Arboriculture & Urban Forestry 45(5): September 2019 urban tree mortality. The earliest of these that we found was a study by Beatty and Heckman (1981) in which the authors surveyed urban forest managers responsible for urban forest programs across the United States on the major causes of tree health and survival issues. The most commonly cited issues were lack of water, nutrient deficiency, and vandal- ism. Some respondents also provided basic mortality information, from which those studies’ researchers concluded that larger cities experience higher mortality, while western states and regions with milder climates experience lower mortality. Notably, such conclu- sions concerning regional mortality trends have not been confirmed or refuted by more recent research. Kirkpatrick et al. (2013) examined residents’ atti- tudes regarding trees in eastern Australia, the amount of removals over the previous five years, and reasons for removals. They found that the main impetus for removal of healthy trees were aesthetic and lifestyle choices. Similarly, Conway (2016) surveyed residents in a suburb of Toronto, Ontario, Canada, to under- stand motivations for planting and removing trees. The author found that most removals were due to concerns about poor tree health, and the second most common reason was property concerns, both perceived risk and actual damage caused by a tree. An example of a one-time inventory is the study conducted by Gilbertson and Bradshaw (1985) on trees in northern England. They observed site factors that could have impacted trees, concluding that van- dalism (18% of dead trees showed signs) and water and nutrient stress (56% of dead trees showed signs) likely played a role, and that it is possible that the stress from weeds and tie strangulation could have been weakening the trees and predisposing them to vandalism. However, it was not made clear whether these observations were made on standing dead trees or severely impaired trees. Seven studies investigated the effects of natural disasters like storms and earth- quakes by conducting one-time inventories following the event. Hauer et al. (1993) conducted an inventory of street tree condition and removals after a major ice storm in Urbana, IL, finding that species, tree form, branch architecture, and the presence/absence of defects all impacted the severity of damage. Jim and Liu (1997) conducted an inventory of trees damaged after a major storm in Guangzhou, China, noting the severity of damage, and found that species, trees size, and development history all influenced storm damage susceptibility. Duryea et al. (1996; 1997; 2007) 181 combined information from surveys of homeowners and residents along with field visits to properties to assess damage done to trees following eight major hurricanes hitting Florida and Puerto Rico. Those studies concluded that taxa, nativity, wood density, crown density, growth form, pruning, and growing in a cluster were all significantly associated with mortal- ity (Table 4). Earthquake damage to trees was inves- tigated by Morgenroth and Armstrong (2012), who studied removal records of trees in city parks in Christchurch, New Zealand. They found that the removed trees comprised of 9% juvenile, 9% semi-mature, 61% mature, and 21% over-mature treesn and that 88% of all leaning trees were mature or over-mature. Leksungnoen et al. (2017) studied trees in Bangkok, Thailand following severe flooding in 2011. The researchers categorized trees as either flood susceptible (> 50% mortality), tolerant (less than or equal to 50% mortality), or highly tolerant (no mortality after the flood). They found 18% of species to be flood susceptible, 75% tolerant, and 7% highly tolerant. In addition to survey-based studies and one-time inventories, we found three other unique studies. Pol- anin (1991) studied a sample of trees that had removal records in Jersey City, NJ, and found that Platanus × acerifolia were most often removed due to sidewalk upheaval (i.e., the tree was removed while it was still alive due to infrastructure conflicts), and Acer plata- noides were more often removed due to death. Helama et al. (2012) conducted a dendrochronology study on Scots pine (Pinus sylvestris L.) trees in a park lawn in Helsinki, Finland, in which they con- cluded that competition from other trees was a likely predisposing factor for mortality and drought as incit- ing factor. Morgenroth et al. (2017) compared the presence and absence of individual trees before and after earthquake-related demolition to examine removals. Using aerial imagery and field visits, they concluded that 78.4% of the original trees remained after demolition activity. Relating Urban Tree Mortality to the Disease-Decline Model To relate the urban tree mortality literature to the Manion’s (1981) disease-decline model, we propose a new framework for urban tree mortality which groups human and biophysical factors as predispos- ing, inciting, and contributing (Figure 6). Some fac- tors that we listed as inciting might function as ©2019 International Society of Arboriculture
September 2019
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