Ungulates decelerate litter decomposition by altering litter quality above and below ground

Abstract

Ungulates can greatly affect forest ecosystems’ functional characteristics. However, limited information is available about their influence on litter decomposition, a major ecosystem process, despite disturbance of ungulates on vegetation through selective browsing and trampling. This study focused on effects of the presence/absence of deer herbivory on decomposition of leaves and roots of three major tree species in a Hokkaido, Japan forest. Our litterbag experiment showed that litter decomposition was significantly faster for both leaves and roots in a deer exclosure than in a control site with deer herbivory. Possible factors for this slowed decomposition because of deer presence include their physical disturbance on soil through trampling. In both sites, the remaining mass of litter was positively correlated with the C:N ratio and lignin content. When analyzed for leaf litter, species with lower C:N ratio and lignin content showed lower litter mass remaining in both sites. Deer generally prefer species with a low leaf C:N ratio and lignin content; the results suggest that leaves of palatable species were less resistant to decomposition. A similar interspecific difference in decomposition was not observed for roots, most likely resulting from the small difference in root litter quality among species. In this forest, tree species with unpalatable leaves, which are becoming predominant, likely decreases leaf litter decomposition, as leaves of palatable plants decompose more rapidly. Roots, however, are not exposed to browsing, regardless of aboveground palatability, and remain within soil as a recalcitrant slowly decomposing litter substrate. These synergetic influences could allow deer herbivory to reduce overall plant decomposition rates aboveground and belowground via changes in plant species composition.