Moose browsing and soil fertility in the boreal forests of Isle Royale National Park

The effect of Alces alces browsing on the nutrient cycles of boreal forests was studied on Isle Royale, Michigan. Soil nutrient availability and microbial activity, including exchangeable cations, total C and N, N mineralization rates, and microbial respiration rates, were uniformly higher in exclosures than outside. These differences were more significant where browsing intensity was high. N mineralization in browsed plots declined with increasing moose consumption rates. Net primary production in exclosures and browsed plots was strongly correlated with N mineralization. N mineralization in turn was positively correlated with litter N return and negatively correlated with litter cellulose content. These differences in litter quantity and quality were caused by an increased abundance of unbrowsed spruce outside the exclosures. Moose pellets alone mineralized less N but more C than soil alone, but pellets combined with soil stimulated N and C mineralization more than the sum of the two separately. However, this did not appear to be sufficient to offset the depression in N and C mineralization in soil resulting from the increased abundance of unbrowsed spruce. 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. The effects of herbivores on ecosystems may be amplified by positive feedbacks between plant litter and soil nutrient availability. -from Authors