Under intensive forestry management, the most vulnerablecommunities are the unique and biologically rich onesassociated with forests older than harvest age (over 20 to100 years depending on forest type and product; Amaranthuset al. 1994, Franklin et al. 1981, Marcot 1997); hardwoods(because repeated cutting of conifers on short rotation cyclesdiscourages the establishment of these late-successional species);and riparian zones, wetlands, and streams (Gregory etal. 1987, Kuenzler 1989,Thomas 1979).Changes in Forest StructureAt the stand level, there are three important differencesbetween natural and harvested forest stands: age, size ofgap openings, and abundance and distribution of large deadwoody debris (Morrison and Swanson 1990, Sharitz et al.1992, Spies and Franklin 1991). Each of these factors plays akey role in functioning and structure of forest ecosystems.Clearcutting results in even-aged regeneration of trees,while natural disturbances such as fire and wind can resultin uneven-aged regeneration. For example, fire creates differenteffects on individual trees in a stand depending ontemperature, time of day, and position in the burn, and italso influences establishment of seedlings. These variablesleave trees of various ages, some partially functioning andothers dead, which contribute to the regeneration of theforest and provide microhabitat for many species.Timber harvesting, especially clearcutting, leaves largeswaths of open area. In contrast, natural disturbances creategaps of mixed sizes depending on cause. These can rangefrom a single tree-fall gap to large blowdowns caused by hurricanesand tornadoes. Tornadoes in boreal forests, for example,may create clearings measuring over 100,000 hectares.
đang được dịch, vui lòng đợi..