Abstract
Potential impacts from climate change and other disturbances expedite the need to address vulnerabilities of urban forests. Low species diversity is a contributor to high urban forest vulnerability, and this study examined 40 public and private tree inventories in the metropolitan area of Milwaukee, Wisconsin, USA. Applying an established vulnerability framework, this study helps to identify the current and future resilience of the urban forest in the face of climate change and other urban forest threats. A Milwaukee metropolitan area tree inventory was compiled and includes 439,974 trees. This inventory then was assessed under 2 climate change models through the end of the century (2070 to 2099). It also was assessed for species diversity under multiple diameter classes, and the Shannon Diversity Index was used to determine correlations between tree size and diversity. The resulting data analysis revealed a poorly diversified urban tree canopy in the Milwaukee metropolitan area. However, when looking at tree size and diversity, diversity increased as tree diameters decreased. Additional analysis revealed that under a low climate change scenario (RCP 4.5), only 9.5% of the overall inventory was within the moderate, moderate-high, or high vulnerability categories through the end of the century. Under a high climate change scenario (RCP 8.5), 55.52% of the inventory fell within those same vulnerability categories. Diameter class did not have a significant impact on vulnerability under either climate change scenario. This data can help inform urban forestry practitioners during species selection for planting trees in their communities.
Introduction
Social, Environmental, and Economic Benefits
Urban trees provide social, economic, and environmental benefits to the urban ecosystem. These potential benefits are reliant on the ability of urban trees to establish and thrive in the typically harsh urban growing environment, an environment that is often subjected to highly disturbed and compacted soils, air pollution, de-icing salts, limited soil volume, infrastructure conflict, vandalism, and more. Additionally, ecological press and pulse disturbances can impact urban tree vulnerability. Pulse disturbances are relatively short-term events, such as an isolated insect outbreak or storm, which can lead to immediate mortality of urban trees, whereas press disturbances are more continuous events, such as climate change, resulting in lasting ecosystem changes and sustained mortality of one or more species (Huff et al. 2020). Building resilient and sustainable urban tree canopies can help mitigate the potential negative social, environmental, and economic impacts from these press and pulse disturbances (Nowak and Greenfield 2018).
Eighty-two percent of North America’s population resides in urban areas (United Nations 2018). In the United States, the number of metropolitan areas exceeding one million people more than tripled from the years 1950 to 2000 (Hobbs and Stoops 2002). With impervious cover in urban areas increasing, this leads to a decrease in urban tree cover; it is estimated that $96 million in urban forest benefits are lost annually in the United States due to urban tree canopy loss (Nowak and Greenfield 2018).
Climate change, and its potential impacts on flora, fauna, and people, is a developing reality (Mora et al. 2013). Particularly in urban environments, trees will continue to be subjected to numerous climate change stressors. This, on top of already harsh growing conditions, threatens to increase urban tree mortality while at the same time impacting the type and amount of viable tree species that can be planted in a given location. Urban trees provide an abundance of ecosystem services including reducing crime (Donovan and Prestemon 2012), increasing human health outcomes (Nilsson et al. 2011; Donovan et al. 2013), increasing property values (Maco and McPherson 2003), reducing stormwater runoff and combined sewer overflows (Xiao et al. 1998; De Sousa et al. 2012), sequestering carbon and air pollution (Nowak and Crane 2002; Nowak et al. 2013), and reducing the urban heat island effect (Livesley et al. 2016). Diversifying urban tree species is a key driver to building a sustainable and resilient urban forest (Raupp et al. 2006; Wood and Dupras 2021).
The Current State of the Urban Forest in the Milwaukee Metropolitan Area
In the Milwaukee metropolitan area, 2 historical events have exposed the vulnerability of the urban forest within the last century. Dutch elm disease (Ophiostoma ulmi, Ophiostoma himal-ulmi, Ophiostoma novo-ulmi) and emerald ash borer (Agrilus planipennis) both caused mass mortality amongst urban trees and resulted in enormous losses in ecosystem benefits (Sivyer 2010; Hauer et al. 2020). Learning from these lessons of the past, diversifying the urban forest should be one of the top goals for urban forestry managers in the Milwaukee metropolitan area and beyond.
