184 species richness variables (total richness, percent native species, and the ratio of native to alien species), and urban canopy char- acteristics (canopy depth and maximum height, depth and height variance to mean ratios, percent canopy cover and percent shrub cover). To approximate normal distributions, percentages were arcsine-square root transformed, while other values were log- or square-root transformed. Because of highly skewed data, and unbalanced design, the non-parametric Kruskal-Wallis test was used to compare species-level basal area of dominant trees sepa- rately within the land use and presettlement vegetation classes. This analysis tests the null hypothesis that all medians are equal, and does not provide an interaction test of factor independence. All GLM analyses were conducted using NCSS (Hintze 2003). Question #4: How have composition, structure, and species distributions changed from presettlement to the modern urban landscape? A GIS layer of U.S. census tract groups (an agglomeration of cen- sus tracts; U.S. Census Bureau 2011) was used to aggregate data for comparison of presettlement and modern vegetation compo- sition. Each tree census and land survey point was assigned to a census tract group based on its spatial location (tract groups had between 1 and 36 tree census points, mean of 10.4 points per tract group). Relative basal area data from each data set was then calculated at the tract group level and combined into a single matrix. An NMS ordination was performed on the tract group matrix with the same settings as above. Presettlement and mod- ern tract groups were connected with “transitional” vectors in the resulting ordination (McCune and Grace 2002). The length and direction of these vectors, which represent the change in species composition from presettlement to the modern landscape, were compared among modern land use categories using multivari- ate analysis of variance (MANOVA) with NCSS (Hintze 2003). Fahey et al.: Origins of the Chicago Urban Forest To assess spatial patterns in species transitions the presence/ absence of each species at the tract group level was determined for the modern tree census data. Researchers then calculated what proportion of tract groups that had a species present in the mod- ern data also had the species present in the PLS data and com- pared this value among species. Distributions of the major tree species were also compared visually between the two data sets. RESULTS Patterns of Species Composition and Structure Twenty species comprised more than 80% of the total basal area sampled by the Urban Tree Census (Table 1). Native non-oak species accounted for 44% of the total basal area, with domi- nance ranging from about 1.4% to 7%. Among this group, soft maple was the leading species, followed by box elder (Acer ne- gundo), cottonwood (Populus deltoides), and green ash. Bur oak, white oak, and red oak accounted for 21.5% of the basal area. Non-native species accounted for an additional 16% of basal area, in which buckthorn (Rhamnus cathartica) was the lead- ing species. Species dominance was much less even in the PLS (Table 1), with white oak accounting for 41.3% of basal area, fol- lowed by bur oak, American elm, red oak, and black walnut (Jug- lans nigra). By 2010, all oaks and American elm had declined in dominance, while soft maple, cottonwood, and white ash in- creased in dominance by several orders of magnitude (Table 1). NMS ordination of modern urban forest species composition (basal area) produced a three-dimensional solution that explained 43.2% of the variation in the original species matrix and was sig- nificant based on Monte Carlo simulation (p = 0.004, Stress = 24.13). None of the potential predictors was strongly related to the ordination axes (all r < 0.03), but Axis 2 was strongly associ- ated with oak dominance (r = -0.763, Figure 2) and also maxi- Table 1. Comparison of basal area and dominance for species sampled by the Urban Tree Census (2010) and by the U. S. Public Land Survey (1820–1830). Dominance is percentage of total basal area and does not sum to 100 because some minor species were not included. Percent change reflects change in dominance from presettlement to modern, very large values may partly reflect differences in sampling strategy between data sets. Species Quercus alba Quercus macrocarpa Quercus rubra Acer saccharinum Acer negundo Populus deltoides Fraxinus pennsylvanica Prunus serotina Juglans nigra Ulmus americana Gleditsia triacanthos Fraxinus americana Acer saccharum Rhamnus cathartica Ulmus pumila Pinus strobus Acer platanoides Robinia pseudoacacia Salix spp. Malus spp. Total Species group Native-oak Native-oak Native-oak Native non-oak Native non-oak Native non-oak Native non-oak Native non-oak Native non-oak Native non-oak Native non-oak Native non-oak Native non-oak Non-native Non-native Non-native Non-native Non-native Non-native Non-native Urban tree census Basal area /hectare 0.80 1.02 0.66 1.09 0.91 0.66 0.57 0.52 0.29 0.29 0.26 0.24 0.23 0.52 0.30 0.25 0.23 0.21 0.18 0.16 9.39 ©2012 International Society of Arboriculture Dominance 6.97 8.86 5.70 9.44 7.89 5.75 4.97 4.55 2.53 2.50 2.21 2.07 1.98 4.55 2.60 2.20 2.02 1.78 1.53 1.37 81.44 Public land survey Dominance 41.28 13.09 6.60 0.01 0.00 0.02 0.00 0.00 2.18 9.03 0.00 0.14 0.97 0.00 0.00 0.00 0.00 0.00 0.00 0.00 73.32 Change in dominance (%) -83.12 -32.29 -13.68 80787.98 -- 36737.79 -- -- 15.79 -72.32 -- 1388.67 104.78 -- -- -- -- -- -- --
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