Large herbivores strongly slow down litter decomposition in temperate forests

In temperate forest ecosystems, the role of large herbivores in litter decomposition, a key nutrient cycling process, is unresolved. Herbivores may first modify litter abundance by affecting plant cover. They may also modify the decomposition process by changing litter quality and altering decomposers9 ability by changing soil abiotic properties and/or decomposer communities. Using two litterbag transplantation experiments in the quasi-experimental situation resulting from the introduction of Sitka black-tailed deer to the forests of Haida Gwaii (Canada), we disentangle the relative importance of the latter modifications on litter decomposition. We demonstrate that deer strongly reduce carbon (C) and nitrogen (N) loss, a 21 and 38 % decrease respectively, mainly by altering plant community composition. We reveal other ecosystem changes induced by deer through feces deposition, abiotic soil modification or decomposer community changes that affect decomposition processes, but are outweighed by deer top-down control on vegetation quality. Thus, the large increase in deer populations in temperate forests worldwide may be considered an important factor modifying global C and N cycling.