Urban Forestry: an Underutilized Tool in Water Management

ARTICLE IN PRESS

Urban Forestry: An Underutilized Tool in Water Management

John T. Van Stan, II*,1, S. Jeffrey Underwoodx and Jan Friesen{ *Department of Geology and Geography, Georgia Southern University, Statesboro, GA, United States x Ofﬁce of the Vice President for Research, California State University, Los Angeles, CA, United States { Department of Catchment Hydrology, Helmholtz Centre for Environmental Research e UFZ, Leipzig, Germany 1Corresponding author: E-mail: [email protected]

Contents 1. Introduction 2 2. Relevance of Urban Forests to Water Management 4 3. Precipitation Partitioning and Its Manageable Inﬂuences 4 3.1 Common Urban Forest Settings 6 3.2 Canopy Trimming Practices 9 3.3 Species Selection 10 4. Extreme WeatherdA Case Study 13 5. Net Precipitation Chemistry and Water Quality 17 6. Concluding Remarks 20 Acknowledgments 21 References 21

Abstract Urban forestry widely affects urban environments, impacting a city’s microclimate, recreational value, and water resources. With regard to water resources, urban trees and forests can, for example, dampen the effects of extreme precipitation or help evaporate precipitation to reduce stormwater runoff. Yet, urban planners rarely consider urban forestry as a tool in integrated water resource management (IWRM). This chapter focuses on how urban forest setting, canopy manipulation, and tree species selection can signiﬁcantly alter urban hydrologic processes, using as an example the ﬁrst interaction between urban forests and the terrestrial hydrologic cycle: canopy precipitation partitioning. We detail the economic relevance of urban canopy precipitation partitioning to IWRM and review research quantifying its connection to manageable urban forest traits. Since many urban forests around the globe face increased extreme storm frequency, a case study of precipitation partitioning among different forest settings during an extreme storm is presented.

Advances in Chemical Pollution, Environmental Management and Protection, Volume 3 ISSN 2468-9289 © 2019 Elsevier Inc. https://doi.org/10.1016/bs.apmp.2018.04.003 All rights reserved. 1 j ARTICLE IN PRESS

2 John T. Van Stan, II et al.

Major factors that inﬂuence the urban forest’s role in stormwater quality are also discussed. Conclusions and future directions on how urban water managers may in- ﬂuence urban canopy precipitation partitioning to assist in achieving management goals are provided.

Keywords: Interception; IWRM; Precipitation; Stemﬂow; Throughfall; Urban forestry

1. INTRODUCTION

Forest cover in urban watersheds around the globe often exceeds 30%, e and can be as high as 62%, of the land area.1 3 Greening initiatives are expanding urban forest cover in most developed regions of the world,3 requiring consideration of forest management during urban development planning.4 Integrating forest management and urban development planning is, however, a complex task. This is because the urban landscapes’ spatial and temporal vegetation patterns are not simply constrained by physical variables (climate, soil types, etc.) but are also intimately tied to political,5 socioeco- nomic,6 public safety,7,8 and cultural (plant selection for privacy screening, aesthetics, etc.) variables.9 The compromise between these variables can result in vegetation patterns that signiﬁcantly alter urban hydrological processes (Fig. 1). This is because management actions select (sometimes inadvertently) for speciﬁc physical and physiological canopy traits, many of which control urban forest water relations.11,12

Figure 1 Urban forests interact with major elements of integrated water resource management (IWRM). Maintenance of urban forests (and other urban vegetation) may require a percentage of the potable water demand. The canopy also alters hydrological processes influencing stormwater management and, thus, the quality and quantity of runoff and rainwater for reuse. For combined urban sewere stormwater systems, urban forests may influence wastewater management via regula- tion of stormwater overflows. However, factors driving urban forestry are numerous and complex.10 ARTICLE IN PRESS

Urban Forestry 3

The need for managers to balance societal water practices (especially stormwater management)13 with the forest hydrological processes that support the multitude of ecosystem services provided by urban forests14 is a relatively recent circumstance (and challenge) in the history of urban watershed management. Water availability, our proficiency in managing it, and the associated costs have long regulated the establishment and expansion of urban landscapes.15 In addition, forests have been linked to critical hydrological processesdsuch as stormwater runoff reduction16 and water filtration17dyet a global assessment of cities revealed that most do not report the extent to which water quality depends on their forest cover, structure, or health.15 This lack of documentation limits monitoring and assessment of the hydrological consequences of urban forest alterations or development. Perhaps the lack of extensive geographical documentation, regarding urban forest interactions with water resources, is in part a product of urban forest management practices that have not been thoroughly integrated into water resource management. Thus, the aim of this chapter is to synthesize results on how urban forest setting, canopy manipulation, and species selection may be used by urban water managers to influence the first interaction between forests and meteoric water (canopy precipitation partitioning). The structure of the chapter begins with Section 2 detailing the economic relevance of urban forestry to IWRM (Fig. 1). Then, Section 3 intro- duces precipitation partitioning and its connection to the urban canopy’s manageable structural traits, including forest setting (Section 3.1), trimming practices (Section 3.2), and species selection (Section 3.3). As many urban forests face increased extreme storm frequency due to hy- e drological intensification,18 22 Section 4 details a case study of precipitation partitioning among different forest settings during an extreme storm. Since precipitation partitioning affects the elemental composition of water entering the urban environment, Section 5 presents major factors that influence canopy capture and exchange of atmospheric deposition. Finally, Section 6 concludes with discussion on how urban water managers can influence forests to assist to achieve management goals. The focus on the waterevegetation interactions that initiate the rain-to- runoff flow path is a critical and logical step toward the integration of urban forestry into IWRM, as it influences the precipitation available to subsequent surficial processes,23 its spatiotemporal patterning,24 and its initial nutrient concentrations.25 ARTICLE IN PRESS

4 John T. Van Stan, II et al.

2. RELEVANCE OF URBAN FORESTS TO WATER MANAGEMENT

Comprehensive economic valuation of the benefits that urban forests provide to communities shows that they can exceed installation and maintenance costs. For example, a survey done in five American cities estimated annual benefits ranging from $1.37 to $3.09 for every invested dollar in urban forest management.13 Recently, a small portion of California’s urban forest (street trees in the public right-of-way for 50 cities) was estimated to have a total asset value of $2.49 billion ($75.1 million, standard error), returning an average benefit of $5.82 per dollar invested.26 This asset value is very high and was based on a wide range of benefits (energy, air pollution mitigation, etc.), including ecosystem services from urban forest impacts on the hydrological processes controlling stormwater runoff.26 Despite the street trees of California’s urban forest being approxi- mately one-third of stocking capacity, their canopies reduced precipitation reaching the soil surface by over 26 million m3/year, returning a monetary value of $41.5 million annually.26 Although that monetary value is very high, it can be considered conservative since street trees in urban forests generally account for only one-fourth of a city’s canopy cover.27 This value, when standardized by the number of trees in statewide urban forest studies, found stormwater control services to be highly valued, ranging from $4.55 to $29.91 tree.26,28,29 Likewise, the inclusion of stormwater quality variables in economic analyses adds to these values: i.e., $39.81 for every 1000 metric tons of reduced suspended sediment, corrected for inflation.30 Conse- quently, urban forest management is of substantial quantitative and economic relevance to IWRM. It is also noted that the stormwater-related economic benefits per tree range substantially by over six times between the maximum and minimum estimated standardized per tree value,26,28,29 indicating managers may manipulate attributes of the urban forest structure to achieve maximal stormwater management benefits.

