182 expect that areas with an agricultural period prior to urban de- velopment (as with much of the Chicago region) may be more likely to be dominated by emergent than remnant stands. Mod- ern land use can also have an important influence on urban veg- etation, but this effect is often not straightforward. The impact of land use is mediated by social factors (e.g., not all residen- tial areas are equivalent; Iverson and Cook 2000; Grove et al. 2006) and can exhibit lag times because of the long life-span of trees relative to patterns of social change (Boone et al. 2010). Interactive relationships between modern land use and his- toric vegetation may also exist, which could complicate interpre- tation of the drivers of urban forest structure and also influence the potential for different sectors of the urban forest landscape to provide ecosystem services. For example, large remnant oaks found in the Chicago region are probably more likely to occur in areas that were forest or woodland in the pre-urban landscape no matter what the modern land use is. However, such trees may also be more common in protected natural areas than in highly developed portions of the urban landscape where lower value emergent forests are more likely to dominate (Zipperer 2002). In a region such as Chicago, where much of the original landscape was not dominated by forest, researchers may expect the distri- bution of emergent forests to be associated with historic prairie locations and the agricultural development that occurred in these areas. Beyond providing lower levels of ecosystem services, these emergent forests may also be more strongly negatively af- fected by invasive exotic species than forest remnants, but the relationship between susceptibility to invasion and intactness of forest ecosystems has been mixed (Martin and Marks 2006). In order to make informed decisions about the future of the urban forest, managers and planners need to understand how the current composition and structure of the forest developed, how it varies across a mosaic of modern land uses and historic ecosystems, and how this composition might affect a provisioning of ecosystem services in the future. The primary objective of this research was to address these needs by investigating patterns in species com- position and forest structure across the urban forest continuum of the Chicago metropolitan region. Specific research questions include: 1) What are the primary gradients in species composition and structure in the urban forest? 2) How do these factors vary with modern land use and presettlement vegetation condition? 3) How do original vegetation pattern and modern land use interact to affect composition and structure across the urban forest? and 4) How have composition, structure, and species distributions changed from presettlement to the modern urban landscape? METHODS Study Area Chicago is the third largest metropolitan area in the United States, with an estimated population of 9.6 million in an area of 24,814 km2 that spreads across three states (Illinois, Indi- ana, and Wisconsin). The study area for this analysis was the Illinois subset of the overall metropolitan region, encompass- ing the seven counties of northeastern Illinois: Cook, DuPage, Kane, Kendall, Lake, McHenry, and Will (Figure 1). The Chi- cago metropolitan region is composed of a wide variety of land uses, cultural and social milieus, population densities, and around 145,686 ha of natural reserves. Chicago is one of ©2012 International Society of Arboriculture Fahey et al.: Origins of the Chicago Urban Forest the largest transport hubs in the country, and for this reason is also a locus for introduction of exotic species, including forest pests like the Asian longhorned beetle (Anoplophora glabripen- nis) and emerald ash borer (Agrilus planipennis), and invasive plant species like European buckthorn (Rhamnus cathartica L.). Figure 1. Map showing the Chicago metropolitan region with the seven counties that constituted the study area outlined. Urban Tree Census plot locations are indicated as shaded areas, repre- senting locations recorded as timber in presettlement surveys. The surficial geology of the Chicago region represents Wood- fordian-aged glacial material deposited in the last 20,000 years with glacial drift (end moraines, till plains, and outwash) in the north, west, and south ,and the former bed of glacial Lake Chicago in the east-central parts of the region (Willman 1971). Predomi- nant substrates include fine-textured silt-loams and clay-loams in glacial till and lake bed deposits; sands in outwash, lake plain de- posits, and beach ridges; coarse-textured gravels in kames, eskers, and valley train deposits; and dolomite bedrock exposed along the major river valleys (Fehrenbacher et al. 1984). The study region has a humid continental climate with mean temperatures of 23°C in July and -6°C in January, and mean annual precipitation of 92 cm (based on climate normals from Illinois State Climatology; Angel 2011). This area is located within the “Prairie Peninsula” (Transeau 1935), a region with unpredictable summer drought (Borchert 1950) that may impact tree growth and survivorship. The presettlement landscape of the Chicago region was about 60%–80% grassland, with the remainder comprising a savanna-
September 2012
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