18 Steenberg et al: A Social-Ecological Analysis of Urban Tree Vulnerability of the >0.1-10.0 cm tree size class was anomalous. It should be noted that multiple-year DBH measure- ments and growth rates derived from there are likely to have high measurement error, which is a potential explanation for this anomaly. Of the measured trees present in both the 2007/2008 and 2014 inventories, Norway maple was the most abundant (Figure 2). White cedar exceeded Norway maple in 2014 in abundance when trees planted during the time between inventories were incorpo- rated. Honeylocust, white cedar, Freeman maple, and littleleaf linden (Tilia cordata) all increased in popu- lation size when planted trees were incorporated, while Norway maple, green ash, silver maple, hor- sechestnut, tree-of-heaven, and white mulberry decreased. No planted green ash, horsechestnut, tree- of-heaven, or white mulberry were observed. Tree- of-heaven had a substantially higher mortality rate than other trees (Table 2), followed by green ash, both of which were higher than the study area average annual mortality rate of 2.4% (Table 2). Green ashes were in the worst condition, which was likely attrib- utable to the ongoing EAB infestation in the study area, while white cedar were consistently in better con- dition. Tree condition of other species was generally reflective of tree size, where consistently larger species (e.g., silver maple and horsechestnut) were in worse condition. The bivariate analysis revealed a number of signif- icant relationships between vulnerability indicators and tree mortality. Land use is known to be an influ- ential driver of urban forest structure and function, which was corroborated by the findings (Table 3). The χ2 test revealed that commercial land uses had a high occurrence of tree mortality (36 observed versus 22 expected), while institutional land uses had a lower occurrence (15 observed versus 20 expected). Distance to the nearest building and building type were other significant built environment indicators, with shorter distances being associated with higher mortality. The five conflict with infrastructure indica- tors all had significant relationships with mortality, yet some were counter to a priori vulnerability assumptions (i.e., increased mortality with conflict). In particular, the presence of conflicts with overhead utility wires had an observed 6 incidences of mortality compared to the expected value of 69. Tree mortality was much higher for trees in the smallest DBH class (67 observed versus 28 expected) and for green ash com- pared to other species (Table 3). Additionally, in-grown ©2019 International Society of Arboriculture trees were far more likely to experience mortality than planted trees. There were no significant relationships between mortality and adaptive capacity indicators. Tree condition had the highest number of signifi- cant relationships with the vulnerability indicators, many of which were associated with increasing inten- sity of the built environment, like land use and site type (Table 3). More impervious surface cover and larger sidewalks and streets were all associated with poor tree condition. Incidences of poor management (e.g., improper pruning, unremoved tethers causing damage), vandalism (e.g., torn branches), and con- flicts with sidewalks were also associated with poor tree condition, while conflicts with buildings were associated with better condition. Tree condition declined consistently with increasing DBH (Table 3). Green ash, silver maple, and horsechestnut were in worse condition, while white cedar and tree-of-heaven were in better condition. There were significant but fairly weak correlations of dwelling value, education, and open greenspace with tree condition (Table 3), although the relationship between education and tree condition was counter to vulnerability assumptions. With tree diameter growth rates, land use, site type, and building type were again found to have sig- nificant relationships (Table 3), with slower growth rates associated with higher-density commercial areas (i.e., commercial land uses and buildings). Multi-family residential land uses and apartment towers were asso- ciated with faster growth rates. Built area intensity was also associated with lower growth rates and greater distances from streets with higher ones. Simi- lar to the counterintuitive mortality results, trees in conflicts with buildings and other types of infrastruc- ture were associated with faster growth rates. As expected, trees in poor condition had slower growth rates and growth rates declined with increasing DBH class (Table 3). The exception to the latter were trees in the smallest DBH class, which combined with the high mortality rate of these trees, is likely explained by transplant shock and establishment failure (Trow- bridge and Bassuk 2004). Open greenspace was asso- ciated with faster growth rates while the presence of stewardship activities (e.g., watering bags) were asso- ciated with lower growth rates (Table 3). The regression models predicting tree condition and growth rates in the multivariate analysis yielded some additional insight (Table 4). The condition model explained 32.1% of the variation in tree condi- tion. Evidence of poor management and DBH were
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