Grasslands around the world experience intense, chronic grazing pressure by herbivores that consume significant proportions of aboveground biomass. The grazing optimization hypothesis predicts that under certain conditions, namely intermediate grazing intensities, grazing can increase aboveground net primary production (ANPP) relative to ungrazed controls. The hypothesis is controversial, however: empirical tests in grasslands around the world show only mixed support, and a general predictive framework for the outcome of a grazing event has remained elusive. While most mechanistic tests of the grazing optimization hypothesis have focused on the contributions of plant functional traits and soil resource levels, recently developed theory suggests that simple density-dependent resource limitation may more accurately predict the response of ANPP to grazing. We conducted a simulated grazing experiment to assess the relative influence of herbivory, plant traits, soil resources, and density dependence on ANPP over two months in the grasslands of Yellowstone National Park. We established 20 paired plots in high and low ANPP communities in May 2018 to test how the effects of an intermediate clipping intensity (50 % biomass removal) on ANPP varied with community-weighted leaf traits, plant-available soil nitrogen and moisture, and initial plot biomass.
Results/Conclusions
ANPP was positively related to soil moisture and showed no relationship to plant-available soil N. Clipping stimulated ANPP more at mesic, productive sites than at dry, unproductive sites. There was no effect of clipping on plant-available soil N or soil moisture across sites. Under the assumptions of density-dependent, resource-limited plant growth, we estimated maximum relative growth rates and steady-state biomass for each community type. In June, the ratio between a plot’s pre-clipping biomass and its estimated steady-state biomass explained over 80% of the variance in ANPP after clipping. Predictive power declined in July, as 63% of the variation of clipping effect on ANPP was explained by a complicated function of soil moisture, soil nitrogen, percent cover of grasses, and pre-clipping biomass relative to steady-state. The effects of density dependence on ANPP in this clipping study mirrored similar patterns in adjacent plots grazed by bison. These results suggest that the predictive ability of the grazing optimization hypothesis may be greatly improved by considering the influence of resource limitation via density dependence at the time of a grazing event.