3. PRECIPITATION PARTITIONING AND ITS MANAGEABLE INFLUENCES

Precipitation over urban watersheds is physically and physiologically “partitioned” by forests (Fig. 2). When precipitation falls over an urban forest canopy, it is physically partitioned into interception (evaporated ARTICLE IN PRESS

Urban Forestry 5

Figure 2 Urban forests partition gross precipitation (Pg) physically and physiologically. Physical partitioning of Pg includes the evaporation (E) of water stored (S) on canopy surfaces31 and from other aerodynamic/splash-related phenomena32 and the resulting “net” precipitation (Pnet) that reaches the surface as throughfall (Pt) and stemflow (Ps). Some Pnet is physiologically partitioned when it is taken up by roots and transpired (T). water and water stored on canopy surfaces), throughfall (water falling from canopy surfaces and through canopy gaps), and stemflow (water that drains to the surface down tree stems). Precipitation that reaches the surface, throughfall plus stemflow, is called “net” precipitation. As the net precipitation enters the soil, some is physiologically partitioned and moves back to the atmosphere via transpiration. On the one hand, some literature exists e on transpiration impacts to urban hydrology33 36 and transpiration demands are integrated into the hobbyist’s,37 professional,38 and govern- mental guidebooks for landscape design.39 On the other hand, few studies have addressed the physical partitioning of precipitation in urban forest canopies, and the authors are unaware of synthesis work focusing on the subject. Information on interception, throughfall, and stemflow across urban forest ecosystems is needed since canopy and meteorological attributes (controlling the partitioning) can drastically affect the magnitude, patterning, and chemistry of net precipitation. In general, precipitation interception returns 10%e50% of annual-scale precipitation back to the atmosphere40 although it can vary drastically across individual storms, maximizing at 100% of precipitation for small storms.41 For green cities in arid and semiarid climates (where small storms are frequent), interception can return substantial amounts of the limited precipitation supply to the atmosphere.42 Throughfall often represents the greatest proportion ARTICLE IN PRESS

6 John T. Van Stan, II et al. of annual precipitation, 60%e90%,43 and its spatiotemporal variability is exceptionally high44 with consequences for the spatial patterning of urban surface water processes.45,46 The smallest proportion of precipitation is typically stemflow, which rarely exceeds 2%.47 However, even modest- seeming stemflow percentages (3%e10% of precipitation) should not be ignored by watershed managers as the outlying canopy acts like a funnel, creating concentrated infiltration points under such conditions.48,49 In natural forests, net precipitation can be further altered through interaction with litter layers,50 yet urban forests are rarely permitted to accumulate litter. Thus, litter interception is not included in the chapter. Precipitation partitioning by forests is a timely and important topic for hydrological forecasting as it is the literal front line between climate change and surface hydrobiogeochemical processes. The canopyeprecipitation interactions controlling net precipitation are linked to storm conditions43 that are being altered by the anthropogenic acceleration of the global water cycle,51 a process that has been intensifying global precipitation52 over the past decade.53 Developing theoretical foundations to support comprehensive integration of urban forestry into IWRM is therefore, urgent, particularly in regions impacted by shifts in precipitation regime. For example, the intensely urbanized (and urban forested) megalopolis of the Northeastern United States has experienced a 70% increase in heavy precipitation events over the past 55 years,21 anditisprojectedto increase.18 Likewise, urban forested regions throughout the world are projected to experience significant changes in precipitation amount and extremes.19,20,22 In these cases, management of precipitation partitioning by urban forest canopies may be an appropriate and practical complement to other climate resiliency and adaptation strategies. Many manageable urban forest attributes have been found to influence the interchange between interception, throughfall, and stemflow, including (1) setting of the urban forests planted, (2) canopy structural manipulation of existing urban forests, and (3) consideration of traits during tree selection. Urban watershed managers may be able to tailor forests to assist in achieving local water goals by considering at least these major urban forest structural factors, along with other urban forestry guidelines.

3.1 Common Urban Forest Settings Because of standard urban planning, engineering, and forestry practices, urban forest settings typically assume one of the several common forms (Fig. 3). Forest fragments have the greatest canopy closure and the lowest ARTICLE IN PRESS

Urban Forestry 7

(A) (B) (C)

(D) (E) (F)

Figure 3 Typical urban forest settings. Urban forests are generally of the following structural types, in order of the degree of canopy closure: (A) forest fragment, (B) park forest, (C) residential forest, (D) open row forest, (E) sheltered row forest, and (F) isolated trees. Each of them may have different impacts on physical precipitation partitioning. internal disturbance, at least locally (Fig. 3A). In park forests, walking paths and lawn spaces form a mosaic of isolated canopies and small copses, or tree clusters (Fig. 3B). Residential, often periurban, forests maintain a distributed pattern of isolated canopies and copses, but the residential canopy space is shared with urban structures: roofs, poles, wiring, etc (Fig. 3C). Row forests represent the most common setting as they line transportation corridors, from the “unsheltered” open highway (Fig. 3D) to inner-city street canyons that can provide many stories of shelter (Fig. 3E). As such, it is no surprise that street trees in row forest settings are the most studied.26,54 Isolated tree canopies are also abundant in the urban forest setting, particularly in locations requiring substantial impervious cover, like parking lots (Fig. 3F). Even though forest fragments (Fig. 3A) are similar to natural closed- canopy forests internally, they can differ signiﬁcantly in perimeter-to-area ratio,55 a metric that has ecological consequences.56,57 For precipitation partitioning, however, no signiﬁcant “edge” effect on throughfall volume and e interception amount has been found yet.58 60 Investigations on whether, ARTICLE IN PRESS

8 John T. Van Stan, II et al. and to what degree, the “edge” effect impacts stemflow volume have not been performed,61 but, as observed for throughfall and interception, stemflow volumes are not expected to significantly respond to forest edge effects. Note that edge effects can significantly alter throughfall and stemflow quality (see Section 4). Forest fragments generally have the highest overall water storage capacity of urban forest settings, which is directly related to canopy closure.62 Reduced canopy ventilation of forest fragments may constrain evaporation rates,63 but greater evaporative surface area will likely increase total interception compared with other urban forest settings. Canopy structure of closed-canopy forests (like the interior of a forest fragment) typically does not favor stemflow (see Section 3.2); thus, throughfall is expected to dominate forest fragment net precipitation. For park and residential forests, the mosaic of isolated trees and copses reduces the competitive factors that pressure canopies to vertically thin- outdi.e., branch drop64 or crown shyness.65 If conditions allow, this setting can increase the density per unit canopy area, which, in turn, increases interception per unit canopy area: e.g., to 20%e60% of precipitation.45,66 The increased interception is likely a product of (1) greater interception surface area enhancing water storage and (2) greater canopy exposure permitting better ventilation and, therefore, increased evaporation rates from leaf and bark surfaces.67,68 Reduction of canopy overlap may also permit greater woody biomass production, which can favor stemflow production.69 Increased storage, evaporation, and stemflow production in urban park and residential forests may result in lower throughfall proportions beneath the canopy. These conditions are also typical for tree rows (Fig. 3D,E) and, of course, isolated trees (Fig. 3F). However, the degree of sheltering by urban structures can alter the canopy’s water, and related energy, budget. Residential and sheltered row forests experience a range of impacts from nearby structures in the “urban canopy layer” that alter their precipitation partitioning. This is because urban structures significantly differ than forest canopies in their interaction with meteorological variables: (1) being impermeable to wind and precipitation; (2) having a greater range of albedo/emissivity values; (3) storing substantially greater heat; and (4) presenting diurnal thermal dynamics linked to engineering controls, such as heating and ventilation.33 Shelter from urban structures reduces the canopy’s access to precipitation and, thus, determines the amount of precipitation intercepted and drained. Once precipitation water is stored on a canopy, being surrounded by urban structures can change the ARTICLE IN PRESS

Urban Forestry 9 meteorological evaporation drivers (wind and energy). Sheltered tree row and residential forests may experience greater shortwave radiation flux from reflection off building structures or increased longwave radiation flux due to overhead exposure to heat-emitting building surfaces rather than the open sky.70 This greater available energy in the canopy can fuel evaporation of entrained precipitation.71 Heated building facades and ground surfaces may also alter the atmospheric circulation patterns,72 which drive evaporation from wet canopies during storms. It is also noteworthy that sheltering of tree row and residential forests can alter a tree’s structural developmentde.g., Moser et al.73 found that crown area, volume, and openness were smallest for Robinia pseudoacacia in urban street canyons than in less-sheltered or unsheltered settings in Bavaria, Germany. These dense, narrow canopies are efficient precipitation interceptors and stemflow generators.

