©2023 International Society of Arboriculture Arboriculture & Urban Forestry 49(5): September 2023 231 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 popula- tion 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 bene- fit was stormwater runoff reduction. Hauer et al. (2020) estimated that potentially 44% of the Milwau- kee Metropolitan Sewerage District’s deep tunnel holding capacity could have been held by the existing American elm street tree population before the intro- duction 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 stor- age, increased air pollution, and decreased energy savings, all point to the importance and value of urban trees (Hauer et al. 2020). DED exposed the vul- nerability within the Milwaukee metropolitan area urban forest. Like many other urban areas, Milwau- kee 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 can- opy 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 con- sisted 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 spe- cies 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 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 ecosys- tem 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 (Lives- ley 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 Milwau- kee 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 poten- tial 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
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