A meta-analysis of the effects of clearcut and variable-retention harvesting on soil nitrogen fluxes in boreal and temperate forests

Abstract

One of the assumed advantages of variable-retention (VR) harvesting over clearcut harvesting is reduced postharvest leaching losses of nitrogen. We test this assumption by synthesizing results from long-term field trials in a meta-analysis. Overall, clearcutting significantly increased soil NO 3 -N concentration, NO 3 -N as a proportion of soluble inorganic nitrogen (SIN), N concentration in leachates, N flux, nitrification rates, and pH, but not total N, NH 4 -N, SIN concentration, ammonification, or N mineralization rate. The proportion of soil NO 3 -N in deciduous forests increased immediately and returned to preharvest levels within five years; the effect was delayed in coniferous forests, but levels remained elevated for several years. Deciduous leaf litter decomposed faster and needle litter decomposed more slowly on clearcut sites than in uncut forests. Single-tree selection caused smaller changes in NO 3 -N than removal of groups of trees (i.e., gap creation) and led to smaller increases in NO 3 -N as a proportion of SIN than clearcut harvesting. High levels of retention (>70%) were required to maintain uncut stand N-cycling characteristics. Postharvest NO 3 -N levels could be predicted from NO 3 -N availability in the uncut forests.