3.2 Canopy Trimming Practices Little research has been performed to quantitatively evaluate how canopy trimming practices affect physical precipitation partitioning,74,75 and no studies known to the authors have been performed in urban forests. Only Kaushal et al.75 evaluate the impact of the three most common canopy trimming practices simultaneously. Most knowledge on canopy structural influences over interception, throughfall, and stemflow is based on research in undisturbed or plantation forests (see review by Pypker et al.76). Still, these results indicate that there are several canopy manipulation methods currently employed for aesthetic and safety purposes that could be used to change the forest’s precipitation partitioning behavior. It is common to (1) thin tree canopies by pruning back to the second-order branch level, (2) pollard canopies by the removal of woody material back to the primary branches or trunk, and (3) cut trees back to the stem base in a process called coppicing. The regrowth response to these varying levels of woody material removal is structurally similar at the pruning sited production of multitudinous, thin, ramified, and fast-growing branchesdwith impacts on the canopy hydrology, which will vary tempo- rally as the cut areas regrow (Table 1). An immediate increase in the proportion of the canopy occupied by gaps, after any level of canopy trimming, produces a short-term increase in throughfall, specifically “gap” throughfall, and a reduction in stemflow, canopy water storage, and evaporation (Table 1). The severity of these short-term shifts in precipitation partitions is linked to the degree of ARTICLE IN PRESS

10 John T. Van Stan, II et al.

Table 1 Hydrological Impacts of Canopy Trimming Hydrological Variable Short Term Long Term Canopy water storage þ Canopy evaporation þ Throughfall þ Stemﬂow þ

Studies on canopy trimming are few,74,75 but this table summarizes direct (þ) and indirect () relationships between the short- and long-term hydrological effects. trimming, where coppicing elicits the greatest changes and pruning of higher-order branches, the least. Due to the production of multitudinous, thin, steeply inclined and ramified branches after pruning,77 the forest canopy eventually becomes efficient at capturing precipitation.75 This long-term increase in efficiently intercepting branches boosts canopy water storage, evaporation, and stemflow generation. Therefore, in forests where canopy trimming has regrown, throughfall decreases (Table 1). It is also important to note that periodic pruning of trees for water management also may reduce subsequent costs accrued during treatment of defects that develop as trees mature.26 Urban forests can host significant vegetation on their canopies as epiphytes (lichens, mosses, bromeliads, etc.) or parasites (mistletoe, Cuscuta, etc.). By increasing the intercepting surface area, the vegetation inhabiting forest canopies generally increases the water storage and evaporation of precipitation while reducing throughfall and stemflow inputs.78 Thus, their trimming (or removal) may be useful in maximizing water receipt at the surface, or for stormwater management, the additional storage in epiphytes may be beneficial.

3.3 Species Selection Urban development typically results in the replacement of native with nonnative plant species,79 although many programs exist promoting reintroduction of native species, and the identification and removal of invasive species, as well as assisting communities and landowners in accom- plishing these tasks.80,81 Whether species selection favors native or nonnative plants, it results in vegetation compositions and structures that impact (perhaps inadvertently at present) the patterning of canopy traits long known to influence precipitation partitioning.76 These canopy traits include species’ leaf shape/configuration,82 bark morphology,83,84 and whether/to what extent they host epiphytes.85,86 Currently, the aesthetic arrangement of ARTICLE IN PRESS

Urban Forestry 11 these canopy traits and its relationship to consumer preferences and public safety design primarily drive species selection and management in urban e forestry.7 9 However, precipitation storage and evaporation estimates exist for most of these canopy traits, so managers can use these alongside aesthetic, safety, and other criteria to aid in tree selection (Table 2). When considering a species’ impact on precipitation partitioning, note that bark and epiphytes merit close attention, as they can store large amounts of water (Table 2). General relationships between species-specific canopy traits and nearly every component of the canopy water balance have been described (Table 3) and may be useful for urban water managers interested in collaborating with urban foresters during species selection. The large ranges in water storage and evaporation for each canopy trait (Table 2) drives in part, differences in precipitation partitioning between species that can be significant.42,46,89 For example, Pinus species with archetypical thick bark and dense evergreen needles tend to intercept the greatest portion of precipitation compared with seasonal broadleaved species with low bark water storage, like some Fraxinus species (Fig. 4). These species-specific canopy traits also translate to low (3% of precipitation) and high (9% of precipitation) stemflow generation for the Pinus spp. versus the Fraxinus spp., respectively (Fig. 4). As mentioned in Section 3, although the precipitation that contacts the canopy becoming stemflow (9%) seems

Table 2 Range of Precipitation Storage Capacity and Evaporation Rates for Major Canopy Traits Evaporation Element Storage (mm) (mm/h) Citation(s) Broadleaves 0.2e2.0* 0.1e4.000 *Leyton et al.,101 André et al.,119 Chen and Li,120 Navar121 Needleleaves 0.1e4.3* 0.1e4.200 *Llorens and Gallart,122 link et al.,123 Navar,121 Van stan et al.68 Bark 1.3e5.9* 0.0e0.300 *Herwitz,83 Van stan et al.68 Epiphytes 0.1e16.6* 0.1e0.500 *Van stan et al.,11,86 Van stan and Pypker,78 Jarvis,124 Ah-Peng et al.125 ^Deadwood/ 1.1e20* 0.1e0.600 *Pypker et al. (2006), Sexton and organics Harmon88, Unsworth et al.,87 Sexton and Harmon88

Depth equivalent units (mm) represent L/m2 surface area. Note that (1) most of these values are from natural (closed-canopy) forests due to the lack of data on urban forests11,68 and (2) deadwood/organics studies cited correspond to “coarse woody debris”87 and “decaying logs.”88 ^In actively managed urban forests, this is minimized. ARTICLE IN PRESS

12 John T. Van Stan, II et al.

Table 3 Hydrological Impacts of Major Species-speciﬁc Canopy Traits Hydrological Variable Canopy Trait Storage Evaporation Throughfall Stemﬂow Leaf Hydrophobic1 þ þ/ Hydrophilic1 þþ þ/þ/ Density/closure/LAI2 þþ Bark Smooth3 þ/þ Rough3 þþ þ/ Branch/Trunk Size (DBH)4 þþ þ/ Inclination4 n.a. n.a. * þ* Biomass/WAI2 þþ þ Epiphyte host?5 þþ

Previous research has found that canopy hydrological variables tend to directly (þ) or indirectly () relate to common leaf, bark, and woody structural traits and the potential for a species to host epiphytes. This summary may be used as a general guide for tree selection. *Inclination over w45 can result in stemﬂow reduction.76 1Rosado and Holder (2013). 2Sadeghi et al.69 3Van Stan et al.126 4Pypker et al.76 5Van Stan and Pypker.78

Figure 4 Example of between-species differences in canopy precipitation partitioning. Four examples of tree species commonly selected for urban forestry around the globe show that the proportion of precipitation partitioned into interception, throughfall, and stemﬂow can signiﬁcantly differ among species. Data from Sadeghi et al.42 and photos from commons.wikimedia.org: from left to right, AlixSaz,90 Luidger,91 Rhinomind,92 Pedro Felipe.93 ARTICLE IN PRESS

Urban Forestry 13 modest, it can produce a significantly concentrated hydrological flux at the base of the stem. For example, the 9% of a 25 mm storm across a modest 30 m2 canopy area results in a stemflow input >60 mm over a 1 m2 area around the trunk. Thus, if water management goals for a region include increasing infiltration, canopy traits favoring stemflow generation should be considered. Although not yet fully quantified, there is a direct relationship between stemflow generation and branch inclination angle to a tipping point whereafter stemflow drips off as throughfall.12,94 Of course, canopy area is also important in diverting precipitation to stemflow. This is apparent for the 7% stemflow generating Cupressus spp. (Fig. 4), whose canopy produces inclined branches, but the narrow canopy area limits its access to precipitation compared with Fraxinus spp. The precipitation interception and drainage efficiency of the species may also depend on less conspicuous canopy traits, such as leaf hydrophobicity.89(Table 3) However, the degree of hydrophobicity can sometimes be intuited by the presence and structure of waxy leaf cuticles.95 Species-specific leaf habits also appear to be linked to temporal trends in leaf hydrophobicity, where longer-lived leaves tend to become less water e repellant over time.96 98

