168 socioeconomic conditions and causal mechanisms. Furthermore, in the urban context, the word “mortality” connotes both death and pre-death removal of trees (Roman et al. 2016); whereas removal is not part of natural (and unlogged) forest systems. In other words, purposeful removals by humans can be a key element of urban tree mortality. Indeed, for many urban tree mortality field monitoring studies, mortality has been defined as a combination of trees observed standing dead plus those observed removed (e.g., Nowak et al. 2004; Lima et al. 2013; Roman et al. 2014a; Ko et al. 2015a; Escobedo et al. 2016; Boukili et al. 2017). Moreover, the site conditions found in urban areas can often be more challenging than those found in natural forest areas, such as compacted soils and low nutrient availability (Urban 2008; Scharenbroch et al. 2017). At the same time, urban trees in maintained and landscaped areas (i.e., not urban trees in closed- canopy wooded park settings or afforested areas) can be given advantages, such as reduced competition for light, as well as supplemental irrigation and fertilizer. Indeed, such tree maintenance is fundamental to arboricultural best practices (Ferrini et al. 2017). Considering the many stresses and advantages for trees growing in the “urban environment” of Man- ion’s (1981) book, a comprehensive literature review of the factors affecting urban tree mortality is war- ranted to re-conceptualize the mortality process in the urban context. Given the complicated nature of the social-ecological systems in which urban trees exist (Pickett et al. 1997; Mincey et al. 2013; Vogt et al. 2015a), factors influ- encing mortality can be described as being human- related, biophysical, or a combination of the two. Biophysical predictive factors of urban tree mortality include species or other taxonomic groups, functional groups (e.g., hardwoods vs. softwoods), drought tol- erance, tree size, and time since planting (e.g., Nowak et al. 2004; Koeser et al. 2014; Roman et al. 2014a; Roman et al. 2014b). Human-related factors include land use, construction and development, and steward- ship or maintenance activities (e.g., Hauer 1994; Nowak et al. 2004; Boyce 2010; Lawrence et al. 2012; Koeser et al. 2014; Roman et al. 2014b). Human-related and biophysical factors can be deeply coupled. For instance, species and site selection choices by tree professionals and residents relate to later susceptibility to drought, but irrigation may enable trees to survive in regions with varying precipitation patterns (Roman ©2019 International Society of Arboriculture Hilbert et al: Urban Tree Mortality: A Literature Review et al. 2014b; Koeser et al. 2014; Mincey and Vogt 2014; Vogt et al. 2015a; Martin et al. 2016). Urban tree mortality can also be classified by life stages, such as establishment-related losses (Richards 1979). The establishment period—the first few years after tree planting (Sherman et al. 2016; Levinnson et al. 2017; Harris and Day 2017; Leers et al. 2018) —is generally viewed as the life stage with the highest mortality for urban trees and has thus been the focus of many mortality studies (e.g., Nowak et al. 1990; Struve et al. 1995; Koeser et al. 2014; Roman et al. 2014b; Roman et al. 2015; Widney et al. 2016). The establishment stage for planted urban trees parallels with classic concepts in forest ecology, where younger and smaller trees have the highest mortality (Franklin et al. 1987; Lines et al. 2010). Trees in natural forests generally have a U-shaped or Type III mortality curve (e.g., Coomes and Allen 2007; Lorimer et al. 2001; Metcalf et al. 2009; Lines et al. 2010). The U-shaped mortality curve has high mortality rates for small trees, low for mid-sized and mature trees, and rising mortality rates for very large trees, whereas the Type III curve similarly has high mortality rates for small trees, and low mortality rates for all other sizes (Harcombe 1987). For either mortality curve shape, forest ecol- ogy studies generally report annual mortality rates of 1 to 3%, or even less, for mature overstory or canopy trees (e.g., Harcombe and Marks 1983; Condit et al. 1995; Lorimer et al. 2001). Following in this reason- ing, Lugo and Scatena (1996) grouped causal factors for mortality in natural forests in the tropics based on intensity levels, with background annual tree mortality less than 5% and catastrophic greater than 5%. In the urban context, catastrophic factors include disease outbreaks (Poland and McCullough 2006), major storms (Staudhammer et al. 2011), and even war (Laçan and McBride 2009; Stilgenbauer and McBride 2010). Background causes are more gradual and could include a tree’s slow decline due to construc- tion-related stress or other adverse site conditions (Koeser et al. 2013). Yet it is possible that the back- ground rate of mortality is higher in urban environ- ments compared to natural forests. For instance, a previous meta-analysis of street tree survival found typical annual mortality to be 3.5 to 5.1% (Roman and Scatena 2011), while Nowak et al. (2004) observed 6.6% annual mortality across all land uses in Balti- more, MD (including both planted trees in landscaped areas and naturally regenerating trees in natural
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