188 Comparison of Presettlement and Modern Land- scape NMS ordination of the tract group species transition matrix pro- duced a three dimensional solution that explained 67% percent of the variation in the original data matrix and was significant based on Monte Carlo simulation (p = 0.004, Stress = 25.82). There was a strong separation of presettlement and modern com- position in the ordination space (Figure 7). However, change in composition from presettlement to modern did not differ among modern land use categories based on MANOVA of transi- tional vectors (all comparisons p > 0.1; vectors not illustrated). Spatial patterns in presettlement and modern species occurrenc- es also illustrate the transitions in species composition and their re- liance on original vegetation. For example, white oak occurrences in the modern landscape largely coincide with locations that had this species present prior to urbanization (Figure 8). In contrast, soft maple occurred only in a few riparian areas in the original landscape, but now is widely spread throughout the region (Figure 8). Also, census tract groups where oak species were present in the modern landscape tended to also have those species present in the presettlement landscape (95% of modern white oak occurrences, bur oak – 74%, red oak – 53%). In comparison, the primary non- oak native species had a far smaller proportion of their modern occurrences associated with presettlement locations (sugar maple – 23%, soft maple – 3%, white ash – 29%, American elm – 29%). Fahey et al.: Origins of the Chicago Urban Forest DISCUSSION Figure 7. Non-metric multidimensional scaling ordination of tree species composition in Tree Census and presettlement land sur- vey combined at census tract group level, location of group cen- troids for species are indicated. Species acronyms: ACNE (Acer negundo), ACPL (A. platanoides), ACRU (A. rubrum), ACSA (A. saccharum), ACSI (A. saccharinum), CAOV (Carya ovata), CEOC (Celtis occidentalis), GLTR (Gleditsia triacanthos), JUNI (Juglans nigra), JUVI (Juniperus virginiana), LOMA (Lonicera maackii), PINI (Pinus nigra), PODE (Populus deltoides), PRSE (Prunus se- rotina), QUAL (Quercus alba), QURU (Q. rubra), QUVE (Q. velu- tina), QUMA (Q. macrocarpa), RHCA (Rhamnus cathartica), ROPS (Robinia pseudoacacia), TIAM (Tilia americana), ULAM (Ulmus americana), ULPU (U. pumila). ©2012 International Society of Arboriculture Influence of Presettlement Vegetation on the Modern Urban Forest This is the first landscape-scale study to illustrate the interac- tive effect of presettlement vegetation pattern (i.e., timber versus prairie) and modern land use on composition and structure of a large metropolitan region urban forest. Previously, McBride and Jacobs (1986) used natural stands as a proxy for presettlement vegetation to show that both original stand condition and urban development affect composition and structure of selected urban forest stands at a local scale. The current data show that presettle- ment prairie and forest vegetation has played an important role, both directly and indirectly, in the development of the modern urban forest landscape of the Chicago region. One of the most important direct influences is the transition of forested areas in the presettlement landscape into residential and natural areas with greater canopy cover, larger trees, higher species richness, and a greater proportion of native tree species than areas devel- oped from prairie vegetation. As a result, remnant forests stem- ming from original woodlands appear to have formed the basis for the parts of the urban forest with the highest value from an ecosystem services perspective (Nowak 1994). These areas also had high levels of oak dominance, and much of the value of these stands is likely derived from the presence of large oaks descend- ing from presettlement vegetation. Such trees are known to pro- vide high levels of important ecosystem services, such as wild- life habitat, shade, and carbon storage (McPherson et al. 1994). The legacy of presettlement vegetation pattern and com- position on the urban forest can also be seen in comparisons of spatial patterns of species occurrence between the original and modern landscapes. Present-day occurrences of oaks (the dominant species in the presettlement landscape) are almost ex- clusively associated with forested areas that had these species present in the original landscape, providing additional evidence for the direct transition of presettlement oak forests into areas with a modern oak component. Patterns of occurrence of indi- vidual oak species in relation to specific species traits and adap- tations are also informative. For example, basal area of red and white oak, primarily forest and woodland species with moder- ate shade tolerance, was greatest in parks. On the other hand, basal area values for bur oak, a savanna species with low shade tolerance, were greatest in agricultural areas. This pattern may result from the presence of bur oak, as occasional trees in the presettlement prairie matrix, or its ability to colonize areas with high light availability associated with agricultural abandonment. Comparisons of spatial pattern and non-native species occur- rence also illustrate the important influence that planting patterns and emergent forests have had on species composition. Native species that are either commonly planted in urban areas (such as soft maple; Figure 8) or opportunistically invade open habitats (e.g., box elder; not shown) had the least congruence between original and modern distribution. The importance of emergent, or successional, stands as a component of urban forests has been illustrated by Zipperer (2002), but the combined effect of original vegetation pattern and land use on the development of these for- ests has not previously been illustrated. As an example, although agricultural and other developed areas (e.g., commercial and in- dustrial) had greater dominance of non-native species than parks
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