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Effects of Soil Disturbance on the Fundamental, Sustainable Productivity of Managed Forests 1

policies in the United States and abroad are reducing timber harvests while wood demand is mounting. Reduced harvesting on public lands means that privately owned lands will be managed with greater intensity in the United States and that wood will be imported from other nations lacking strong environmental safeguards. It is imperative, therefore, that both public and private forest lands be managed to sustain their productivity, and many nations are seeking effective monitoring methods. Central to this is our ability to estimate a site's fundamental capacity for growing vegetation, and to detect changes in this capacity caused by management. Because soil is the factor of a site modified most easily and profoundly by management, and because soil largely is independent of the current condition of vegetation, soil-based variables offer our most effective and practicable indices of sustainable productivity. The North American Long-Term Soil Productivity cooperative research program (LTSP) is the world's most extensive coordinated effort to address questions of sustainable productivity in managed forests. Early findings from the 12 LTSP sites in California illustrate the physical importance of organic soil cover in reducing soil erosion and maintaining favorable soil temperature and moisture relations during summer drought. Findings also show that the biological significance of soil compaction depends on soil texture. Moderate compaction degrades vegetative growth on fine-textured soils but can enhance growth on coarse-textured soils where drought is a factor. Impacts of soil compaction on tree growth often are masked by effects of competing vegetation. Measurements taken under operational conditions show that compaction associated with mechanized thinning can reduce soil rooting volume by as much as one-half. Subsoiling seems to mitigate the effect. Root damage caused by subsoiling did not adversely affect the growth of residual trees. Results are providing practicable field methods for monitoring management impacts on sustainable productivity.