Acceleration of vegetation turnover and element cycling by mammalian herbivory in riparian ecosystems

Abstract

Summary 1 We examined the effects of browsing by moose and snowshoe hares on vegetation structure, species composition, plant demography and element cycling in 25 riparian (willow) vegetation stands along the Tanana River, interior Alaska, across a 250-km gradient that represented a fivefold range in moose densities (0.2–1.0 km−2). 2 Browsing frequency was much greater in areas with high moose densities. The combined browsing pressure (% of annual browse production consumed by herbivores) of moose (Alces alces) and snowshoe hares (Lepus americanus) on forage species was over fivefold greater in the high moose density areas (33%) than the low moose density areas (6%), reflecting the fivefold difference in moose densities. 3 Above-ground biomass of preferred willow species was lower in high moose density areas, and the age structure of the vegetation was significantly skewed towards younger age classes. This shift in age structure was accompanied by higher proportion of dead to live ramets, indicating higher mortality under increased browsing pressure. Increasing browsing pressure favoured later-successional species (Alnus tenuifolia and Populus balsamifera) on the landscape, as shown by both a reduction in the number and size of willow stands in high-density areas. 4 High rates of fecal input resulted in a doubling of above-ground N input in high moose density areas. The rates of carbon and nitrogen input from willow communities to the soil as mediated by moose herbivory were approximately fivefold and eightfold greater in high moose density areas than in low moose density areas, respectively. 5 Synthesis. The effects of herbivory were manifested at several ecological hierarchies, including the individual, the community, and the landscape. Across these spatial scales herbivory appears to accelerate the ecosystem turnover of carbon and nitrogen by a combination of both plant- and animal-based processes.