Page 665 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 19 ecosystem essentials 629
Clear cuts on private land
Forest Service land
episodes that triggered partial extinctions over the past 440 million years overcame the resilience of plant and animal communities, destabilizing the ecosystem. When an ecosystem crosses such a threshold, it moves toward a new stable state.
In an ecosystem, a population of organisms can be stable yet not resilient. A tropical rain forest is a diverse, stable community that can withstand most natural dis- turbance. (An ecosystem with inertial stability has the ability to resist some low-level disturbance.) Yet this ecosystem has low resilience in terms of severe events; a cleared tract of rain forest will recover at a slower rate than many other communities because most of the nu- trients are stored in the vegetation rather than in the soil. Furthermore, changes in microclimates may make regrowth of the same species difficult. In contrast, a midlatitude grassland, although less diverse than a rain forest, has high resilience because the system can cope
▲Figure 19.22 Clear cut landscape on Vancouver Island, British Columbia. This highly fragmented landscape of the klanawa Valley resulted from decades of logging on southwest Vancouver island where conflict between resource extraction and old-growth forest conservation continues. [Chris Cheadle/alamy.]
◀Figure 19.21 Disruption of a forest community. an example of clear-cut timber harvesting that devastated a stable forest community and produced
drastic changes in microclimatic conditions. about 10% of the U.S. northwest’s old-growth forests remain, as identified through satellite images and giS analysis. [Bobbé Christopherson.]
with a range of disturbances and recover quickly. For example, after a fire, rapid regrowth oc- curs from the extensive root systems of grassland species.
In the context of ecosystem stability and resilience, consider the elimination of a full section of forest ecosystem on private land in southern Oregon, south-southeast
of Crater Lake, shown in Figure 19.21. The practice of clear- cutting (the complete removal of timber) can cause a dis- turbance that overcomes a forest community’s resilience and prevents it from returning to its natural stable state. Adjacent lands administered by the U.S Forest Service use more sustainable harvesting regimes, including partial tree removal and forest thinning, that are not as likely to desta- bilize forest communities. Clearcutting results in conflict over resource extraction and ecosystem stability between foresters and environmentalists in many areas of old-growth forest, like Vancouver Island (Figure 19.22).
agricultural Ecosystems When humans purposely eliminate biodiversity from an area, as they do in most ag- ricultural practices, the area becomes more vulnerable to disturbance. An artificially produced monoculture com- munity, such as a field of wheat or a tree plantation, is vul- nerable to insect infestations or plant diseases (Figure 19.23).
▲Figure 19.23 Pine tree plantation in Ontario. [J. David andrews/Masterfile.]