4. EXTREME WEATHERdA CASE STUDY

Many urban forests face an increase in extreme storm frequency due e to hydrological intensiﬁcation.18 22 The authors are unaware of research comparing precipitation partitioning across different urban forest settings during extreme storms. Little research exists on this subject in general,99 likely because canopy water storage is rapidly overwhelmed (see Table 2) by intense storms. Canopy structure controls more than storage; however, as it affects the time for precipitation to drain and micrometeorological conditions driving evaporation. Thus, here we share observations from an example extreme rain event, w100 mm, starting February 3, 2016, with maximum 5-min intensities >50 mm/h (Fig. 5)acrossthreesites along a natural-to-urban gradient in the Southeastern United States, near Statesboro, Georgia: 32.45 N, 81.78 W(Table 4). The relevance of this case study is that, for the past half decade, the underlying synoptic conditions have become very common in the winter: an extensive ridging of height contours in approximate alignment with the US west coast (Figs. 6A). This circulation anomaly has been observed in a long-lived ARTICLE IN PRESS

14 John T. Van Stan, II et al.

Figure 5 Storm meteorological conditions for an example rain event (starting 1700 EST, February 3, 2016) during which precipitation partitioning was monitored across a natural-to-urban gradient in forest setting. Meteorological conditions are plotted every 5 min, and a summary of even-scale rain conditions is provided in Table 4.

Table 4 Summary of Rainfall and Precipitation Partitioning Characteristics Across a Natural-to-Urban Forest Structural Gradient in Georgia (United States) for February 3, 2016 Site Storm Characteristics Natural Fragment Row Rainfall Amount (mm) 97.8 88.1 105.8 Storm intensity (mm/h) 5.3 4.9 5.9 Maximum intensity (mm/h) 55.2 66.1 77.6 Throughfall % of rain amount 83.8% 81.6% 75.3% % of storm intensity reduction 17.0% 28.6% 32.2% Interception % of rain amount 16.2% 18.4% 24.7% Storage (mm) 1.2 1.7 2.8 Evaporation (mm/h) 0.11 0.14 0.18

Total storage and event maximum evaporation rate were estimated via Aston100 and Leyton et al.101 Stemﬂow was negligible in these pine stands41 and was excluded. ARTICLE IN PRESS

Figure 6 Upper troposphere and surface plots for example storm. (A) 500 hPa geopoten- tial height ﬁeld observations (50 gpm contour interval) and superimposed dashed and solid lines to indicate trough and ridge axes, respectively. (B) In surface analyses where station models are standard, fronts are shown as blue (cold) and red (warm), and contours are MSLP at 4 hPa intervals. (C) NOAA HYSPLIT back trajectories of air masses proximal to the study sites. Red, blue, and green trajectory traces are 1000, 3000, and 6000 m AGL, respectively. Below trajectory map is a time graph illustrating relative humidity estimates for each of the trajectory traces at 6 h intervals for the 72 h before the storm starts. ARTICLE IN PRESS

16 John T. Van Stan, II et al. quasistationary position so frequently that it has been called the “ridicu- lously resilient ridge.”102 With this configuration, the North American dipole (alternating continental pattern of surface temperature) has been observed in a rather intense phase: with warmer than normal temperature coinciding with the west coast ridge and colder than normal temperature observed in conjunction with the downstream trough.103 Increasing frequency of this configuration with a poleward displacement of the Bermuda High suggests that storms like in this case study may become more frequent. The storm intensity in the case study was extreme, but its duration was also anomalous (Fig. 5). Rainfall was forced by a frontal boundary (Fig. 6B); however, the lack of atmospheric translation provided a very slow advance of the cold frontal boundary across the state of Georgia. In addition to the slow movement of the tropospheric wave, cold air advected southward to the Gulf of Mexico behind the front, and the air in advance of the cold front was warm and extremely moist (Fig. 6C). The warm moist air was moved into southeast Georgia by the return flow around the surface high pressure ridge over the western Atlantic Ocean (Fig. 6C). The air masses at 1000 and 3000 m were highly charged with moisture at their origins and remained >50% relative humidity for the 72-h period of the analysis (Fig. 6C). Thus, Fig. 6AeC shows the interconnectivity of scales that is becoming more frequent and resilient than in preceding decades,102,104 with the continental scale wave impacting the synoptic scale frontal boundary and mesoscale moisture advection (and precipitation) across southeast Georgia. It is this configuration that altered the advancement speed of surface fronts and allowed for long-duration rainfall (as observed in this case study). One can, thus, argue that the change in frequency of west coast ridging along with the poleward shift of the Bermuda high during the winter season (i.e., Fig. 6) portends more extreme rainfall events across the southeast portion of the United States. Storm conditions were similar across sites: Rain amount was 88e 106 mm and total storm intensity was 5e6mm/h(Table 4). Sites were Pinus species in a natural, fragment, and tree row forest setting (Fig. 5, bottom panels). No sites produced significant stemflow. Throughfall intensity across sites was lower than rain intensity, but intensity “dampening” differed: 33%, 28%, and 17% beneath the tree row, fragment, and natural forest, respectively (Table 4). Wet canopy evaporation rates per unit canopy area (derived indirectly from rain v. interception regressions) were higher for the row, 0.18 mm/h, than for the fragment, 0.14 mm/h, and ARTICLE IN PRESS

Urban Forestry 17 the natural setting, 0.11 mm/h (Table 4). Water storage per unit canopy area was highest and lowest for the row and natural forest, respectively (Table 4). Rainfall interception percentages per unit canopy area decreased from urban-to-natural stand structures, 25%e16% (Table 4). Rain-to-throughfall intensity reductions of the magnitude observed at these forest sites have been reported previously from modeling work, where the throughfall response to extreme storms (>80 mm rainfall) with short-duration, high-intensity rainfall periods reduced intensity by up to w30%.99 This intensity reduction for large magnitude extreme storms appears to be driven primarily by the way water transfers through thecanopyandsecondarilybyevaporation.99 The pathways and timing of rainfall transfer through the canopy are tied to canopy structure,44,105 and thus, it is reasonable that the assumedly lengthier rainfall transfer through the densest, most “layered” tree row canopy would result in the greatest intensity reduction (Table 4). Tree row canopies are better venti- lated/moreexposedtowindcomparedwiththefragmentandnatural stand, in part explaining higher evaporation rates per unit canopy aread more details on evaporation from this site can be found in Van Stan et al.68 Settings that permit dense canopy development (parks, residential, rows, isolated canopies: Fig. 3) may best reduce rain intensity during extreme storm events. Rain intensity reduction is connected to several important surface processes in urban IWRM. For example, on impervious surfaces, rain and throughfall intensity can control, in part, the rapidity of the runoff and streamwater discharge response. In parks, intense throughfall can saturate soils, aiding the destabilization and uprooting of trees.106 Careful consideration of urban forest setting, alongside other landscaping requirements, therefore, may be useful in mitigating the runoff response and associated sediment loads from extreme storms.

5. NET PRECIPITATION CHEMISTRY AND WATER QUALITY

The journey of precipitation water from first canopy contact to net precipitation (throughfall and stemflow) often results in substantial chemical alteration.25 During the dry period between storms, particulate and gaseous aerosols (including air pollutants) suspended in the passing winds are deposited on the urban canopy surfaces (Fig. 7). Deposited aerosols can undergo significant bio- and photochemical transformations on canopy surfacesda notable example is sulfur.107,108 Dry deposition also consists of materials ARTICLE IN PRESS

18 John T. Van Stan, II et al.

Figure 7 Major processes that alter precipitation quality during contact with the canopy. Atmospherically transported materials that inﬂuence stormwater quality, including natural and pollutant aerosols, solutes, and gasses, are deposited onto urban forest canopies during dry and wet periods. Many of these materials are exchanged with the canopy during wash-off by precipitation, resulting in net precipitation quality being drastically altered compared with bulk precipitation.