The rich history of urban forestry in the Milwaukee metropolitan area, which includes long established urban forestry departments, educational institutions, and research, provides valuable information and data on how the current state of the urban forest came to be. This data can help combat future threats to the resilience of the area’s urban forest and expose potential vulnerabilities in its makeup. The lack of species diversity in the current tree population decreases its resiliency (increases its vulnerability) to both pulse and press disturbances across various temporal and spatial scales (Krouse 2010; Sivyer 2010; Hauer et al. 2020). With the potential impacts of climate change through the end of the century, paired with the ever-present possibility of future invasive pests and diseases, it becomes necessary to look to the past to understand the potential consequences of having a vulnerable urban forest.
The introduction of Dutch elm disease (DED) to the Milwaukee metropolitan area in the late 1950s had a devastating impact on the urban forest. Over a 40-year period from 1956 to 1996, nearly all of the 106,732 American elms (Ulmus americana) along Milwaukee city streets were either killed by DED or removed after showing symptoms (Hauer et. al. 2020). Through economic modeling, Hauer et al. (2020) estimated a $250 million loss in net present value over a 40-year simulated period. This modeling included associated maintenance costs, which Vogt et al. (2015) point out are a crucial step in the economic analysis of tree benefits.
The loss of the American elm urban tree population created a major void in the urban tree canopy, which in turn led to a significant loss of ecosystem services provided. An important lost ecosystem benefit was stormwater runoff reduction. Hauer et al. (2020) estimated that potentially 44% of the Milwaukee Metropolitan Sewerage District’s deep tunnel holding capacity could have been held by the existing American elm street tree population before the introduction of DED to the region in the 1950s. In total, the region has spent $4 billion to mitigate stormwater issues, some of which may have been abated through forest canopy retention. These environmental and economic implications, along with lost carbon storage, increased air pollution, and decreased energy savings, all point to the importance and value of urban trees (Hauer et al. 2020). DED exposed the vulnerability within the Milwaukee metropolitan area urban forest. Like many other urban areas, Milwaukee lacked tree diversity, which is one of the most important tools for creating a resilient urban forest in the face of exotic pests and diseases (Raupp et al. 2006).
Similar to DED, the discovery of emerald ash borer (EAB) in the area in 2008 again exposed the vulnerabilities of the urban forest canopy. The lack of species diversity led to a large loss of urban tree canopy cover, and thus huge amounts of lost ecosystem values. At the time EAB entered the region, around 17% of the City of Milwaukee’s urban forest consisted of EAB-susceptible ash species (Fraxinus spp.) (Souci et al. 2009). A 2008 i-Tree UFORE analysis revealed a $221 million structural value of ash species within the city alone (i-Tree 2008). With a nearly 100% mortality rate from EAB over the last 2 decades, southeastern Wisconsin, along with many communities across the United States, has been nearly depleted of all untreated ash species. The loss of these species equates to billions of dollars of lost ecosystem services, mitigation costs, and economic burdens (Kovacs et al. 2010; Vannatta et al. 2012).
Climate Ready Trees
Until recently, there has been a lack of discussion around climate change and urban trees, despite the role urban trees play in mitigating, adapting to, and being impacted by climate change (Krajter Ostoić and Konijnendijk van den Bosch 2015). Several studies have looked at the potential impacts of climate change on things like temperature, cold hardiness zones, and heat zones around the world, and the implications these changes will have on urban trees already burdened with urban stressors (Yang 2009; Ordóñez and Duinker 2014; Brandt et al. 2017; McPherson et al. 2018; Swanston et al. 2018; Brandt et al. 2021).
Though many studies on urban trees and climate change focus on species composition, it is important to note that a vast array of other factors influence urban tree vulnerability. These other factors include organizational capacity, maintenance practices, structural composition, and economic considerations (Brandt et al. 2021). It is not always as simple as identifying a species that is adaptable and well suited for future climate projections and then planting that species. For example, several elm cultivars and hybrids are extremely adaptable and suitable for a wide range of climatic zones, but also require significant structural pruning. Municipalities and organizations may not have the allocated resources, labor, expertise, and/or budget to keep up with the maintenance needs of trees like those elms (Giblin and Johnson 2017). Additionally, the commercial introduction of new cultivars and hybrids can make it difficult for urban forestry practitioners to predict adaptability and zone suitability across different cultivars, and different regionally sourced specimens, all within the same species. Furthermore, vulnerability to invasive pests and diseases is an ongoing concern, regardless of their adaptability to urban environments and their suitability to projected climate change.
