Managing Forests for Yield

Management of forest vegetation and its relationship to water yield has long been a lively issue. The USDA Forest Service has been criticized in northern Colorado for failing to have a forest management strategy that includes consideration of water yield for downstream users, including fish and wildlife (Swanson 1998). Some say it is possible to get substantial increases in runoff with modest vegetation management; others contend that even aggressive vegetation removal is likely to have only localized effects. An excellent historical review of this subject is available (Ziemer 1986) and is recommended reading. The Santiago Agreement, which addresses criteria and indicators for sustainable management of forests (“Sustaining the World’s Forests,” 1995), lists “enhancement of ability to predict impacts of human intervention on forests” as one indicator of progress. The work presented here attempts to build toward such an improved capability. We have examined one aspect of the issue—quantity of the change in annual runoff in response to forest thinning—for a large portion of the Sierra Nevada Mountains in central California (fig. 1). For the remainder of this article, the term water yield is understood to represent the annual cycle, unless otherwise stated. Examination of expected changes in water yield for a large area where vegetation thinning has been proposed in the Sierra Mountains of California indicates that the size of the area has an important bearing on annual runoff. At the smallest scales, some treated areas may have easily measured changes in water yield, with the potential for impacts on aquatic biota and water quality. The average changes in annual runoff per unit area for large tracts, however, are too small to be measured directly and thus must be quantified by using models to extrapolate existing knowledge. Forest Hydrology GIS