Moose Browsing and Soil Fertility in the Boreal Forests of Isle Royale National Park

Abstract

Selective foraging by moose on hardwoods and avoidance of conifers alters community composition and structure, which in turn can affect nutrient cycles and productivity. The effect of moose browsing on the nutrient cycles of boreal forests was studied using three 40—yr—old exclosures on Isle Royale, Michigan. Two alternative mechanisms by which moose affect ecosystems were tested: (1) moose depress both the quantity and quality of litter return to the soil, and hence N mineralization and net primary productivity, by browsing on hardwoods and avoiding conifers; (2) moose stimulate N mineralization, and hence net primary productivity, by opening the canopy and by dropping fecal pellets. 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 browsing intensity was low. 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 nitrogen and carbon mineralization in soil resulting from the increased abundance of unbrowsed spruce. We conclude that, 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.

Short‐ and long‐term effects of changes in reindeer grazing pressure on tundra heath vegetation

“…This is not surprising as the effects of herbivory on forage quality and nutrient cycling (Pastor et al 1993;de Mazancourt et al 1998;Sirotnak & Huntly 2000) differ at different temporal scales. However, short-term grazing enhancement proved to be an effective indicator of long-term effects, although the same was not true of short-term grazing reduction.…”

Direct and Indirect Effects of Herbivores on Nitrogen Dynamics: Voles in Riparian Areas

“…Many studies have documented the stimulation of nitrogen cycling by herbivores (e.g., Ruess andMcNaughton 1987, Frank andGroffman 1998), and some have reported a deceleration of nitrogen cycling as an indirect consequence of selective herbivory (Pastor et al 1993, Ritchie et al 1998), but no studies have examined both mechanisms simultaneously in a site. We found that the net effect of vole herbivory on soil organic matter quality differed between two study sites.…”

“…Selective foraging on high-quality plants, e.g., plants with a relatively low C:N ratio or with low concentrations of recalcitrant chemicals, can decrease the abundance of these plants relative to plants of lower quality (McInnes et al 1992, Ritchie et al 1998). The relatively low-quality plant litter mineralizes nitrogen at a slower rate than would occur in the absence of herbivory (Pastor et al 1988(Pastor et al , 1993. The objective of this study was to determine the effects of voles (Microtus, Rodentia: Microtinae) on nitrogen dynamics in riparian meadows in Yellowstone Park.…”

Insect Herbivory in Tropical Forests

“…By consuming plant material, herbivores influence decomposition and nutrient cycling in communities (Pitelka 1964;Schultz 1964;Kitchell et al 1979;Swank et al 1981; Pastor and Cohen 1997). Experimental studies havc confirmed the role of vertchrate herbivores as mcdiators of decomposition processes in terrestrial systems iMc1n11cs et al 1992; Molvar et al 1993;Pastor et al 1993;Ritchic ct al. 1998).…”