Pastor et al. studied the effect of moose (Alces alces) browsing on the nutrient cycles of boreal forests using three, 40-year old exclosures on Isle Royale, Michigan. Soil nutrient availability and microbial activity, including exchangeable cations, total carbon and nitrogen, nitrogen mineralization rates, and microbial respiration rates were uniformly higher in exclosures than outside. These differences were more significant where browsing intensity was high and less often significant where was browsing intensity was low. Differences in litter quantity and quality were caused by an increased abundance of unbrowsed spruce outside the exclosures. In the long term, high rates of moose browsing depress N mineralization and net primary production through the indirect effects on recruitment into the tree stratum, and subsequent depression of litter N return and litter quality. These results suggest that the effects of herbivores on ecosystems may be amplified by positive feedbacks between plant litter and soil nutrient availability.
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