Influence of partial cutting on site microclimate, soil nitrogen dynamics, and microbial biomass in Douglas-fir stands in western Washington

Abstract

Partial cutting as an alternative to clear-cutting is being practiced in many Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) stands in the Pacific Northwest United States. The objectives of this study were to determine the influence of (i) partial cutting (dispersed green-tree retention) and clear-cutting on air and soil temperatures and soil moisture, on surface mineral soil net ammonification, nitrification, and total N mineralization rates, and on soil microbial biomass and (ii) the proximity to trees in the green-tree retention areas on these factors. The study was conducted in 60- to 70-year-old stands in the Cedar River Watershed, about 70 km southeast of Seattle, Wash., between July 1994 and August 1995. Air and soil temperatures and soil moisture in the green-tree retention treatments were generally intermediate between the uncut forest and the clearcut treatments. There were no significant differences in net ammonification, nitrification, and total N mineralization rates and in soil microbial biomass among treatments. Soil moisture influenced net ammonification and total N mineralization rates and microbial biomass. Nitrogen dynamics were generally not influenced by nearness to trees in the green-tree retention treatment. Average microbial biomass, however, was higher near to green trees (1 m) than far (6 m).