94 have a high natural resistance to hurricanes. Moreover, St. John’s routinely experiences high winds (but not hurricane events), and so native species may be expected to have evolved some resis- tance to wind disturbance. At the time the hurricane struck, the deciduous trees were still in full-leaf, and researchers did not expect crown streamlining to have a major effect between co- nifers and deciduous species. Overall, the authors expected to see differences between native and non-native species as well as between conifers and non-conifers. Across species, larger trees (both in terms of height and DBH) and those that were less slender (lower height to DBH ratio) were expected to be more susceptible to falling or snapping. Age and condition were also expected to be a factor with older trees and those with rot or re- cent spanworm damage (Fry et al. 2008) being more susceptible. METHODS Study Area The study area included the cities of St. John’s and Mount Pearl, and the town of Paradise, on the Avalon Peninsula, is- land of Newfoundland, Canada (Figure 2). The population of this region is approximately 190,000 (Statistics Canada 2011 census). While the downtown city of St. John’s is row housing, and there are apartment dwellings throughout the urban areas, most of the sample sites fell within areas with suburban devel- opment and single-family dwellings, in nearby public parks, or in small tracts of forest in close proximity to subdivisions. Locations of all sample sites are shown in Figure 3; locations were sampled with a handheld GPS unit (Garmin 76map). Wiersma et al.: Factors Influencing Tree Fall in an Urban Setting to control for microsite variation (e.g., slope, topographic po- sition) because the study authors were interested in how trees vary in their response by species across the larger area of impact. At each site, data were recorded on a randomly selected fallen (uprooted or snapped) tree, as well as on the nearest standing tree. The assumption was that trees in close proximity to each other would have been exposed to the same localized wind pat- terns. The authors were not able to control for sampling adja- cent trees in the same wind direction. Where there was a group of trees that had fallen together and evidence that this was due to root interlocking (often part of the root mass was exposed) or one tree knocking down a neighbour, data were measured from the nearest tree still standing. Thus the pair-wise analy- sis of a single fallen and single adjacent standing tree was used to test whether tree properties of species, height, DBH, slen- derness, and age contributed to the propensity for a tree to be uprooted or snapped. Data collected included species [Ryan (1978) and Farrar (1995) were used for reference], location (as sampled with a GPS unit), height/length, diameter, and distance to nearest standing tree. Once data were entered, the slender- ness (ratio of height to diameter was calculated). The same data were recorded for standing trees. In addition, for the trees that were uprooted, root depth and diameter were measured. Any notes on condition (evidence of past spanworm damage, core rot, etc.) were also noted for both standing and downed trees. Following compilation of field data, all sites were revisited to verify species identification and to take a tree core for ag- ing. All cores were counted with a 10× hand lens by a single member of the team. Cores were recounted (blind) by a second researcher. In the event of a difference >5 years between age es- timates by the two researchers, the second researcher re-counted a second time (again blind); and if second re-count was within <5 years of first re-count, then the mean of the two re-counts of the second researcher (who had more experience) were taken. Statistical Analysis Researchers carried out all statistical analysis using MINITAB (v. 15). Trees were categorized into native/non-native classes and into coniferous/deciduous in two separate analyses. Bi- nary logistic regression was carried out to test for differences in category (native/non-native or coniferous/deciduous) and in properties of height, DBH, slenderness, age, and distance to nearest standing tree. Two sets of models were run within each analysis: one including tree height and DBH but not slender- ness, and the other with slenderness included. This was to ac- count for the fact that many species do not show a high degree of variation in slenderness (Kane 2008). Post-hoc goodness- of-fit tests (Pearson χ2 ) were applied and measures of associa- Figure 2. Map of the study area (dark rectangle) on the north- east Avalon Peninsula, on the island of Newfoundland, Canada. The inset map shows the location of Newfoundland relative to the rest of Canada. Tree Sampling Tree sampling was carried out on the weekend following the storm (September 25–26, 2010); researchers randomly selected sites to visit in the greater St. John’s area. There was no attempt ©2012 International Society of Arboriculture tion tested. A second set of tests tested against age combined with a quadratic term for age to test whether the youngest and oldest trees had a different response than mid-aged trees. RESULTS A total of 35 sites were visited, for a total of 70 trees surveyed (35 standing and 35 downed). Dominant species were bal- sam fir (24 trees; 13 of which were uprooted or snapped) and black spruce (13 trees; 7 of which were uprooted or snapped). Of the 35 uprooted/snapped trees, 24 were conifers, and 28 were native species. However, 23 of the 35 undamaged trees
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