Litter Decomposition in Yellowstone Grasslands: The Roles of Large Herbivores, Litter Quality, and Climate

Plant litter decomposition is a key process that influences carbon and nutrient cycling in terrestrial ecosystems. Litter decomposition is controlled by a number of factors such as temperature, moisture, litter quality, and the composition of soil microbial communities. Large herbivores can affect litter decomposition by modifying each of these factors. The goal of this study was to examine the contribution of herbivores in determining decomposition rates across an ecosystem with a steep climatic gradient. We examined the effect of migratory grazing ungulates (primarily elk) on litter decomposition by comparing rates of mass loss over 1.5 years inside and outside fenced exclosures at 10 grassland sites that spanned gradients of temperature, precipitation, plant community composition, and grazing intensity in Yellowstone National Park (YNP). Based on previous studies indicating positive feedbacks of herbivory on soil carbon and nutrient processes in grazed YNP grasslands, we predicted that YNP grazers should also improve litter quality and stimulate litter decomposition. Instead, we found that variation in precipitation was the predominant driver of litter decomposition across the ecosystem. Grazing decreased litter lignin:nitrogen ratio (improved quality), but did not affect decomposition rates. Our results suggest that while grazers can influence the quality of litter, climate is a more important driver of litter decomposition across the YNP ecosystem.