Arboriculture & Urban Forestry 45(1): January 2019 decline of ash might also inflate the influence of some other exposure indicators. Nonetheless, this study’s findings suggest that quantifying known biological exposures would be beneficial in future vulnerability assessments, given the high levels of decline and mortality of green ash attributable to EAB. One finding that ran contrary to the a priori vulnerability assumptions was the exceed- ingly high survival rate of trees in conflict with over- head utility wires, compared to those that were not. This may be due to the hardiness of the species selected for street tree plantings. However, the study authors offer the one theory requiring further investi- gation: that trees most often in conflict were the municipally-owned, larger trees in the public right- of-way. Despite the conflict with utility wires, more frequent maintenance of these trees by urban forest practitioners could potentially explain this trend, but this requires further research to be substantiated. Urban forest structural elements that characterize sensitivity were found to be valuable in examining overall vulnerability. Specifically, tree condition was a highly influential predictor of mortality and diame- ter growth. This finding confirms existing research supporting condition as an effective predictor of mor- tality (Koeser et al. 2013). This finding also suggests that more detailed frameworks for assessing tree con- dition (i.e., not just percent crown dieback) are valu- able. Conversely, the findings also highlight important drivers of condition decline, such as poor manage- ment and vandalism, where poor management was most often identified as improper pruning practices and vandalism as torn branches on smaller trees (Lu et al. 2010). Decline, mortality, and vulnerability of the studied trees were likely a function of the compo- sition and age distribution of the neighborhood and tolerance of individual species to urban conditions (e.g., high tolerance of honeylocust, and therefore low sensitivity and minimal mortality; Burns and Honkala 1990). One notable species-level effect was the much higher likelihood of mortality for in-grown species (e.g., tree-of-heaven), which emphasizes the importance of differentiating between planted and in-grown trees in urban forest vulnerability assessment. Tree size was a highly influential metric of urban forest sensitivity, both in its interaction with expo- sures and as a predictor of tree condition. Trees in the smallest size class had by far the highest mortality rates, as might be expected (Roman and Scatena 2011). 21 Additionally, assessed trees were consistently in poor condition with increasing diameter. Larger sites with more open greenspace, and those that were farther from adjacent buildings, were more likely to have larger trees, and therefore trees in poor condition, despite more suitable growing conditions than higher- density commercial areas. Again this highlights the influence of specific conditions in Harbord Village, and subsequently limits further generalizations. How- ever, declining tree condition with age is an estab- lished pattern (Nowak et al. 2004), which suggests higher sensitivity and subsequent vulnerability of mature urban forest ecosystems, often found in older, established residential neighborhoods. Importantly, it may also reveal that the processes driving decline in tree condition may sometimes differ from those driv- ing mortality. Overall, adaptive capacity indicators were less influential on ecological change and vulnerability than exposure and sensitivity indicators. For one, they were limited by the scale of available socioeco- nomic data (i.e., census dissemination areas as opposed to households). However, this limitation does not preclude them from being important in long- term urban forest vulnerability. Many studies support a strong positive relationship of both urban forest structure and stewardship with the socio-demographic characteristics of city residents at broader spatial scales (e.g., Grove et al. 2006; Troy et al. 2007; Con- way et al. 2011; Greene et al. 2011; Schwarz et al. 2015). Political processes are also important drivers of urban forest distribution and stewardship. For instance, Kendal et al. (2012) found that income inequality in tree cover distribution was more pro- nounced in public streetscapes than in residential properties. Füssel (2010) emphasizes that while observed empirical data are more objective and reli- able, they cannot reveal all aspects of system vulner- ability, especially long-term risks. It is likely that the comparatively short time span (e.g., six to seven years) between tree inventories in this study, as well as the spatial scale of data used in the analysis, might explain this lower influence of adaptive capacity on ecological change. Importantly, the quantitative nature and specific indicators of the Steenberg et al. (2017a) vulnerability framework restrict the concep- tion of adaptive capacity considerably, especially given its emphasis on census data, and might imply that adaptive capacity is restricted to affluent communities. ©2019 International Society of Arboriculture
January 2019
| Title Name |
Pages |
Delete |
Url |
| Empty |
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success. You will be contacted by Washington Gas with follow-up information regarding your request.
This process might take longer please wait
