230 Rogers et al: Species Diversity and Urban Forest Resilience in the Milwaukee Metropolitan Area Arboriculture & Urban Forestry 2023. 49(5):230–246 https://doi.org/10.48044/jauf.2023.017 Examining Species Diversity and Urban Forest Resilience in the Milwaukee, Wisconsin (USA) Metropolitan Area By Elton C. Rogers, Paul D. Ries, EdD, and Daniel C. Buckler Abstract. Potential impacts from climate change and other disturbances expedite the need to address vulnerabilities of urban forests. Low spe- cies diversity is a contributor to high urban forest vulnerability, and this study examined 40 public and private tree inventories in the metropol- itan 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 cor- relations between tree size and diversity. The resulting data analysis revealed a poorly diversified urban tree canopy in the Milwaukee metro- politan 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 vulnera- bility 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. Keywords. Climate Change; Disturbance; Inventories; Resilience; Urban Forestry. Scientific Journal of the International Society of Arboriculture INTRODUCTION Social, Environmental, and Economic Benefits Urban trees provide social, economic, and environ- mental benefits to the urban ecosystem. These poten- tial benefits are reliant on the ability of urban trees to establish and thrive in the typically harsh urban grow- ing environment, an environment that is often sub- jected to highly disturbed and compacted soils, air pollution, de-icing salts, limited soil volume, infra- structure conflict, vandalism, and more. Additionally, ecological press and pulse disturbances can impact urban tree vulnerability. Pulse disturbances are rela- tively short-term events, such as an isolated insect outbreak or storm, which can lead to immediate mor- tality 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, envi- ronmental, 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 exceed- ing one million people more than tripled from the years 1950 to 2000 (Hobbs and Stoops 2002). With impervi- ous 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
