232 Rogers et al: Species Diversity and Urban Forest Resilience in the Milwaukee Metropolitan Area 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 stud- ies 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, struc- tural composition, and economic considerations (Brandt et al. 2021). It is not always as simple as iden- tifying a species that is adaptable and well suited for future climate projections and then planting that spe- cies. For example, several elm cultivars and hybrids are extremely adaptable and suitable for a wide range of climatic zones, but also require significant struc- tural 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 spe- cies should be identified and utilized in the urban landscape to avoid large potential losses in urban can- opy 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 Milwau- kee metropolitan area, specifically exploring tree diversity across various tree diameter classes. This inventory data was then analyzed for climate readi- ness, 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 cli- mate 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 sce- narios, 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 organiza- tions 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
