Many of the effects of environmental forcing factors on plant and ecosystem processes are general enough to permit extra­polation to many terrestrial ecosystems. The quantitative ecosystem conse­quences of any particular combination of environmental forcing factors are influenced by interactions among the EFFs, the resource status of the site, and characteristics of the organisms present. 3. Plant responses to resource imbalances caused by EFFs tend to com­pensate for the imbalance, at least partially. The dominant mechanisms of compensation are changes in the biochemical capacity for resource uptake, changes in biomass and nutrient allocation, and changes in rates of tissue loss.

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Compensation for changing resource balance allows plants to make growth a function of resource availability over a wide range of resource balances. The plasticity or compensation ability of any particular species is, however, limited and effective compensation for major changes in resource balance caused by EFFs seems to require changes in species composition. When appropriate species are available, changes in species composition will not dramatically disrupt the response of growth to resource availability, but the availability of appropriate species may be compromised by EFFs, espe­cially changing land use patterns (Field et al., 1992). 4.

Ecosystem responses to altered resource balance caused by environ­mental forcing factors are often limited by certain constraints. The tendency for ecosystems to amplify deficiency or excess of nutrients through the feedback of tissue chemistry on decomposition and nutrient availability can exaggerate carbon-nutrient imbalance When nutrients are limiting, EFFs that increase carbon availability tend to further decrease nutrient availability through this feedback. This usually makes natural ecosystems less respon­sive to increased atmosplicric CO2 than many agricultural ecosystems or plants in controlled environments.

The decomposition feedback also ampli­fies the effect of EFFs that directly increase nutrient availability (Field et al., 1992). 5. Ecosystem and plant processes provide little buffering against the effects of EFFs that tend to decrease primary production. The tissue chemis­try/decomposition feedback and the tendency for ecosystems to lose abun­dant nutrients and water further weaken the buffering.

These mechanisms increase the likelihood of decreased primary production in ecosystems im­pacted by pollutants, UV-B, and loss of soil fertility (Field et al., 1992). 6. Most ecosystems are exposed to several atmospheric EFFs and in­creased intensity of human utilization.