(1) respired and exuded from the canopy, particularly for potassium109,110 and dissolved organic matter,111 (2) excreted from canopy fauna,112 or (3) from microbial communities hosted on canopy surfaces.113 Precipitation, or wet deposition, brings further solutes and particulates to the urban forest canopy while also scouring dry deposited compounds from canopy surfaces, in a process called “wash-off” (Fig. 7). During transport through the canopy as throughfall and stemflow, the chemically enriched precipitation water ex- changes with leaf, bark, and epiphyte surfaces (Fig. 7). The result of these chemical alterations is, generally, that the concentration of many solutes þ can be enhanced by 500 (e.g., K ) to 5000 (e.g., NO3 ) times that of gross precipitation.114 Canopyeprecipitation interactions controlling throughfall and stemflow chemical quality often generate 100 s kg/ha year of nutrient supply to the surfaces below the urban forest canopy (e.g., dissolved organic carbon.111 In addition to generating nutrient supply to the surface, these processes have been found to capture air pollutants and to magnify their deposition to the surface via throughfall115 and stemflow.116 Although no edge effects on throughfall and stemflow volume have been found (see Section 3.1), the edge of forest fragments, and assumedly copses in park and residential settings, have been found to affect the chemical ARTICLE IN PRESS

Urban Forestry 19 quality of throughfall60 and stemflow acid neutralizing capacity.117 For throughfall, solutes most significantly affected by the edge effect were primarily from atmospheric deposition, such as sea salts, Cl , and potentially 2 60 acidifying ions, SO4 and NO3 . An “edge enhancement” factor developed by Wuyts et al.60 observed that sea salt concentrations could double along the edge compared with the interior and that potentially acidifying ions increased up to 1.5 times along the edge. Species should also be considered in edge effects for urban forest planning, as coniferous trees have been found to capture and concentrate aerosols more effectively along the forest edge than broadleaved trees.60 The effect of potentially acidifying ions on stemflow, along the edge, may be offset by increased acid neutralization capacity (ANC) of trees along the edge.117 In fact, edge enhancement factors for potentially acidifying ions are generally <2,60,118 but Shiklomanov and Levia117 reported an edge-to-interior ANC multiplier of 2.44 for stemflow. Studies on edge-related shifts in throughfall chemistry are numerous enough to have merited a review a decade ago118; however, research on edge effects for stemflow is limited. Insights from the limited stemflow literature, but ample throughfall literature, indicate that magnification of aerosol pollutant load along urban forest edges may be substantial (throughfall is the bulk of precipitation reaching the surface). Thus, managers may consider minimizing the perimeter length of urban forests. These quality-related findings regarding urban forest canopy interactions with precipitation raise many important questions for IWRM. Principally, how does the urban canopy’s scavenging of aerosol and chemical exchange with precipitation influence stormwater quality? Unfortunately, this is a knowledge gap for urban forests. If air pollutants are spatially magnified by throughfall and stemflow processes, should urban forest planning and management consider common air mass provenances and trajectories for storms that carry these pollutants when determining where and what to plant? Alternatively, can the nutrients transported by net precipitation from the canopy to soils (in the absence of impervious surfaces, like in parks) improve the urban soil ecosystem services? The answers to these questions are not known yet and need further research. This is because, to the authors’ knowledge, connections between the urban forest’s alteration of precipitation chemistry and surface hydrological processes (soilwater, runoff, and streamwater quality) have not been previously researched. This may be an important nexus between urban forestry and IWRM for urban catchments (see Fig. 1), and investigations on this topic are arguably necessary for the holistic integration of these two fields. ARTICLE IN PRESS

20 John T. Van Stan, II et al.

6. CONCLUDING REMARKS

Urban forestry is a valuable, but underutilized, tool for managing the amount, quality, and timing of precipitation supply to the surface of urban watersheds. The physical partitioning of precipitation by canopies into interception, throughfall, and stemflow can be affected by the (1) urban forest setting, (2) trimming of canopy structures, and (3) consideration of traits’ hydrological interactions when selecting tree species. It is important to note that manipulating canopies’ precipitation partitioning only addresses hydrologic processes at start of the terrestrial water cycle, and urban forestry practices can manipulate vegetation-related factors further down the rainfall-to-runoff pathway (understory cover, transpiration, root growth forms, etc.). Although the literature is sparse, it appears that by considering at least these major urban forest structural factors, along with other urban forestry guidelines, urban watershed managers may be able to tailor forests to assist in achieving local stormwater goals. When the aim is net precipitation reduction, past research indicates that managers should select tree species with traits that favor water storage and evaporation such as thicker and rougher bark, denser leaf coverage that persists throughout the year, and relatively flat branching architecture that hosts epiphytes. An urban forest setting that allows maximum canopy biomass production (greater between-tree spacing) may allow denser canopy development and, therefore, greater interception efficiency. If the goal is to maximize net precipitation, managers can consider canopy traits, setting and trimming techniques to maximize either the areally diffuse flux (throughfall) or the concentrated flux (stemflow), depending on whether deep infiltration (via stemflow) is desired. Promotion of throughfall versus stemflow will primarily depend on branching architecture, where a greater number of steeper branches with minimal bark thickness will promote stemflow, and the opposite for throughfall. Although increased stemflow production may result in deeper infiltration of precipitation, there are many considerations to enable meaningful stemflow management. In fact, excellent recommendations regarding urban forestry and stemflow have been made by Schooling and Carlyle-Moses94 “to promote stemflow production while also minimizing stormwater runoff in urban environments.” Recommendations from Schooling and Carlyle-Moses,94 in addition to those discussed in this chapter, include ensuring that sufficient infiltration capacity be provided at the trunk base and considering the ARTICLE IN PRESS

Urban Forestry 21 regional storm regime and mature size of selected species during planning (i.e., smaller trees will generate stemflow under lower storm sizes). It is also prudent for managers to consider precipitation projections, including increased extreme storm size and frequency, when considering the urban forest setting, species selection, and canopy management. In the extreme storm case study for the southeastern United States, the tree row setting more effectively stored and evaporated rainfall and reduced rainfall intensity per unit canopy area than the fragment or natural stand structure. Further research should examine the precipitation partitioning of the various urban forest settings and their common canopy manipulations during extreme storms. Regarding stormwater quality, urban managers should be mindful that precipitation’s interaction with the urban forest canopy can (1) significantly increase or deplete solute concentrations, (2) transfer substantial bacteria and fungal spores to the surface, (3) concentrate nutrients/pollutants to near-stem soils via stemflow, and (4) direct greater aerosol deposition along forest edges. Other questions regarding urban forests’ role in urban stormwater quality remain, particularly across scales (i.e., city-scale forest interactions with synoptic meteorological conditions in the face of anthropogenic change), and where throughfall and stemflow intersect with soil, riparian, and stream processes.

ACKNOWLEDGMENTS Collaboration between Van Stan, Underwood, and Friesen on this chapter was supported by US-NSF (EAR-1518726).

