ABSTRACT
Site availability includes not only sites created by disturbances (broad scale) (Pickett and White 1985), but also the death of an individual and existing site conditions (fine scale) (Brand and Parker 1995). A disturbance is defined as “any discrete event in time that disrupts [landscape], ecosystem, community, or population structure and changes resources, substrate availability, or physical environment” (Pickett and White 1985). Natural disturbances that affect urban landscapes include wind and ice storms, fire, and flooding as well as herbivory (e.g. insect outbreaks), pathogens, and stress-induced mortality. A disturbance can be classified by its seasonality, distribution, frequency, magnitude, and severity (Pickett and White 1985) and, collectively, all of the disturbances affecting a landscape are called the disturbance regime. The primary site disturbance in the urban landscape is land clearing for development. Site clearing can range from an area less than 0.25 hectare for a single structure, thus possibly leaving the adjacent vegetation relatively intact, to clearing the entire area for an entire subdivision (e.g. >50 ha). Site clearing can include not only removing existing vegetation cover but also removing soil and possibly altering drainage patterns (Effland and Pouyat 1997). Such extensive clearing can
create a new substrate as A and B horizons of the soil profile are removed. Successional processes on these sites would be defined as primary succession rather than secondary succession since this new substrate has not previously supported vegetation, does not contain a seed bank, and possibly has not accumulated organic matter. In general, primary succession is associated with glaciated and volcanic sites rather than urban sites. In the urban landscape, primary succession is associated with stone and brick walls, demolition sites, spoil heaps from industrial waste, and abandoned roads and sidewalks (Rebele 1992, 1994). Contrary to vegetation dynamics in adjacent rural areas where secondary succession dominates, both primary and secondary successional processes are important ecological processes in urban landscapes. The disturbance regime changes as a landscape is urbanized. Some disturbances such as fires and flooding are suppressed because of their devastating effects on property and the potential for loss of life. For instance, fire suppression reduces the frequency and alters the intensity of fire events. A subsequent outcome is a shift away from fire-dependent species to more mesic species and new plant communities. In the Chicago Metropolitan area, fire suppression has changed the oak savannah from a community dominated by Quercus spp. and grasses to one containing Acer saccharum and exotic species (Kline 1997). Similarly, humans introduce new disturbances. Sharpe et al. (1986) observed an increase in frequency of fire because of arson, trampling, dumping of yard and garden waste, and vandalism in urban woodlands in the Milwaukee metropolitan area. A study of reforested vacant lots in Syracuse, New York revealed that 80 percent of sites sampled were disturbed intensively and extensively by human activities such as vandalism, fire, trampling, and dumping (Zipperer 2002). Matlack (1993) called this human effect a sociological-edge effect because most human activities occurred within 82 m of the edge. Even though these novel disturbances create new sites for colonization, they affect the germination and growth of species. For instance, trampling is a novel, small-scale, frequent disturbance which many native species have not evolved mechanisms to cope with. Without natural barriers such as high stones and down-woody debris to created micro-safe sites, trees did not regenerate in remnant forest in Helsinki, Finland because of trampling (Lehvävirta et al. 2004). Management activities can also be considered a type of disturbance. The type, frequency, and intensity of management will significantly influence vegetation dynamics (Zipperer et al. 1997). For instance, if a vacant lot is regularly managed by mowing, its structure and composition would differ from a vacant lot that is left unmanaged (sensu Godefroid and Koedam 2007). Overall, site management activities, such as mowing, clearing, and weeding, inhibit or alter successional processes. Brand and Parker (1995) recognized that seasonality, environmental heterogeneity within the site, and environmental factors of the site can influence germination and growth. Environmental factors particularly important to urban landscapes include urban heat island, atmospheric pollution (e.g. nitrogen deposition and ozone), altered soil properties (physical and chemical), soil moisture, and light availability (Rebele 1994; Pickett et al. 2001) (see species performance section for more detail). Another aspect of site condition is its landscape structure and social context. Landscape structure refers to site location in an urban landscape with respect to adjacent land uses as well as site configuration (e.g. shape, size, and orientation). Both attributes have been shown to influence site composition (see Godefroid and Koedam 2003). Social context refers to the social attributes of the site and adjacent sites. A study of plant diversity in Phoenix, Arizona, a city in the Sonoran desert of the United States, showed that in addition to elevation, former land use, home owner income and housing age were significant site attributes influencing vegetation dynamics (Hope et al. 2006).
