study conducted by Webb & Kaunzinger (1993) on the invasion of Norway

22 maple within the Drew University Forest Preserve in New Jersey they conclude that two factors that influence the likelihood that an introduced species will become invasive: characteristics of a site and the life history of the species (Webb & Kaunzinger, 1993). Similar to the site in this study, the Drew University site is a small (18 ha) natural area with a disturbance history (Webb & Kaunzinger, 1993). The study also states that Norway maple, can invade natural areas within proximity to urban landscapes (Webb & Kaunzinger, 1993; Nowak & Rowntree, 1990; Lapointe & Brisson, 2011). Like the Drew University study, the Norway maple invasion within the Wilket Creek results from not recognizing and not managing invasive species within natural areas within the first few decades of their introduction (Webb & Kaunzinger, 1993). This study shows that over a period of 70 years, Norway maple has spread throughout the entire study area and has an ideal reverse J curve indicating successful regeneration. If not managed effectively, invasive Norway maple will continue to proliferate and expand further into Wilket Creek ravine (Dong, 2015). This poses harm to the ravine’s ecological integrity, which is also identified as an Ecologically Significant Area (North-South Environmental Inc, 2002) and creates an even more costly predicament for future generations (Webb & Kaunzinger, 1993).

Conclusion The study’s objectives were to improve our understanding of the Norway maple invasion within the Wilket Creek ravine and determine when, where, and how this invasive species was introduced. From the collected data and results of the analysis this study has determined that Norway maple was introduced into the study area in the 1940s due to the increase in housing development around the ravine and the extensive gardens and recreational areas developed on the property by Rupert Edwards (City of Toronto, n.d.; Toronto Botanical Garden, 2020). Analysis of the 172 mapped mature Norway maple within the study area show that the invasive species has a density per hectare of 34.4 trees with a basal area of 1.31m² per hectare. Norway maple regeneration is indeed abundant in almost all areas of the property and if not managed. it will continue to harm the ravine’s ecological integrity.

Recommendations Selecting the appropriate treatment is often difficult when managing forests that have an abundance of invasive woody species, such as the Norway maple. With the knowledge obtained from this study we have developed three recommendations to address the Norway maple invasion and associated impacts within Wilket Creek study area and prevent its introduction in other natural areas. The first recommendation would be to use mechanical and chemical methods to remove and control the Norway maple within the site. Mechanical control of Norway

23 maples would involve removing saplings and seedlings from the forest (including all root systems) and cutting mature Norway maple trees close to the ground, or girdling (removing bark and phloem layer from 10cm around the trunk) (Webb, Pendergast, & Dwyer, 2001; CABI, 2020). Wood from the removed trees would be distributed and left as snags or downed woody debris throughout the forest where appropriate (Webb, Pendergast, & Dwyer, 2001; CABI, 2020). The use of chemical control is also recommended for Norway maple management and removal. Chemical control would involve applying herbicide to stumps or girdled trees, or to the base of saplings that are <10cm DBH (CABI, 2020). This method would require prior monitoring to determine which individual trees to apply the herbicide too, as well as post application monitoring as multiple treatments are often needed for success (CABI, 2020). With chemical control, besides using the appropriate chemicals, it is also imperative to take into account proximity to water and surrounding flora and fauna before application (CABI, 2020). The mechanical and chemical removal of Norway maple within site would be time-consuming and expensive, costing roughly $1000 per mature tree to remove and approximately $4000 per acre to use herbicides (Invasive Plant Control Inc, 2016; Al Miley & Associates, 2020). However, this is the consequence when invasive species are ignored and not properly managed at the start of their establishment. The only way to manage the invasion is to slowly remove all the Norway maple trees and deplete the seedbank with on-going management. The second recommendation would be to create strong legislation concerning the growth, distribution, and planting of known invasive species within TBG and the City of Toronto. The development of an invasive species policy at TBG ensuring that invasive plants are not planted within the gardens and that any invasive trees are to be replaced with native species would assist in keeping the ecological integrity of the ravine. Creating concrete bylaws ensuring only native and/or non-invasive trees can be planted within new and established suburbs would play a large role in managing and eliminating invasive trees within natural areas (Roussy, Kevan, Dale, & Thomas, 2008). Prohibiting the sale of Norway maple and Norway maple cultivars in tree nurseries and promoting native tree species throughout cities and private lands would also help prevent the spread of Norway maples in surrounding ecosystems (Roussy, Kevan, Dale, & Thomas, 2008). The third and final recommendation is public education and outreach at TBG. The public needs to be aware that Norway maples planted in their own backyards can negatively impact urban woodlots and forests (Roussy, Kevan, Dale, & Thomas, 2008). Communities should be educated on the ecological and economic issues caused by Norway maples on both public and private properties (Roussy, Kevan, Dale, & Thomas, 2008). Fortunately, the study site is home to the Toronto Botanical Garden (TBG). The TBG is where many Torontonians come to expand their ecological knowledge (Toronto Botanical Garden, 2020). TBG’s mission is to “connect people to plants, inspiring us to

24 live in harmony with nature” (Toronto Botanical Garden, 2020). Since invasive exotic species are harmful to the natural environment, invasive species education should be an important component of this mission. It is believed that public education through the botanical garden would greatly impact on the public view of the Norway maple and its cultivars. Partnering with the City of Toronto and implementing educational signage along the Edwards Gardens and TBG trails would be a great way to highlight why Norway maples are so problematic (Roussy, Kevan, Dale, & Thomas, 2008). Norway maple focused outreach events hosted by the TBG would also be extremely beneficial. Some of these events could include native vs. non-native education workshops and seminars geared explicitly towards backyard trees and how they influence nearby natural areas, or even hands-on events that have volunteers remove Norway maple seedlings and saplings throughout the study site.

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Appendix 1: Statistical Regression Results (Summary)

y= 3.026112 + 0.019360x R² =0.5569

Appendix 2: Archival Aerial Photographs of Study Site (City of Toronto, n.d.)

Map of Study Site in 1938

(http://jpeg2000.eloquent-systems.com/toronto.html?image=ser97/s0097_fl0009_id0008.jp2)

Map of neighbourhood just west of study site in 1938

(http://jpeg2000.eloquent-systems.com/toronto.html?image=ser97/s0097_fl0009_id0007.jp2)

Map of Study Site in 1961 Map of Study Site in 1971

http://jpeg2000.eloquent- http://jpeg2000.eloquent-

systems.com/toronto.html?image=ser12/s00 systems.com/toronto.html?image=ser12/s001 2_fl1971_it0117.jp2 12_fl1961_it0145.jp2

Map of Study Site in 1981 http://jpeg2000.eloquent- systems.com/toronto.html?image=ser12/s 0012_fl1981_it0031.jp2

Appendix 3: Tools & Materials The equipment and resources used to conduct this study were provided by the University of Toronto Daniels Faculty of Architecture, Landscape, and Design. Data Collection Equipment: Data Analysis Equipment: DBH tape (x2) Sandpaper (80, 250, and 400 grit) Rangefinder (x2) Palm sander Clinometer (x2) Wooden mount blocks Increment borer (x2) Wood glue Plastic straws (x80) CooRecorder Software Sharpie marker (x2) R statistical software Collection bag Microsoft Excel Masking tape (x2) Geographic Information System (GIS) mapping Tablet (x2) software