REFERENCES 1. Konijnendijk CC. A decade of urban forestry in Europe. For Policy Econ 2003;5:173e86. 2. Nowak DJ, Stein SM, Randler PB, Greenﬁeld EJ, Comas SJ, Carr MA, et al. Sustaining America’s urban trees and forests. United States Department of Agriculture, Forest Service; 2010. General Technical Report 2010; NRS-62. 3. McGovern M, Pasher J. Canadian urban tree canopy cover and carbon sequestration status and change 1990e2012. Urban For Urban Green 2016;20:227e32. 4. Miller RW, Hauer RJ, Werner LP. Urban forestry: planning and managing urban greenspa- ces. 3rd ed. Long Grove: Waveland Press; 2015. 5. Campbell LK. Constructing New York City’s urban forest: the politics and governance of the Million Trees NYC campaign. In: Sandberg L, Bardekjian A, Butt S, editors. Urban forests, trees & greenspace, a political ecology perspective. New York: Routledge; 2014. p. 242e60. 6. Avolio ML, Pataki DE, Gillespie TW, Jenerette GD, McCarthy HR, Pinceti S, et al. Tree diversity in southern California’s urban forest: the interacting roles of social and environmental variables. Front Ecol Evol 2015;3:73. 7. Turner DS, Mansﬁeld ER. Urban trees and roadside safety. J Transp Eng 1990;116. https://doi.org/10.1061/(ASCE)0733-947X(1990)116:1(90). ARTICLE IN PRESS

22 John T. Van Stan, II et al.

8. Wolf KL, Bratton N. Urban trees and traffic safety: considering US roadside policy and crash data. Arboric Urban For 2006;32:170e9. 9. Clarke LW, Jenerette GD, Davila A. The luxury of vegetation and the legacy of tree biodiversity in Los Angeles, CA. Landsc Urban Plan 2013;116:48e59. 10. Steenberg JWN, Millward AA, Nowak DJ, Robinson PJ. A conceptual framework of urban forest ecosystem vulnerability. Environ Rev 2017;25:115e26. 11. Van Stan JT, Levia DF, Jenkins RB. Forest canopy interception loss across temporal scales: implications for urban greening initiatives. Prof Geogr 2015a;67:41e51. 12. Gotsch SG, Draguljic D, Williams CJ. Evaluating the effectiveness of urban trees to mitigate storm runoff via transpiration and stemflow. Urban Ecosyst 2017. https:// doi.org/10.1007/s11252-017-0693-y. 13. McPherson EG, Simpson JR, Peper PJ, Maco SE, Xiao Q. Municipal forest benefits and costs in five US cities. J For 2005;103:411e6. 14. Livseley SJ, McPherson EG, Calfapietra C. The urban forest and ecosystem services: impacts on urban water, heat, and pollution cycles at the tree, street, and city scale. J Environ Qual 2016;45:119e24. 15. McDonald RI, Douglas I, Revenga C, Hale R, Grimm N, Groenwall J, et al. Global urban growth and the geography of water availability, quality, and delivery. Ambio 2011;40:437e46. 16. Xiao Q, McPherson EG. Rainfall interception by Santa Monica’s municipal urban forest. Urban Ecosyst 2003;6:291e302. 17. Neary DG, Ice GG, Jackson CR. Linkages between forest soils and water quality and quantity. For Ecol Manage 2009;258:2269e81. 18. Horton R, Yohe G, Easterling W, Kates R, Ruth M, Sussman E, et al. Northeast. In: Melillo JM, Richmond T, Yohe GW, editors. Climate change impacts in the United States: the third national climate assessment. Washington: US Global Change Research Program; 2014. p. 371e95. 19. Madsen H, Lawrence D, Lang M, Martinkova M, Kjeldsen TR. Review of trend analysis and climate change projections of extreme precipitation and floods in Europe. J Hydrol 2014;519D:3634e50. 20. Melillo JM, Richmond T, Yohe GW. Climate change impacts in the United States: the Third National Climate Assessment. Washington: US global change research program. 2014. 21. Walsh J, Wuebbles D, Hayhoe K, Kossin J, Kunkel K, Stephens G, et al. Our changing climate. In: Melillo JM, Richmond T, Yohe GW, editors. Climate change impacts in the United States: the third national climate assessment. Washington: US Global Change Research Program; 2014. p. 19e67. 22. Loo YY, Billa L, Singh A. Effect of climate change on seasonal monsoon in Asia and its impact on the variability of monsoon rainfall in Southeast Asia. Geosci Front 2015;6: 817e23. 23. Savenije HHG. The importance of interception and why we should delete the term evapotranspiration from our vocabulary. Hydrol Process 2004;18:1507e11. 24. Keim RF, Skaugset AE, Weiler M. Temporal persistence of spatial patterns in throughfall. J Hydrol 2005;314:263e74. 25. Parker GG. Throughfall and stemflow in the forest nutrient cycle. Adv Ecol Res 1983; 13:57e133. 26. McPherson EG, van Doorn N, de Goede J. Structure, function and value of street trees in California, USA. Urban For Urban Green 2016;17:104e15. 27. McPherson EG, Nowak D, Heisler G, Grimmond S, Souch C, Grant R, et al. Quan- tifying urban forest structure, function and value: the Chicago urban forest climate project. Urban Ecosyst 1997;1:49e61. ARTICLE IN PRESS

Urban Forestry 23

28. Davey Resource Group. Indiana’s street tree benefits summary. 2010. http://www.in.gov/ dnr/forestry/files/Fo-INSpeciesDistributionUrbanTrees709.pdf. 29. Treiman T, Kuhn N, Gartner JT, Koenig A. Missouri’s 2010 street tree economics. Columbia, MO: Missouri Department of Conservation; 2010. 30. Holmes TP. The offsite impact of soil erosion on the water treatment industry. Land Econ 1988;64:356e66. 31. Klaassen W, Bosveld F, de Water E. Water storage and evaporation as constituents of rainfall interception. J Hydrol 1998;212e213:36e50. 32. Dunkerley DL. Evaporation of impact water droplets in interception processes: histor- ical precedence of the hypothesis and a brief literature overview. J Hydrol 2009;376: 599e604. 33. Oke TR. The micrometeorology of the urban forest. Phil Trans R Soc Lond B 1989; 324:335e49. 34. Kjelgren R, Montague T. Urban tree transpiration over turf and asphalt surfaces. Atmos Environ 1998;32:35e41. 35. Bartens J, Day SD, Harris JR, Wynn TM, Dove JE. Transpiration and root development of urban trees in structural soil stormwater reservoirs. Environ Manage 2009;44: 646e57. 36. Pataki DE, McCarthy HR, Litvak E, Pincetl S. Transpiration of urban forests in the Los Angeles metropolitan area. Ecol Appl 2011;21:661e77. 37. Rubin G, Warren L. The drought-defying California garden. Portland: Timber Press; 2016. 38. Ingels JE. Landscaping: principles and practices. 7th ed. Clifton Park: Delmar Cengage Learning; 2009. 39. California Department of Water Resources. A guide to estimating irrigation water needs of landscape plantings in California. Sacramento: University of California Cooperative Extension; 2000. 40. Carlyle-Moses DE, Gash JHC. Rainfall interception loss by forest canopies. In: Levia D, Carlyle-Moses D, Tanaka T, editors. Forest hydrology and biogeochemistry. Heidelberg: Springer; 2011. p. 407e23. 41. Van Stan JT, Coenders-Gerrits AMJ, Dibble M, Bogeholz P, Norman Z. Effects of phenology and meteorological disturbance on litter rainfall interception for a Pinus elliottii stand in the Southeastern US. Hydrol Process 2017a;31:3719e28. 42. Sadeghi SMM, Attarod P, Van Stan JT, Pypker TG. The importance of considering rainfall partitioning in afforestation initiatives in semiarid climates: a comparison of common planted tree species in Tehran, Iran. Sci Total Environ 2016;568:845e55. 43. Levia DF, Keim RF, Carlyle-Moses DE, Frost EE. Throughfall and stemflow in wooded ecosystems. In: Levia D, Carlyle-Moses D, Tanaka T, editors. Forest hydrology and biogeochemistry. Heidelberg: Springer; 2011. p. 425e43. 44. Voss S, Zimmermann B, Zimmermann A. Detecting spatial structures in throughfall data: the effect of extent, sample size, sampling design, and variogram estimation method. J Hydrol 2016;540:527e37. 45. Guevara-Escobar A, Gonzalez-Sosa E, Veliz-Chavez C, Ventura-Ramos E, Ramos- Salinas M. Rainfall interception and distribution patterns of gross precipitation around an isolated Ficus benjamina tree in an urban area. J Hydrol 2007;333:532e41. 46. Inkil€ainen ENM, McHale MR, Blank GB, James AL, Nikinmaa E. The role of the residential urban forest in regulating throughfall: a case study in Raleigh, North Carolina, USA. Landsc Urban Plan 2013;119:91e103. 47. Van Stan JT, Gordon DA. Mini-review: stemflow may be more famine than feast for forests’ near-stem soil communities. Front Plant Sci 2018. https://doi.org/10.3389/ fpls.2018.00248. ARTICLE IN PRESS

