Arboriculture & Urban Forestry 38(5): September 2012 mum tree diameter (r = -0.408). There was not a strong separation among land use categories in the ordination space, but presettle- ment vegetation condition did show a relatively strong gradient (Figure 2). MRPP found no significant difference in species com- position among the land use categories (A = 0.0001, p = 0.35), but there was a significant, but not especially strong, difference among presettlement vegetation classes (A = 0.004, p < 0.001). PCA of canopy variables yielded a two dimensional solution that explained 65.3% of the variation in the original data matrix and illustrated two strong gradients in canopy structure (Figure 3). Principal components 1 and 2 explained 45.2% and 20.1% of the variation, respectively, and were both highly significant based on randomization tests (eigenvalues 7.22 and 3.28, and both p = 0.001). The first axis was strongly associated with all of the structural variables except percent shrub cover and crown light exposure, and illustrates a strong separation of plots in variables associated with overall canopy volume (Figure 3). The second axis was strongly related to crown light exposure and displayed a dichotomy between plots with large open-grown trees and those with high total canopy volumes. However, none of the pre- dictor variables were strongly associated with either axis (all r < 0.1) and there was not strong separation among land use or presettlement categories in the ordination or based on MRPP analysis (land use: A = 0.001, p = 0.76; presettlement: A = 0.012, p < 0.001; A is the chance-corrected within-group agree- ment, values of <~0.1 are considered to indicate low associa- tion among group differences even with statistical significance). All species groups showed a strong negative exponen- tial distribution, although oaks had a much longer “tail” (i.e., greater abundance in large size classes; Figure 4a; Figure 4b). The magnitude of the stem densities in each size class dif- fered greatly between species groups, with smaller size classes strongly dominated by non-native and opportunistic native spe- cies such as buckthorn, box elder, and black cherry (Figure 4a; Figure 4c). Some oaks occurred in smaller size classes, but oaks only made up a majority of the stem density in the larg- est size classes (>70cm DBH; Figure 4b). Diameter distribu- tions of the three species categories differed significantly among land use categories (oak χ2 oak χ2 = 99.57, p < 0.001; non-native χ2 (r2 = 0.7582), and percent canopy cover (r2 = 15.69, p = 0.047; native non- = 133.37, p < 0.001). Basal area had significant (p < 0.001) log-linear positive corre- lations with canopy depth (r2 = 0.7571), maximum canopy height = 0.6581). Similar cor- relations with each canopy metric were also significant (p < 0.001) at the land use level for agriculture (r2 > 0.7124), parks (r2 > 0.7043), and residential (r2 > 0.4725), development (r2 > 0.6348). Be- cause these relationships were non-linear, a proportionally great- er loss of BA corresponded to a linear loss of canopy structure. Drivers of Composition and Structure Mean BA of oaks, native non-oaks, and non-native species var- ied significantly among land use and presettlement vegetation categories (Table 2). Native species had greater BA in parks and lower BA in developed areas, while non-native species had greater BA under agricultural land use. Only oak BA varied sig- nificantly between the PLS vegetation types, with greater BA in former timber habitat (Figure 5b). Only non-native species had a significant interaction, as their BA was greater in former tim- ber under agricultural land use, but greater in parks and residen- Figure 3. Principal components analysis ordination of canopy structure variables in Tree Census data, with symbols coded by presettlement condition of plots and location of group centroids for canopy structure variables indicated. Canopy variance is the variance to mean ratio of crown volumes within a plot. tial habitats that were formerly prairie (Table 2). Oak and non- native dominance varied significantly (Table 2) and inversely (Figure 5a), with greater oak dominance in parks and in former timber, and lower non-native dominance in parks and in timber. Among measures of species diversity, total species rich- ness and percent native richness varied significantly across the land use and vegetation types, with greater values in parks and former timber habitats, and lower values in developed ar- eas (Table 2). The ratio of native to alien species exceeded 1.0 only in parks, but with marginal (P = 0.08) significance. All ©2012 International Society of Arboriculture 185 Figure 2. Non-metric multidimensional scaling ordination of tree species composition in Tree Census data with symbols coded by presettlement condition of plots and correlated (r > 0.3) variables overlaid.
September 2012
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