Readiness of Milwaukee’s Urban Forest
Going forward, adaptable and climate-ready tree species should be identified and utilized in the urban landscape to avoid large potential losses in urban canopy due to climate change, invasive pests and diseases, and other disturbances. These climate-ready tree lists should be tailored to local species diversity, tolerance to local urban stressors, and nursery stock availability. The first step in this process is to survey existing inventories to identify their vulnerability to climate change and other disturbances. This study began by collecting, organizing, and analyzing tree data from multiple organizations within the Milwaukee metropolitan area, specifically exploring tree diversity across various tree diameter classes. This inventory data was then analyzed for climate readiness, and an established vulnerability framework was applied to the inventory to identify the current and future resilience of the urban forest in the face of climate change and other urban forest threats through the end of the century. Finally, additional analysis was performed to look at the relationship between tree diameter classes and resilience to climate change. By compiling all available inventories, assessing the current diversity of the species contained in them, and assessing those trees for various climate change scenarios, urban forest professionals will be able to use these analyses to make better informed planting decisions.
Materials and Methods
Study Area
The Milwaukee metropolitan area (Figure 1) as defined by the US Census Bureau (Census Reporter 2021) includes Milwaukee, Ozaukee, Washington, and Waukesha counties. The study area is 2,341 km2 and is home to 1.5 million people (Census Reporter 2021). The entire Milwaukee metropolitan area was selected for this study for a number of reasons. Many previous vulnerability studies and tree inventory analyses have focused on either individual cities or on multiple cities spanning large geographical areas. However, this study addressed multiple organizations that exist in close proximity. Topography, watersheds, and ecosystems are not confined by municipal and/or organizational boundaries, and all of the organizations within the Milwaukee metropolitan area interact with each other on a variety of levels. In particular, many of the municipalities used in this study are part of the Milwaukee Metropolitan Sewerage District, and a large concern for this area is water quality and the historical effects of combined sewer overflows and flooding. The entire Milwaukee metropolitan area contributes to the health of the watersheds and water quality, and large amounts of money have been invested in stormwater mitigation efforts.
The extent of the Milwaukee metropolitan area, covering 4 counties in southeastern Wisconsin. The points represent inventoried trees that were collected for this study.
Inventory Data Collection
Tree inventory data were collected from 40 organizations with known spatially referenced tree inventories, including towns, villages, cities, cemeteries, colleges, and county park systems. The vast majority of the trees were located along streets, and the majority of the organizations were publicly managed municipalities. Tree inventories were collected directly from the municipality or organization, from private consulting companies, or from the Wisconsin Department of Natural Resources (DNR) through the Wisconsin Community Tree Map project. It was important to ensure that the data collected were as up-to-date and thorough as possible in order to get an accurate overview of the urban forest. As discussed earlier, the Milwaukee metropolitan area is recovering from the invasion of EAB, and any inventories containing pre-EAB data would not have given an accurate snapshot of the current urban forest. Of the inventories collected, more than 97% of the inventories had been actively updated within the last 5 years. All inventories had received updates within the past 8 years.
There were large discrepancies in the type of data available in each inventory, such as the lack of a single common field shared across all inventories. This required a large amount of standardized “data cleaning.” The aim of the inventory collection was to account for all spatially referenced living trees in the Milwaukee metropolitan area. Thus, all vacant sites, stumps, standing dead trees, and no-plant zones were removed from the dataset. As with any tree inventory system of this magnitude in a highly managed urban environment, trees are likely to be removed every single day for things such as storm damage, disease, and construction. However, given the currency of the data, this inventory provides a highly accurate snapshot of the Milwaukee metropolitan area over the last 5 to 10 years. Any tree species that made up less than 0.05% (219 trees) was eliminated from the final dataset before applying the vulnerability framework. However, these species were still included in the total count for calculating percentages. This was also the case for data points that had “unknown” species, as well as trees identified only to the genus level, with the exception of apple/crabapple (Malus spp.), which has a vulnerability score within the framework. Specific cultivars that were not individually scored in the vulnerability framework were lumped together by their species (e.g., Norway maple [Acer platanoides] includes its ‘Crimson King’ and ‘Deborah’ cultivars).
