Forests are nature's most bountiful and versatile renewable resource, providing simultaneously a wide range of economic, social, environmental and cultural benefits and services. The worldwide demand for their numerous functions and outputs is increasing with the expanding population, while the global forest resource is shrinking either as a result of overharvesting, deforestation and permanent conversion to other forms of land use in many tropical regions, or as a consequence of forest decline associated with airborne pollutants in temperate regions.
Forests represent a unique situation in terms of global environmental issues. Physically, they are located within the territories of sovereign states, yet their environmental role extends beyond their borders at both transboundary and regional as well as global levels. For example, the management, or mismanagement, of watershed forests of international rivers has transboundary implications in terms of soil and water conservation in neighbouring countries. Similarly, airborne pollutants generated in one country may be transported across the boundary and cause forest decline in others. The role of forests in global ecological cycles highlights the environmental significance of forests beyond the boundaries of the nations where they are located. In this context, they are being viewed as global commons similar to the atmosphere and oceans.
While forests are resilient ecosystems, there are limits to their ability to withstand environmental change, and they degrade when these limits are exceeded. Understanding these limits also allows us to enhance various forest outputs through silviculture. There are many examples of thinning, selective harvesting and manipulation of watershed forests to increase the yield of wood, water and wildlife without any apparent negative ecological impact. On the other hand, some industrial forestry activities have been associated with a number of environmental stresses on forest ecosystems. These activities include harvesting; road construction; manipulation of cover types and species through silviculture and reforestation and through the use of mechanical, biological and chemical technologies for protection against fire, insects, diseases and competing vegetation.
Forests are also exposed to environmental stresses associated with other human activities such as industrial manufacturing and the use of fossil fuels. The impacts of some of these stresses are restricted and local, others are global. For example, while forest decline in certain parts of Europe is attributed to airborne pollutants, all types of the world's forests would be exposed to the anticipated global warming associated with an increase in the concentration of greenhouse gases in the atmosphere (Main), 1989b). A managed natural forest, a forest plantation or an ecological reserve to study natural processes would be of limited value, if any, in the vicinity of a manufacturing facility emitting stressful pollutants.
Sustainable forest development, therefore, means recognizing the limits of forests to withstand environmental change, individually and collectively, and in managing human activities to produce the maximum level of benefits obtainable within these limits. A number of parameters may be used to assess the status of forests with regard to individual species and ecosystems (Jordan, 1989; Rapport et al., 1985; Woodwell, 1970). The above definition of sustainable forest development recognizes three critical parameters: productive capacity, renewal capacity and species and ecological diversity.
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