Responses of available soil nitrogen and litter decomposition to openings of different sizes in dry interior Douglas-fir forests in British Columbia

Selection of an appropriate silvicultural system that maintains productivity requires an understanding of the effects of harvest type on post-harvest changes in nitrogen availability. The effects on inorganic nitrogen availability and litter decomposition of various harvesting systems, including partial canopy removal and a range of clearcut opening sizes, were investigated in a dry Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) forest in southern British Columbia. We measured concentrations of ammonium and nitrate after 6-week buried bag incubations of forest floor and mineral soil samples in replicated plots within uncut forest, within partially harvested forest, and in openings of 1.7, 0.4, and 0.1 ha, from two to seven growing seasons after harvest. Post-incubation nitrate and the proportion of inorganic N as nitrate in forest floor and mineral soil were elevated from 2 to 7 years after harvesting in the 1.7 ha openings. There was no consistent trend of higher ammonium concentration in either uncut forest or 1.7 ha openings. The proportion of inorganic N as nitrate was consistently the lowest in the uncut forest, and often significantly different from any opening treatment, including partial cutting. The increased proportion of inorganic N as nitrate occurred within I I m of the uncut forest edge. At very low moisture contents, nitrate concentrations tended to zero only in the uncut forest. Leaching of the increased available nitrate is unlikely because of the large increase in grass biomass that occurs after harvesting, and the dry soil moisture conditions during the growing season. Decomposition of Douglas-fir, lodgepole pine and trembling aspen litters, or forest floor was inconsistently, or not affected by any harvest treatment or by distance from, or aspect of, the edge of the forest. The post-harvest increases in nitrate concentrations cannot be attributed to faster decomposition. Crown Copyright (C) 2003 Published by Elsevier B.V. All rights reserved.