Herbivory-induced alteration of ecosystem biogeochemistry: the case of domestic sheep herbivory in the Norwegian mountains

Herbivory can have a major impact on the stocks and fluxes of elements in an ecosystem. As herbivores forage for limiting nutrients, the preferential consumption of certain plants over others shifts plant community composition. The well-defended plants that can resist herbivory have specialized traits often leading to lower quality litter that is slow to decompose. The dominance of well-defended species promotes a greater quantity of low-quality litter to enter the soil which then can slow the mineralization of limiting elements. When soil fertility is low, increased dominance of well-defended plant species can slow nutrient cycling leading to an herbivory-induced deceleration of ecosystem biogeochemistry. Here we present results from a 23-year fencing experiment in the south-western mountains of Norway that compares the effect of high density grazing with the effect of excluding domestic sheep. We complement the experiment with a series of natural and human-created islands in two hydroelectric reservoirs that have excluded sheep-grazing for at least sixty years and therefore can serve as a natural control. The low-alpine site (~850–1050 m) is characterized by a wet oceanic climate with a nutrient-poor granitic parent material creating a mixture of sandy soils of histosols, gleysols and podzols often with moist, deep, and acidic O-horizons.

We discovered that herbivory impacted both the plant community and ecosystem biogeochemistry in the stocks, concentrations, and ratios of silicon (Si) and phosphorus (P) in plants and soils. Our results demonstrate that sheep herbivory was associated with the dominance of herbivory-resistant grass Nardus stricta while the ungrazed islands harbored herbivory-susceptible grasses and forbs such as Deschampsia flexuosa and Solidago rigida. N. stricta was found to have low quality plant leaves with a high Si to P ratio (Si:P). Its dominance scales this high Si:P stoichiometry to the bulk aboveground plant biomass leading to a higher stock of Si under mainland herbivory compared to the island control. In comparison, the island vegetation was found to be relatively enriched in P. There were no treatment differences in the Si:P ratio between the mainland fencing treatment. N. stricta remained dominant inside many fences, suggestive of negative feedback towards the high-grazing state; however, one site transitioned to low Si:P ratio plant biomass with high D. flexuosa abundance and was classified with the islands. The present case suggests a mechanism of plant-soil-herbivory interactions where herbivory, through increasing dominance of a well-defended plant species, impacts ecosystem biogeochemistry via Si and P. Our empirical results inform theory on the role of herbivores in generating stabilizing negative feedback among ecosystem states that can aid to scale and implicate zoogeochemistry into Earth system models. We will discuss our results in the context of theory that describes herbivory-induced deceleration of ecosystem nutrient cycling. A deep understanding of herbivory-induced plant-soil feedbacks, expanded to include the stoichiometry of elements beyond carbon and nitrogen, is essential for efforts to model animals in the Earth system.