Emulating Natural Disturbance

There is a growing interest in emulating natural disturbance and using knowledge of the landscape dynamics associated with natural disturbances as a guide for conducting management practices in National Forests in the United States (e.g., Swanson et al. 1997, Wallin et al. 1996, Wimberly et al. 2004; Zasada et al. 2004). The underlying assumption is that forest ecosystems have intrinsic properties that are related to the frequency, duration, and intensity of disturbance. If management impacts fall within the range of variability defined by historical natural disturbance, it is thought that the managed forest ecosystems are more likely to be sustainable (Landres et al. 1999). Thus, emulating natural disturbance has emerged as a means for achieving forest sustainability (Perera et al. 2004).

The general concepts that define this approach have taken several forms, including silvicultural applications (Bergeron and Harvey 1997; McRae et al. 2001), disturbance and forest dynamics (Armstrong 1999; He et al. 2004a), decision-support systems (Hessburg et al. 2004), and forest harvesting patterns (Franklin and Forman 1987; Gustafson and Crow 1996; Li et al. 1993). Landscape ecologists have made significant contributions to these topics.

The Augusta Creek Study, conducted in the Willamette National Forest in western Oregon, is a good example of applying the concept of emulating natural disturbance in the field. Here, a spatially and temporally explicit landscape plan was developed for a 7600-ha area (18 780 acres) with the primary objectives of maintaining native species, ecosystem processes, and landscape structures, and of maintaining long-term ecosystem productivity in a landscape where much of the area is allocated to timber management (Cissel et al. 1998). Although this intermediate step is a common operational step in the forest planning process, there are three aspects that make the Augusta Creek Study a useful guide for others.

First, historical fire regimes are used as the basis for vegetation management. Past fire frequencies, intensities, and spatial patterns were used as a template to guide rotation lengths, harvest rates, green-tree retention levels, and the spatial pattern of timber harvests. As in all such applications, the underlying assumption is that native species are adapted to the range of patterns created by historical disturbances. A second feature of the Augusta Creek Study is the integration of terrestrial and aquatic management objectives through the use of a landscape perspective. Specifically, the management of aquatic ecosystems was designed to be complemented by upslope management practices and patterns given both the larger landscape prescriptions and local conditions (Cissel et al. 1998). As this suggests, a third element was the linkage of management objectives across spatial scales. Local decisions were set in a regional and National Forest-scale context (Cissel et al. 1998). Such an approach is being applied in National Forest planning in the Great Lakes region and elsewhere in the United States.

Moving from concept to practice in emulating natural disturbance as a guide for forest management is hampered by inadequate knowledge (Cleland et al. 2004). Disturbances occur at widely different magnitudes, frequencies, and intensities and these differences produce varied responses and outcomes. For example, at many locations within the Augusta Creek landscape, there is the possibility of low, mixed, or high fire severity, producing differences in the structure and composition of the vegetation. Natural disturbances are caused by many factors—including diseases, insects, wind, ice, extreme temperatures, fire, prolonged drought, landslides, and floods—that operate at many temporal and spatial scales. Furthermore, natural disturbances often interact with human-caused disturbances such as timber harvesting and other land uses (He et al. 2004b; Loehle 2004; Shang et al. 2004). Better understanding of the nature of these interactions is a critical need in landscape and disturbance ecology.

Was this article helpful?

0 0

Post a comment