Vulnerability Framework
Multiple studies have shown that vulnerability can be assessed in the urban forest in order to identify adaptive planning to increase resilience and identify vulnerabilities (Ordóñez and Duinker 2014; Brandt et al. 2017; Steenberg et al. 2017). The vulnerability framework (Figure 2) used in this study was created and deployed in Brandt et al. (2021). The framework combines an individual species’ exposure and sensitivity to projected hardiness zones and heat zones with the species’ ability to adapt to disturbances and biological factors in the urban environment (Brandt et al. 2021).
Vulnerability framework reproduced from Brandt et al. 2021.
United States Department of Agriculture hardiness zones and American Horticultural Society heat zones are useful markers for predicting zone suitability of tree species (Brandt et al. 2017). Hardiness zones are calculated using the annual minimum temperature, and heat zones are calculated based on the number of days above 30 °C. A dataset published in 2018 shows projected hardiness and heat zones for multiple timeframes throughout the end of the century (2070 to 2099)(Matthews et al. 2018). In the Brandt et al. (2021) study, this existing dataset was used to create statistically downscaled values at a one-eighth degree resolution. Brandt et al. (2021) used 2 different climate change scenarios in order to project future hardiness and heat zones. Each scenario was derived by pairing global climate models with Representative Concentration Pathways (RCP), trajectories of greenhouse gas concentrations based on potential social, economic, and political conditions. The low climate change scenario used the Community Climate System Model and paired it with the RCP 4.5 storyline of global greenhouse gas emissions peaking midcentury and atmospheric concentrations stabilizing shortly after 2100 (Moss et al. 2008; Gent et al. 2011; Brandt et al. 2021). Though RCP 4.5 is considered an intermediate pathway by the Intergovernmental Panel on Climate Change, this analysis did not use a more dramatic reduction in greenhouse gas concentrations (e.g., RCP 2.6) due to urban areas already being hotter than less developed, rural spaces (Moss et al. 2008; Brandt et al. 2021). The high climate change scenario used the RCP 8.5 storyline of increasing greenhouse gas emissions throughout the 21st century and was paired with the Geophysical Fluid Dynamics Laboratory CM3 model (Donner et al. 2011). The resulting low and high climate change scenarios in the Brandt et al. (2021) study will be referred to as Low and High for the remainder of the text.
The adaptive capacity of a species in Brandt et al. (2021) used qualitative scoring that builds upon adaptability assessments from Brandt et al. (2017) and Matthews et al. (2011). The adaptability scoring system utilizes 20 modification factors (Table 1), and each species is scored based on each modification factor’s influence on positive or negative effects of establishment, growth, and survival (Brandt et al. 2021). The scores for each modification factor were derived from horticultural, silvicultural, and street tree literature, including Gilman and Watson (1993), Burns and Honkala (1990a, 1990b), Hauer et al. (2006), and Duryea et al. (2007). The resulting scores were then subjected to expert review. For a full description of the framework process, consult Brandt et al. (2021). A few new species were scored specifically for this study, as they had not been previously scored.
Adaptability modification factors, codes, and descriptions. This table is an abbreviated version of Table 2 from Brandt et al. (2021).
For this study, 2 scenarios were applied to the collected inventory, a Low climate change scenario and a High climate change scenario. Both scenarios were projected to the end of the century (2070 to 2099). Because the study area was so large, the tree inventories that were collected spanned multiple hardiness and heat zones under different climate scenarios (Figure 3). For the initial analysis, a hardiness zone of 6 was projected for the entire Milwaukee metropolitan area under a Low climate change scenario, and this was paired with a heat zone projection of 7. For the High climate change scenario, zones 7 and 9 were used for the cold hardiness zone and heat zone respectively. Additional climate projections were run as well to assess variability among different communities within the Milwaukee metropolitan area.
