Nitrogen dynamics across silvicultural canopy gaps in young forests of western Oregon

Abstract

Silvicultural canopy gaps are emerging as an alternative management tool to accelerate development of complex forest structure in young, even-aged forests of the Pacific Northwest. I investigated patterns of nitrogen (N) availability along transects through 0.1 and 0.4 ha silvicultural gaps in three 50-70 year old Douglas-fir forests of western Oregon. Six indices of N availability in forest floor and mineral soil and several factors related to N cycling were measured from November 2005 to February 2007, approximately 6-8 years after gap creation. Results indicate that mineral soil pools of extractable ammonium (NH4⁺) and nitrate (NO3⁻), rates of net N mineralization and nitrification, and concentrations of ion-exchange resin NH4⁺ and NO3⁻ were significantly elevated in gaps relative to adjacent forest. Gap-forest differences in forest floor layers were less clear. For the majority of response variables, magnitudes and trends were similar in the centers of both gap sizes. N availability in gap edge positions more often resembled levels in the forest than in the gap interior, and there were few significant differences between positions north and south of gap centers. Forest floor and mineral soil percent moisture did not significantly differ along gap transects, nor did decomposition rates of wooden tongue depressors. Litterfall carbon (C) inputs and litterfall C:N ratios in gaps were significantly lower than in the forest. Reciprocal transfer incubations of mineral soil samples between gap and forest positions revealed that sample origin had a significant effect on net nitrification rates, while incubation environment did not. Variability of several indices of N availability also increased in gaps.

The overall increase of N availability in 6-8 year old silvicultural gaps may be due more to the quality and quantity of litterfall inputs than temperature and moisture conditions. Increased quality of litterfall in gaps, as indicated by lower C:N ratios, may increase rates of decomposition and net N mineralization, while overall lower litterfall C inputs may lead to C-limitation of microbial immobilization, resulting in increased accumulation of inorganic N in soil. While environmental factors have been shown to drive N availability soon after gap creation, litter inputs from early-seral species may perpetuate increased N availability into early stages of vegetative succession. From a management perspective, increased N availability in gaps may increase tree productivity, but at the same time, increase the likelihood of invasion by exotic species. Gap creation may also increase gap-scale heterogeneity of available N in the short-term, while increasing stand-scale heterogeneity in the long-term.