Deer slow down litter decomposition by reducing litter quality in a temperate forest

International audienceLitter decomposition is a key process that allows the recycling of nutrients within ecosystems. In temperate forests the role of large herbivores in litter decomposition remains a subject of debate. To address this question, we used two litterbag experiments in a quasi-experimental situation resulting from the introduction of Sitka black-tailed deer Odocoileus hemionus sitkensis on forested islands of Haida Gwaii (Canada). We investigated the two main pathways by which deer could modify litter decomposition: change in litter quality and modification of decomposer communities. We found that deer presence significantly reduced litter mass loss after one year, mainly through a reduction in litter quality. This mass loss reflected a 30 and 28 % lower loss of carbon (C) and nitrogen (N), respectively. The presence of deer also reduced the ability of decomposers to breakdown carbon, but not nitrogen. Indeed, litter placed on an island with deer lost 5% less carbon after one year of decomposition than did litter decomposing on an island without deer. This loss in ability to decompose litter in presence of deer was outweighed by the differences in mass loss associated with the effect of deer on litter quality. Additional effects of feces deposition by deer on the decomposition process were also significant but minor. These results suggest that the effects continental-scale dramatic increases in deer populations may have on broad-scale patterns of C and N cycling deserve closer attention