The current and projected climatic conditions in the 4-county Milwaukee metropolitan area. Figure 3A shows plant hardiness zones, with hardiness zones corresponding to the minimum projected temperature an area would reach in the winter. Figure 3B shows heat zones, reflecting the number of days per year projected to have maximum temperatures ≥ 30 °C (86 °F). The Low (RCP 4.5) scenario anticipates significant reductions of greenhouse gas emissions over the 21st century, while the High (RCP 8.5) scenario assumes a business-as-usual continuous increase in emissions. Data for the figures from Matthews et al. (2018) and Matthews et al. (2019). Note that the modeled climate data has gaps that were filled for visualization purposes.
Results and Discussion
Inventory Data
The 10 most common species in the inventory collected were as follows: Norway maple (25.93%), honey locust (Gleditsia triacanthos)(9.46%), green ash (Fraxinus pennsylvanica)(9.40%), common hackberry (Celtis occidentalis)(3.75%), littleleaf linden (Tilia cordata)(3.08%), Freeman maple (Acer × freemanii)(2.75%), crabapple (2.59%), sugar maple (Acer saccharum)(2.37%), white ash (Fraxinus americana)(2.23%), and American basswood (Tilia americana)(2.21%)(Figure 4).
The top 20 most utilized trees in the Milwaukee metropolitan area inventory.
Although not necessarily part of this study, it was interesting to note the variance in species diversity based on the size of the organization, type of organization (cemetery, park system, municipality, etc.), and population density. Notably, Norway maples made up 38% of the City of Milwaukee’s inventory, which was 12% higher than the entire Milwaukee metropolitan area inventory. There were also large differences in the percentage of ash species among the organizations. Those municipalities that are actively treating large portions of their population of ash trees expectedly had much higher percentages than those that removed most of their ash trees or let them die as they were attacked by EAB.
Species diversity appears to increase dramatically when looking at smaller diameter trees. When considering only trees < 25.4 cm in diameter at breast height (DBH), no species made up more than 13.87% of the total inventory. The 10 most common species under 25.4 cm DBH were as follows: Norway maple (13.87%), honey locust (7.41%), Freeman maple (4.93%), hackberry (4.85%), crabapple (4.37%), Japanese tree lilac (Syringa reticulata)(4.24%), Callery pear (Pyrus calleryana)(3.62%), swamp white oak (Quercus bicolor)(3.13%), Kentucky coffeetree (Gymnocladus dioicus)(3.01%), and littleleaf linden (2.96%)(Figure 5).
The top 20 most utilized trees under 25.4cm DBH. Note the flattening of the inverse “J” shape in comparison to Figure 4.
Additional diversity data were analyzed for multiple diameter classes at 12.7-cm intervals (Figure 6). Note the flattening of the inverse “J” shape coinciding with smaller DBH classes. These diameter classes were then used to calculate the Shannon Diversity Index of each individual diameter classification (Figure 7).
Five different diameter classes are overlaid to show the differences in tree diversity.
The Shannon Diversity Index (H = −Σπ × ln[π]) of each diameter class. The larger the value, the greater the diversity. Notice the increase in values for lower diameter classes amongst the Milwaukee metropolitan tree inventory data. The Hutcheson t-test was also performed on each DBH class and showed statistically significant (P < 0.001) differences between each diameter class’s diversity (H). The error bars represent confidence intervals (95%) from the Hutcheson t-test.
Diameter has been used as a proxy for tree age (McPherson et al. 2016). Thus, the data may imply that younger trees and newer plantings represent higher species diversity. However, DBH to age relationships varies across different tree species (Morgenroth et al. 2020). Additional analyses are necessary to understand whether or not tree diversity increases with newly planted trees, or if the variations between the maximum growth diameters of different species, and differences in survivability to maturity, impact the differences in diversity across diameter classes.
