Arboriculture & Urban Forestry 37(5): September 2011 not be determined. Excluding the embedded trees and saplings, t-tests for differences among means as well as Pearson’s product moment correlations were calculated for basal area, root plate diameter, root plate depth, and percent slope measurements us- ing the classifications of soil slide or rockslide as well as surface root system species or tap and heart root systems species. The χ2 tests, Pearson’s product moment correlation analysis, and t- tests were performed using PASW Statistics (formerly SPSS Sta- tistics) version 17 and the significance level selected was 0.05. RESULTS test results did not reveal significant differences for spe- cies of trees and saplings among the rockslides, soil slides, and Natural Area survey. Sugar maple, common hackberry, and white ash had the first, second, and third highest number of trees, re- spectively, in the Natural Area survey as well as the rockslides and soil slides. The rockslides and soil slides had significantly more trees and fewer saplings than found in the Natural Area survey. Also, the soil slides had significantly more saplings and fewer trees than the rockslides (Figure 1). The numbers of sugar maple trees and saplings was equal in soil slides, but there were nearly four times as many trees as saplings in rockslides, which accounts for most of the difference between saplings and trees for rockslides and soil slides. The five taxa comprising the surface root system species (red maple, sugar maple, common hackberry, sugarberry, and willow) had slightly more tree and sapling stems than the 21 species with tap or heart root systems. The surface root system species comprised 74% of the saplings (Table 1). Species Composition Changes Following Landslides The χ2 215 Root Plates Differences for Soil Slides and Rockslides The means for root plate diameter, root plate depth, and percent slope were significantly smaller for soil slides than rockslides; however, mean basal area was not significant- ly different between soil slides and rockslides (Table 2) even though there were more than twice as many saplings in soil slides than rockslides (Table 1). The linear correla- tions among basal area, root plate diameter, and root plate depth were significant for both rockslides and soil slides. In contrast, there were no significant linear correlations for basal area, root plate diameter, and root plate depth with percent slope for rockslides and soil slides (Table 3). Root System Type and Urban Landslides Basal area and root plate depth means were significantly smaller for the surface root systems species than tap and heart root systems species, but root plate diameter and per- cent slope means were not significantly different (Table 4). Each linear correlation analysis of basal area, root plate diameter, and root plate depth for surface root system spe- cies and tap and heart root systems species resulted in significant positive correlation coefficients. As might be expected, an increase in tree size is associated with an in- crease in root plate mass for the surface root system species as well as the tap and heart root system species. The cor- relation analysis of percent slope with basal area, root plate diameter, and root plate depth for surface root system spe- cies and tap and heart root systems species showed the re- sults were not significant except for percent slope with root plate depth for tap and heart root systems species (Table 5). Table 2. Means and standard deviations for basal area, root plate diameter, root plate depth, and percent slope for trees and saplings in rockslides and soil slides. Rockslides (n = 124) Mean Standard deviation Basal area (dm2 ) Root plate diameter (m) 3.8 Root plate depth (m) Percent slope z 14.6 1.6 31.1 13.7 2.4 0.8 11.1 Soil slides (n = 129) Mean Standard deviation 12.2 3.1z 1.3z 21.1z 13.6 1.8 0.8 15.8 Mean for soil slides is significantly (<0.05 level, two-tailed t-test with different numbers of samples and variances) smaller than mean for rockslides. Table 3. Linear correlation coefficients for basal area, root plate diameter, root plate depth, and percent slope for trees and saplings in rockslides (below the center diagonal of the matrix) and soil slides (above the center diagonal of the matrix). Basal area Root plate Root plate Percent diameter depth Basal area (dm2 ) Root plate diameter (m) 0.59z Root plate depth (m) Percent slope 0.59z 0.07 Figure 1. Trees and saplings in soil slides, rockslides, and Radnor Lake State Natural Area survey (Loeb et al. 2010). Natural Area survey results were transformed from the original totals with division by 22. ©2011 International Society of Arboriculture 0.70z 0.50z -0.01 0.53z 0.47z 0.10 z Correlation coefficient is significant (<0.05 level, two-tailed test). slope 0.05 -0.05 -0.02
September 2011
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