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. We investigated patterns of N availability along transects through 0.1 and 0.4 ha restorative gaps in three 50-70 year old Douglas-fir forests across 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/Conclusions
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 due to high variability. In mineral soil and forest floor, N availability in gap edge positions more often resembled that 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 and temperature did not significantly differ along gap transects, nor did decomposition rates of cellulose tongue depressors. Total litterfall inputs and litterfall C:N in gaps were significantly lower than in the forest. Reciprocal transfer in situ incubations of mineral soil samples between gap and forest positions revealed that net nitrification in samples originating from gap centers was significantly greater than in forest samples, while incubation environment did not have a significant effect on N cycling. Although environmental factors are often thought to contribute to changes in N availability immediately after forest gap creation, our results suggest that early seral species may perpetuate increased N availability into the initial stages of vegetative succession.
