190 and residential areas, these modern land uses had much higher non-native basal area when they developed from former prairies. In most cases it is unlikely that these opportunistic species in- vaded prairies directly. Rather, these species were either planted during urban development, or were established in open habitat following agricultural abandonment. The lack of an impact of presettlement vegetation on buckthorn is interesting, and may be related to the more recent introduction of this species or to life- history traits that allow it to invade intact forests (such as bird dis- persal and high shade tolerance). In developed areas, the relative abundance of emergent stands, and the dominance of invasive and opportunistic species in them, likely accounts for the relative lack of canopy structure and high proportion of non-native species. Variation in Forest Characteristics Across the Urban Forest Continuum Although some strong gradients in urban forest composition and structure were evident, there was not a clear link between these characteristics and modern land use. This result is not especially surprising given the strong influence of presettlement condition illustrated here, and previous work in Chicago and other urban systems that has shown inconsistent modern land use-vegetation relationships (Rowntree 1984b). Urban forest condition can be strongly influenced by patterns of early settlement and devel- opment (Nowak 1993), and in the case of the Chicago region, modern land uses may reflect a wide variety of development patterns. For example, many residential and commercial areas in the Chicago suburbs were in agricultural land use until very recently, while those in the interior are likely to have been in urban land uses for well over a century. Such lags in develop- ment could influence factors such as planting preferences, soil compaction, and likelihood of remnant versus emergent forests. Another factor that may limit the power of modern land use classes to predict forest composition and structure is the wide variety of site conditions within these categories (Cadenasso et al. 2007). Part of this effect is the nature of cat- egorical data, which necessarily lumps areas with somewhat different characteristics into classes. For example, not all residential areas are equivalent, as areas with high property or income values have been shown to have greater canopy cover (Iver- son and Cook 2000). However, there can also be a lag between social factors and forest composition and structure because of the rapid pace of social change relative to forest stand processes, such as tree maturation and mortality (Boone et al. 2010). Although there were significant differences among land use categories in the dominance of spe- cies groups, neither the more desirable (e.g., oaks, native mesic forest species) nor problematic (e.g., invasive exotics and native opportunists) species were absent from any categories. Both native- and exotic-dominated stands can be found across the urban forest continuum. This finding suggests that management focusing completely on either natural forests or traditionally defined urban forests will miss parts of the urban forest continuum that either have important ecosystem value or have the poten- tial to be invasion loci for exotic species. However, ©2012 International Society of Arboriculture Fahey et al.: Origins of the Chicago Urban Forest canopy and stand structure varied strongly across land use categories, suggesting that the value of the stands in some parts of the urban forest is generally greater than others. De- veloped areas (e.g., commercial, industrial, transportation, utility, and institutional land uses) had the lowest levels of all relevant metrics, including native species proportion, spe- cies richness, canopy cover, and canopy structure. These data appear to support the findings of Zipperer (2002), in that compared to emergent forests, remnant forests have more complex canopy structure and greater diversity in size struc- ture, both of which can be associated with important ecosys- tem services such as shading (McPherson et al. 1997), and could lead to greater resilience (McBride and Jacobs 1986). The Future of the Chicago Urban Forest: A Forest in Transition The Chicago urban forest appears to be in a transitional state (Figure 9), which could have very important implications for the future of the region and could affect the management para- digm for the regional forest. The size structure evident in this data indicates a likely shift in composition away from oak dominance, as all of the oak species were much less abundant than non-native and opportunistic-native species in smaller size classes. This finding fits with results of studies conducted in natural areas in the region, which have consistently shown a trend of decreasing oak dominance at the expense of mesic- site associated species, such as maple (Bowles et al. 2005). The oak resource in the region is largely even-aged and com- posed of pre and immediate post-settlement trees (Bowles et al. 2005). The even-aged structure and maturity of the large oaks in the urban forest (~200 years; Bowles and Jones 2008) sug- gest a potential precipitous decline in oak canopy dominance in the near future, as these trees reach their natural life-span. Across the urban forest, smaller size classes are highly domi- nated by exotic and opportunistic or mesic forest-associated na- Figure 9. Conceptual model illustrating generalized development of Chicago re- gion urban forest and possible future trajectory.
September 2012
| Title Name |
Pages |
Delete |
Url |
| Empty |
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success. You will be contacted by Washington Gas with follow-up information regarding your request.
This process might take longer please wait
