The degradation of environmental assets, especially soils, air and water, severely challenges the productivity of agriculture and forest resources (Pinstrup-Andersen and Pandya-Lorch, 1998, Price et al., 1999a,b). In the post-World War II period, approximately 23% of the world's agricultural and forestlands were classified as degraded by the United Nation's Environment Programme (Oldeman et al., 1991). Irrigated land is particularly vulnerable, although the expansion of irrigation is slowing.
In forest ecosystems, where the life cycle of the dominant vegetation is measured in decades and centuries, the impacts on ecosystem productivity depends on both the change in regional environmental conditions and the history of those changes over the past century. An asymmetry in the rates of mortality or degradation and the processes of regeneration and renewal (Kurz et al., 1995) strongly influences the achieved productivity at the landscape scale by introducing an age-class legacy effect (Bhatti et al., 2000) that constrains opportunities for human intervention (Kurz and Apps, 1999). In some regions adverse impacts of climate change on productivity are associated with local climatic extremes, late or early frost (Repo et al., 1996, Ogren et al., 1997, Columbo, 1998), or increases in fire, insect or disease (Hogg and Schwarz 1997; Kurz et al., 1995). Where higher temperatures are associated with increased drought, as appears to be happening in continental parts of the boreal zone, even in the absence of fire, net primary productivity is reduced as a result of lowered photosynthetic rates associated with reduced stomatal conductance (Sellers et al., 1997). Some of these adverse influences may be offset by increases in growing season length (Myneni et al., 1997), nutrient cycling (Tian et al., 1998), and more favorable temperatures (Bugmann, 1997). Although at present the global forests on aggregate appear to have increased productivity (i.e., are acting as a net sink of atmospheric carbon), the changes are not spatially uniform. Moreover on the time dimension, there is a growing consensus that the present net positive increment (Schimel et al., 2001) in the global average in response to global change is not sustainable, and will decline or disappear as global change pressures continue.
Although the economic impacts of the long term environmental degradation of forest and agricultural systems are difficult to determine, the general consensus is that they will eventually begin to undermine the necessary expansion of food and fiber production if allowed to increase at current rates.
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