Grazing intensity differentially regulates ANPP response to precipitation in North American semiarid grasslands

Grazing intensity elicits changes in the composition of plant functional groups in both shortgrass steppe (SGS) and northern mixed-grass prairie (NMP) in North America. How these grazing intensity-induced changes control aboveground net primary production (ANPP) responses to precipitation remains an open question, especially in light of predicted climate changes. Here, we evaluated effects of four levels (none, light, moderate and heavy) of long-term (>30 yrs) grazing intensity in SGS and NMP on 1) ANPP, 2) precipitation use efficiency (PUE, ANPP:precipitation) and 3) precipitation marginal response (PMR, slope of a linear regression model between ANPP and precipitation). We advance prior work by examining 1) the consequences of a range of grazing intensities (more than grazed vs. ungrazed), and 2) how grazing-induced changes in ANPP and PUE are related both to shifts in functional group composition and physiological responses within each functional group. Spring (April-June) precipitation, the primary determinant of ANPP, was only 12% higher in NMP than in SGS, yet both ANPP and PUE were 25% higher. Doubling grazing intensity in SGS and nearly doubling it in NMP reduced ANPP and PUE by only 24% and 33%, respectively. Increased grazing intensity reduced C3 graminoid biomass and increased C4 grass biomass in both grasslands. Functional group shifts affected PUE through biomass reductions, as PUE was positively associated with the relative abundance of C3 species and negatively with C4 species across both grasslands. At the community level, PMR was similar between grasslands and unaffected by grazing intensity. However, PMR of C3 graminoids in SGS was eight-fold higher in the ungrazed treatment than under any grazed level. In NMP, PMR of C3 graminoids was only reduced under heavy grazing intensity. Knowledge of the ecological consequences of grazing intensity provides valuable information for mitigation and adaptation strategies in response to predicted climate change. For example, moderate grazing in SGS would sequester the same amount of aboveground carbon as light grazing because ANPP was nearly the same. In contrast, reductions in grazing intensity in NMP from moderate to light intensity would increase the amount of aboveground carbon sequestrated by 25% because of increased ANPP.