ABSTRACT
All forested ecosystems are influenced by natural disturbances. Disturbance in plant communities affects the maintenance of species richness and the process of community development (Bendix 1997; Collins and Pickett 1987; Connell 1978; Cooper-Ellis et al. 1999; Drury and Nisbet 1973; Foster 1988; Foster and Boose 1992; Grime 1979; Grubb 1977; Huston 1979; Pickett and White 1985; Platt and Weis 1977; Smith 1972; Watt 1947; Whittaker 1969; Whittaker and Levin 1977). Controls exerted by fine-scale, local disturbances interact with coarse-scale, regional disturbances in complex ways (Beatty 1991, 2003; Frelich and Reich 1995; Glitzenstein et al. 1986; Lertzman 1992; Lertzman and Fall 1998; Pastor and Broschart 1990). Disturbances are caused by both physical and biological events. Physical disturbances include generally stochastic processes that disrupt some aspect of the physical environment; such disturbances include wind downbursts, flooding, fire, avalanche, or landslide. Biological disturbances are often endogenous processes that stem from influences of native or exotic species in the ecosystem, and may include death from insect outbreaks, plant pathogens, or impacts caused by nonnative species (e.g., excessive herbivory, competition, or predation). In this chapter, we take a broad view of what qualifies as a disturbance, expanding on the definition of Pickett and White (1985) to include biological disturbances that often act as ongoing stress, but depending on the severity of the interaction, can also be discrete disturbance events. Both physical and biological disturbances operate at the coarse scale, such as hurricane blowdowns or gypsy moth defoliation, and at the fine scale, such as individual treefall or excessive deer browse. Coarse-scale phenomena are generally responsible for initiating successional regeneration responses that can take decades to centuries (stand-destroying fire, for example), while the fine-scale phenomena are responsible for maintenance stand regeneration responses that usually take years to decades (seedling recruitment to the canopy in a light gap, for example) (Allen and Walsh 1996; Bendix 1997; Clark et al. 1999; Kupfer and Malanson 1993; Parker and 1-56670-635-1/05/$0.00+$1.50 © 2005 by CRC Press 62
Bendix 1996;Pitman et al. 1999). The frequency of disturbance also varies with spatial scale, as coarser-scale events occur less frequently but are more intense (Drury and Nisbet 1973; Watt 1947). Disturbance, therefore, plays a different role at fine vs. coarse scale. If the incorporation of natural disturbance regimes into restoration and forest management is a long-term goal, then an understanding of forest dynamics at different spatial scales and for physical versus biological disturbances will be necessary. The many unanswered questions about the role of disturbance in forest dynamics present a challenge to those who wish to incorporate disturbance into a forest restoration plan as we will illustrate in two case studies, from needle-leaf forests in the western U.S., and northern hardwood forests of the eastern U.S.
