Clearcutting and deer browsing intensity interact in controlling nitrification rates in forest floor

Major forest disturbance such as clearcutting may increase nitrification rates in the forest floor, but the magnitude of this NO3-flush varies among different forest ecosystems. Here, we tested the hypothesis that this variation could partly be due to differences in deer-browsing intensity. More specifically, we hypothesized that nitrification rates would decrease as deer browsing increased and that this effect would be higher in clearcuts than in forests. Our experimental design consisted of 3 replicated blocks, each consisting of 3 plots (10-40 ha) in which white-tailed deer densities were kept at 0, 7.5, and 15 individuals.km(-2) for 6 y and a fourth plot in which natural deer densities varied between 27 and 56 deer.km(-2). In the first year, 70% of mature balsam fir (Abies balsamea) forests in each plot were clearcut harvested. Average potential nitrification in forest floor samples was higher in clearcut than in forest plots. In clearcut plots, potential nitrification at the high deer density was about 2.5 times higher than at lower deer densities, thereby contradicting our first hypothesis. In forest plots, potential nitrification was negatively related to indices of soil available C but was unrelated to deer density. The shrub and herb vegetation in clearcut plots was dissimilar to, and more altered by, deer browsing than the understory vegetation in forest plots. In clearcut plots, increasing deer density reduced the percent cover of nitrophilous herbaceous species and increased the percent cover of graminoid plants. Possible mechanisms that could explain the interaction between forest disturbance and deer browsing intensity in controlling the magnitude of the NO3-flush are (1) soil N inputs via feces and urine, (2) loss of soil NO3-sinks due to the disappearance of nitrophilous plant species, and (3) browser-induced succession towards rangeland plant communities that stimulate rhizodeposition and soil N dynamics.