24 John T. Van Stan, II et al.

48. Johnson MS, Lehmann J. Double-funneling of trees: stemflow and root-induced pref- erential flow. Ecosci 2006;13:324e33. 49. Livesley SJ, Baudinette B, Glover D. Rainfall interception and stemflow by eucalypt street trees e the impacts of canopy density and bark type. Urban For Urban Green 2014;13:192e7. 50. Gerrits AMJ, Savenije HHG. Forest floor interception. In: Levia D, Carlyle-Moses D, Tanaka T, editors. Forest hydrology and biogeochemistry. Heidelberg: Springer; 2011. p. 445e54. 51. Huntington TG. Evidence for intensification of the global water cycle: review and synthesis. J Hydrol 2006;319:83e95. 52. Trenberth KE. Changes in precipitation with climate change. Clim Res 2011;47:123e38. 53. Coumou D, Rahmstorf S. A decade of weather extremes. Nat Clim Chan 2012;2:491e6. 54. Soares AL, Rego FC, McPherson EG, Simpson JR, Peper PJ, Xiao Q. Benefits and costs of street trees in Lisbon, Portugal. Urban For Urban Green 2011;10:69e78. 55. Hurd JD, Wilson EH,Lammey SG, Civco DL. Characterization of forest fragmentation and urban sprawl using time sequential Landsat imagery. Proc ASPRS Ann Conv April 2001:1e12. 56. Game M. Best shape for nature reserves. Nat Lond 1980;287:630e2. 57. Laurance WF, Yensen E. Predicting the impacts of edge effects in fragmented habitats. Biol Conserv 1991;55:77e92. 58. Neal C, Robson AJ, Bhardwaj CL, Conway T, Jeffrey HA, Neal M, et al. Relationships between precipitation, stemflow and throughfall for a lowland beech plantation, Black Wood, Hampshire, southern England: findings on interception at a forest edge and the effects of storm damage. J Hydrol 1993;146:221e33. 59. Klaassen W, Lankreijer HJM, Veen AWL. Rainfall interception near a forest edge. J Hydrol 1996;185:349e61. 60. Wuyts K, De Schrijver A, Staelens J, Gielis L, Vandenbruwane J, Verheyen K. Com- parison of forest edge effects on throughfall deposition in different forest types. Environ Pollut 2008;156:854e61. 61. Levia DF, Germer S. A review of stemflow generation dynamics and stemflow-environment interactions in forests and shrublands. Rev Geophys 2015;53:673e714. 62. Fathizadeh O, Hosseini SM, Zimmermann A, Keim RF, Boloorani AD. Estimating linkages between forest structural variables and rainfall interception parameters in semi-arid deciduous oak forest stands. Sci Total Environ 2017;601e602:1824e37. 63. Pereira FL, Valente F, David JS, Jackson N, Minunno F, Gash JH. Rainfall interception modelling: is the wet bulb approach adequate to estimate mean evaporation rate from wet/saturated canopies in all forest types? J Hydrol 2016;534:606e15. 64. Harris RW. Summer branch drop. J Arboric 1983;9:111e3. 65. Putz FE, Parker GG, Archibald RM. Mechanical abrasion and intercrown spacing. Amer Midl Nat 1984;112:24e8. 66. Xiao Q, McPherson EG, Ustin SG, Grismer ME, Simpson JR. Winter rainfall interception by two mature open-grown trees in Davis, California. Hydrol Process 2000;14:763e 84. 67. Pereira FL, Gash JHC, David JS, Valente F. Evaporation of intercepted rainfall from isolated evergreen trees: do the crowns behave as wet bulbs? Agric For Meteorol 2009; 149:667e79. 68. Van Stan JT, Norman Z, Meghoo A, Friesen J, Hildebrandt A, Coté^ J-F, et al. Edge-to- stem variability in wet-canopy evaporation from an urban tree row. Bound-Layer Mete- orol 2017b;165:295e310. 69. Sadeghi SMM, Van Stan JT, Pypker TG, Friesen J. Canopy hydrometeorological dynamics across a chronosequence of a globally invasive species, Ailanthus altissima (Mill., tree of heaven). Agric For Meteorol 2017;240e241:10e7. ARTICLE IN PRESS

Urban Forestry 25

70. Harman IN, Best MJ, Belcher SE. Radiative exchange in an urban street canyon. Bound-Layer Meteorol 2004;110:301e16. 71. van Dijk AIJM, Gash JH, van Gorsel E, Blanken PD, Cescatti A, Emmel C, et al. Rain- fall interception and the coupled surface water and energy balance. Agric For Meteorol 2015;214e215:402e15. 72. Allegrini J, Dorer V, Carmeliet J. Wind tunnel measurements of buoyant flows in street canyons. Build Environ 2013;59:315e26. 73. Moser A, Roetzer T, Pauliet S, Pretzsch H. Structure and ecosystem services of small- leaved lime (Tilia cordata Mill.) and black locust (Robinia pseudoacacia L.) in urban environments. Urban For Urban Green 2015;14:1110e21. 74. Giacomin A, Trucchi P. Rainfall interception in a beech coppice (Acquerino, Italy). J Hydrol 1992;137:141e7. 75. Kaushal R, Kumar A, Alam NM, Mandal D, Jayaparkash J, Tomar JMS, et al. Effect of different canopy management practices on rainfall partitioning in Morus alba. Ecol Eng 2017;102:374e80. 76. Pypker TG, Levia DF, Staelens J, Van Stan JT. Canopy structure in relation to hydrological and biogeochemical fluxes. In: Levia D, Carlyle-Moses D, Tanaka T, editors. Forest hydrology and biogeochemistry. Heidelberg: Springer; 2011. p. 371e88. 77. Follett M. Branch growth response to pruning: development and testing of a new approach to measuring branch extension in trees [M.Sc. thesis]. University of Quebec; 2016. 59 pp.. 78. Van Stan JT, Pypker TG. A review and evaluation of forest canopy epiphyte roles in the partitioning and chemical alteration of precipitation. Sci Total Environ 2015;536: 813e24. 79. Kowarik I. On the role of alien species in urban flora and vegetation. In: Pysek P, Prach K, Rejmanek M, Wade PM, editors. Plant invasionsdgeneral aspects and special problems. Amsterdam: SPB Academic; 1995. p. 85e103. 80. Hawthorne TL, Elmore V, Strong A, Bennett-Martin P, Finnie J, Parkman J, et al. Mapping non-native invasive species and accessibility in an urban forest: a case study of participatory mapping and citizen science in Atlanta, Georgia. Appl Geog 2015;56: 187e98. 81. Brancalion PHS, Garcia LC, Loyola R, Rodrigues RR, Pillar VD, Lewinsohn TM. A critical analysis of the Native Vegetation Protection Law of Brazil (2012): updates and ongoing initiatives. Nat Conserv 2016;14:1e15. 82. Horton R. Rainfall interception. Mon Weather Rev 1919;47:603e23. 83. Herwitz SR. Interception storage capacities of tropical rainforest canopy trees. J Hydrol 1985;77:237e52. 84. Herwitz SR. Raindrop impact and water-flow on the vegetative surfaces of trees and the effects on stemflow and throughfall generation. Earth Surf Proc Landf 1987; 12:425e32. 85. Blum OB. Water relations. In: Ahmadjian V, Hale ME, editors. The lichens. New York: Academic Press; 1973. p. 381e400. 86. Van Stan JT, Stubbins A, Bittar T, Reichard JS, Wright KA, Jenkins RB. Tillandsia usneoides (L.) L. (Spanish moss) water storage and leachate characteristics from two mari- time oak forest settings. Ecohydrol 2015b;8:988e1004. 87. Unsworth MH, Phillips N, Link T, Bond BJ, Falk M, Harmon ME, et al. Components and controls of water flux in an old-growth Douglas-firdwestern hemlock ecosystem. Ecosyst 2004;7:468e81. 88. Sexton JM, Harmon ME. Water dynamics in conifer logs in early stages of decay in the Pacific Northwest, USA. Northwest Sci 2009;83:131e9. 89. Holder CD, Gibbes C. Influence of leaf and canopy characteristics on rainfall interception and urban hydrology. Hydrol Sci J 2017;62:182e90. ARTICLE IN PRESS