Note that these figures include some genera besides crabapple, unlike the vulnerability data below. Hybrid and cultivated elm species, for example, make up a large portion of both the overall Milwaukee metropolitan area inventory as well as the smaller DBH classes. However, because there are so many commercially available cultivars and hybrids, no single elm cultivar or hybrid made it into the top 20 species in either inventory above. Most of these elm species have similarly high adaptive capacity and low vulnerability to climate change. Overall, it appears that species selection and diversification is increasing as urban forest management practices improve over time.
Vulnerability Projections
Using the initial Low climate change projection (cold hardiness zone 6 and heat zone 7), only one species, red pine (Pinus resinosa), was classified as having high vulnerability by the end of the century, which only accounted for 0.08% of the inventory. No species fell under moderate-high vulnerability with the Low projection. Some notable species classified as having moderate vulnerability under the Low climate projection included white ash, northern catalpa (Catalpa speciosa), tulip poplar (Liriodendron tulipifera), black cherry (Prunus serotina), and black alder (Alnus glutinosa). The moderately vulnerable species combined made up 3.08% of the overall inventory, or 13,551 trees. Overall, under a Low climate projection scenario, 90.15% of trees in the Milwaukee metropolitan area fell under the low to low-moderate vulnerability scores; this was true for all 30 of the most planted tree species with the exception of white ash (Table 2). When applying the High scenario to the Milwaukee metropolitan area inventory, the number of trees that fell under the low to low-moderate vulnerability score dropped to 34.72%. An additional 2 species, black alder and northern catalpa, entered the high vulnerability class, and a total of 16.06% of species fell under the moderate-high to high vulnerability rankings. Notable species classified as having moderate-high vulnerability under the High projection include sugar maple, American basswood, Callery pear, silver maple (Acer saccharinum), Colorado blue spruce (Picea pungens), northern white-cedar (Thuja occidentalis), English oak (Quercus robur), London planetree (Platanus × acerifolia), and white oak (Quercus alba). Perhaps most alarming, 55.52% of the trees inventoried in the Milwaukee metropolitan area (244,273 trees) are considered moderately, moderately-high, or highly vulnerable to the High climate change projection (Figure 8).
Vulnerability scores for the 30 most-planted species across the Milwaukee metropolitan area tree inventory.
Vulnerability comparison of the inventory under Low (RCP 4.5) and High (RCP 8.5) climate scenarios.
Additional vulnerability scores were applied for 2 distinct diameter classes, trees that were under 25.4 cm DBH and those under 12.7 cm DBH, and a comparison was made between the 3 diameter classes as they relate to vulnerability within the canopy under a Low scenario (Figure 9A). Likewise, a comparison was made between the 3 diameter classes as they relate to vulnerability within the canopy under the High scenario (Figure 9B). Vulnerability does not appear to be a strong function of diameter within the inventory. Percentages varied among the 3 diameter classes as they relate to vulnerability to zone suitability and adaptive capacity among both climate scenarios. For example, while the percentage of trees that fell under the moderate, moderate-high, and high vulnerability categories combined decreased slightly within smaller diameter classes in the High scenario, the percentage of trees that fell under moderate-high and high vulnerability categories actually increased within smaller diameter classes under the High scenario. There was no significant association observed when comparing the mean and median of DBH within each vulnerability category for the entire inventory under both scenarios.
(A) Vulnerability comparison of different diameter classes under a Low (RCP 4.5) climate scenario. (B) Vulnerability comparison of different diameter classes under a High (RCP 8.5) climate scenario.
It is important to note that the only genus-level vulnerability score available is for crabapples, and due to the inconsistency in data collection methodologies across inventories, 7.03% of the inventory was only identified to an unscorable genus level, with the most prominent being Ulmus and Tilia.
Since the Milwaukee metropolitan area spans multiple projections of hardiness and heat zones, additional models for various zone projections were also run for the study area. This process can be used in the future to allow individual organizations and communities within the larger Milwaukee metropolitan area to assess the vulnerability of their trees. When running the entire inventory through these variations based on the geographical location of each individual tree, the vulnerability percentages of the entire inventory vary slightly within High projections. They did not change in the Low projections.
