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

Simon Chollet,Jean‐Louis Martin,Sue J Grayston,Morgane Maillard,Juliane Schörghuber

doi:10.1002/ecy.3235

Abstract

Litter 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 quasiexperimental 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 1 yr, 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 break down carbon, but not nitrogen. Indeed, litter placed on an island with deer lost 5% less carbon after 1 yr of decomposition than did litter decomposing on an island without deer. This loss in ability to decompose litter in the 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 dramatic continental-scale increases in deer populations may have on broad-scale patterns of C and N cycling deserve closer attention.

Highlights

Deer and the functioning of temperate forests By the mid-20th century wild ungulates, essentially deer, were largely missing from temperate forests of Europe and eastern North America, a result of hunting and/or loss of favorable land cover (McShea et al 1997, Apollonio et al 2010)
Deer slow down decomposition mainly by changing understory plant communities In our study, we demonstrated that deer, through their impact on the understory vegetation, control the overall quality of litter reaching the forest floor, and the decomposition process
Our results show that in temperate forests abundant deer may play an important role in ecosystem functioning, modifying aboveground characteristics, and those belowground, and reducing nutrient cycling

Summary

Introduction

Deer and the functioning of temperate forests By the mid-20th century wild ungulates, essentially deer, were largely missing from temperate forests of Europe and eastern North America, a result of hunting and/or loss of favorable land cover (McShea et al 1997, Apollonio et al 2010). More recent studies focused on aboveground understory community functioning (Horsley et al 2003, Royo et al 2010) with a focus on effects cascading on invertebrates, birds and small mammals (e.g. Chollet and Martin 2013, Foster et al 2014)]. In the latter part of the 20th century, the effect of large herbivores on processes of nutrient cycling in temperate forests drew attention (Pastor et al 1993, Frank and Groffman 1998, Ritchie et al 1998, Olofsson and Oksanen 2002, Tanentzap and Coomes 2012). Contrary to grasslands or boreal forests, there are still only a few studies on how large herbivores affect litter decomposition in temperate forest ecosystems

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