26 John T. Van Stan, II et al.

90. AlixSaz, 2011: https://commons.wikimedia.org/wiki/File:%D0%AD%D0%BB%D1% 8C%D0%B4%D0%B0%D1%80%D1%81%D0%BA%D0%B0%D1%8F_%D1%81% D0%BE%D1%81%D0%BD%D0%B0_-_Pinus_eldarica.jpg. 91. Luidger, 2006: https://commons.wikimedia.org/wiki/File:20061227-Figueres_ Teatre-Museu_Dal%C3%AD_MQ.jpg. 92. Rhinomind, 2017: https://commons.wikimedia.org/wiki/File:Vestergade_(Aarhus) _03.jpg. 93. Pedro Felipe, 2013: https://commons.wikimedia.org/wiki/File:Avenida_Ciudad_de_ Lima_-_Bogot%C3%A1.JPG. 94. Schooling J, Carlyle-Moses DE. The influence of rainfall depth class and deciduous tree traits on stemflow production in an urban park. Urban Ecosyst 2015;18:1261e84. 95. Rosado BHP, Holder CD. The significance of leaf water repellency in ecohydrological research: a review. Ecohydrol 2012;6:150e61. 96. Crockford RH, Richardson DP. Partitioning of rainfall in a eucalypt forest and pine plantation in southeastern Australia: III. Determination of the canopy storage capacity of a dry sclerophyll eucalypt forest. Hydrol Process 1990;4:157e67. 97. Schreuder M, van Hove LWA, Brewer CA. Ozone exposure affects leaf wettability and tree water balance. New Phytol 2001;152:443e54. 98. Sase H, Takahashi A, Sato M, Kobayashi H, Nakata M, Totsuka T. Seasonal variation in the atmospheric deposition of inorganic constituents and canopy interactions in a Japanese cedar forest. Environ Pollut 2008;152:1e10. 99. Keim RF, Skaugset AE, Link TE, Iroumé A. A stochastic model of throughfall for extreme events. Hydrol Earth Syst Sci 2004;8:23e34. 100. Aston AR. Rainfall interception by eight small trees. J Hydrol 1979;42:383e96. 101. Leyton L, Reynolds ERC, Thompson FB. Rainfall interception in forest and moorland. In: Sopper WE, Lull HW, editors. International symposium on forest hydrology. New York: Pergamon Press; 1967. p. 163e78. 102. Swain DL, Singh D, Horton DE, Mankin JS, Ballard TC, Diffenbaugh NS. Remote linkages to anomalous winter atmospheric ridging over the Northeastern Pacific. J Geo- phys Res Atmos 2017;122:12194e209. 103. Singh D, Swain DL, Mankin JS, Horton DE, Thomas LN, Rajaratnam B. Diffenbaugh, NS. Recent amplification of the North American winter temperature dipole. J Geophys Res Atmos 2016;121:9911e28. 104. Seager R, Hoerling M, Schubert S, Wang H, Lyon B, Arun K, et al. Causes of the 2011e14 California drought. J Clim 2015;28:6997e7024. 105. Keim RF, Link TE. Linked spatial variability of throughfall amount and intensity during rainfall in a coniferous forest. Agric For Meteorol 2018;248:15e21. 106. Schaetzl RJ, Johnson DL, Burns SF, Small TW. Tree uprooting: review of terminol- ogy, process, and environmental implications. Can J For Res 1989;19:1e11. 107. Baldocchi DA. multi-layer model for estimating sulfur dioxide deposition to a deciduous oak forest canopy. Atmos Environ 1988;22:869e84. 108. Erisman JW, Draaijers G. Deposition to forests in Europe: most important factors influencing dry deposition and models used for generalisation. Environ Pollut 2003;124: 379e88. 109. Lindberg SE, Shriner DS, Hoffman WA. The interaction of wet and dry deposition with the forest canopy. In: D’Itri FM, editor. Acid precipitation: effects on ecological systems. Ann Arbor: Ann Arbor Science Publications; 1982. p. 385e410. 110. Lovett GM, Lindberg SE. Dry deposition and canopy exchange in a mixed oak forest as determined by analysis of throughfall. J Appl Ecol 1984;21:1013e27. 111. Van Stan JT, Stubbins A. Tree-DOM: dissolved organic matter in throughfall and stemflow. Limnol Oceanogr Lett 2018. https://doi.org/10.1002/lol2.10059. ARTICLE IN PRESS

Urban Forestry 27

112. Michalzik B, Levia DF, Bischoff S, N€athe K, Richter S. Effects of aphid infestation on the biogeochemistry of the water routed through European beech (Fagus sylvatica L.) saplings. Biogeochem 2016;129:197e214. 113. Bittar T, Pound P, Whitetree A, Moore LD, Van Stan JT. Estimation of throughfall and stemflow bacterial flux in a subtropical oak-cedar forest. Geophys Res Lett 2018;45: 1410e8. https://doi.org/10.1002/2017GL075827. 114. Germer S, Zimmermann A, Neill C, Krusche AV, Elsenbeer H. Disproportionate sin- gle-species contribution to canopy-soil nutrient flux in an Amazonian rainforest. For Ecol Manage 2012;267:40e9. 115. Rodrigo A, Avila A, Roda F. The chemistry of precipitation, throughfall and stemflow in two holm oak (Quercus ilex L.) forests under a contrasted pollution environment in NE Spain. Sci Total Environ 2003;305:195e205. 116. Klucariakova D, Marton P, Pichler V, Marton E, Tunyi I. Pollution detection by mag- netic susceptibility measurements aided by the stemflow effect. Water Air Soil Pollut 2008;189:213e23. 117. Shiklomanov AN, Levia DF. Stemflow acid neutralization capacity in a broadleaved deciduous forest: the role of edge effects. Environ Pollut 2014;193:45e53. 118. De Schrijver A, Devlaeminck R, Mertens J, Wuyts K, Hermy M, Verheyen K. On the importance of incorporation forest edge deposition for evaluating exceedance of critical pollutant loads. Appl Veg Sci 2007;10:293. 119. André F, Jonard M, Ponette Q. Precipitation water storage capacity in a temperate mixed oak-beech canopy. Hydrol Process 2008;22:4130e41. 120. Chen Y-Y, Li M-H. Quantifying rainfall interception loss of a subtropical broadleaved forest in Central Taiwan. Water 2016;8:14. 121. Navar J. Fitting rainfall interception models to forest ecosystems of Mexico. J Hydrol 2017;548:458e70. 122. Llorens P, Gallart F. A simplified method for forest water storage capacity measurement. J Hydrol 2000;240:131e44. 123. Link TF, Unsworth MH, Marks D. The dynamics of rainfall interception by a seasonal temperate rainforest. Agric For Meteorol 2004;124:171e91. 124. Jarvis A. Measuring and modelling the impact of land-use change in tropical hillsides: the role of cloud interception to epiphytes. Adv Environ Model 2000;1:118e48. 125. Ah-Peng C, Cardoso AW, Flores O, West A, Wilding N, Strasberg D, et al. The role of epiphytic bryophytes in interception, storage, and the regulated release of atmospheric moisture in a tropical montane cloud forest. J Hydrol 2017. https://doi.org/10.1016/ j.jhydrol.2017.03.043. 126. Van Stan JT, Lewis ES, Hildebrandt A, Rebmann C, Friesen J. Impact of interacting bark structure and rainfall conditions on stemflow variability in a temperate beech- oak forest, central Germany. Hydrol Sci J 2016;61:2071e83.