Study Limitations
Several limitations exist with a study of this magnitude. As previously noted, urban tree canopies are constantly changing as general maintenance activities occur, and trees fail and die for a myriad of reasons. Thus, a tree inventory will never be a perfect model for the actual canopy at any given time. One of the biggest limitations to obtaining a true vulnerability percentage for a given canopy is the lack of quality data in species identification. Over 7% of the 439,974 trees were only identified to the genus level (this percentage does not include the “scoreable” Malus genus), which inhibits the ability to score these species to the vulnerability framework. Other limitations included some inventories lacking tree diameter data. This study highlights the importance of high quality and standardized data collection and organization within tree inventories. It should be noted that when looking at smaller diameter classes, and those species planted more recently, the percentage of overall trees identified at a species level increased when compared to the overall inventory.
Conclusion
The Milwaukee metropolitan area tree inventory data reveal a poor distribution among urban forest canopy species with a large percentage of the overall inventory consisting of a select few species. However, the negative correlation between diversity and DBH may indicate an increase in diversity amongst the more recently planted trees. The increased evenness of species distribution can potentially increase the urban forest’s future resilience to pulse and press disturbances. Future press disturbances that impact a species, or perhaps 2 or more species in the same genus, may not have the same economic, environmental, and social impacts as past press disturbances such as DED and EAB.
Climate change has the potential to act as a press disturbance to a wide variety of tree species through the end of the century. This study applies the latest vulnerability framework described in Brandt et al. (2021) to the tree inventory compiled for the Milwaukee metropolitan area. This framework scores individual species based on their adaptability to a wide range of modification factors, as well as their suitability to projected changes in cold hardiness and heat zones through the end of the century. This is the first study that the authors are aware of that applies this vulnerability framework to an entire metropolitan area, made up of up-to-date inventories from a wide range of municipalities and other public and private organizations. Similar to previous studies, under Low climate change projections the vast majority of the tree species, and the vast majority of the overall inventory (90.15%) that make up the Milwaukee metropolitan area, are not considered vulnerable. However, under the High climate change projection, over half of the inventoried trees (55.52%) fell between moderately and highly vulnerable. Additional analysis shows that smaller diameter classes are equally vulnerable under the High climate change projection. This highlights the fact that although urban forestry practitioners appear to be diversifying their new plantings, they are not necessarily planting species that can withstand a High climate change scenario.
The Milwaukee metropolitan area is dealing with the effects of emerald ash borer. As evidenced by the tree inventories collected, there remains a large percentage of living susceptible ash trees (11.63%). Regardless of these ash species’ vulnerability to climate change, they will not be viable species for new plantings, unless EAB-resistant varieties are propagated and planted in the future. This means that over the next century, over 11% of the inventory will become available for new species plantings. Urban forest practitioners should use this opportunity to increase species diversification, as well as select species that are not vulnerable under climate change projections. The Wisconsin DNR and Brandt et al. (2021) have both identified underutilized climate ready tree species for Wisconsin communities, and these should be considered when selecting species for future plantings.
Although this study did not look at vacant planting sites specifically, there was evidence of a large portion of the Milwaukee metropolitan area urban forest available for new plantings. This may be a result of large numbers of ash removals due to EAB. The City of Milwaukee alone had 4,617 sites (2.26% of their inventory) available for planting. Similar percentages were seen with other organizations that tracked planting site data. When a conservative 2% planting space projection is applied to the entire Milwaukee metropolitan area inventory, it equals 8,799 potential planting sites. Again, this presents an opportunity for the selection of a diverse and climate-resilient species composition for future plantings. As evidenced by the tree inventory analysis, it appears that urban forestry practitioners in the Milwaukee metropolitan area have already been diversifying their species selections for new plantings over the last few decades.
Managed urban tree inventories should continue to be evaluated for their adaptability and vulnerability over the next century. More consistent and reliable recommendations will continue to be developed with new climate models and with an improved understanding of different species’ ability to adapt to urban conditions.
Conflicts of Interest
The authors reported no conflicts of interest.
Acknowledgements
The authors would like to thank all of the organizations that provided their tree data for this study. The authors would also like to extend thanks to Dr. Leslie Brandt and Dr. Adrienne Keller for providing guidance and resources for this project and to Annamarie Rutledge for data curation.
- © 2023 International Society of Arboriculture
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