Grazing effect on grasslands escalated by abnormal precipitations in Inner Mongolia

Grazing effects on arid and semi-arid grasslands can be constrained by aridity. Plantfunctional groups (PFGs) are the most basic component of community structure(CS) and biodiversity &ecosystem function (BEF). They have been suggested asidentity-dependent in quantifying the response to grazing intensity and droughtseverity. Here, we examine how the relationships among PFGs, CS, BEF, and grazingintensity are driven by climatic drought. We conducted a manipulative experimentwith three grazing intensities in 2012 (nondrought year) and 2013 (drought year).We classified 62 herbaceous plants into four functional groups based on their lifeforms. We used the relative species abundance of PFGs to quantify the effects ofgrazing and drought, and to explore the mechanisms for the pathway correlationsusing structural equation models (SEM) among PFGs, CS, and BEF directly or indirectly. Grazers consistently favored the perennial forbs (e.g., palatable or nutritiousplants), decreasing the plants' relative abundance by 23%-38%. Drought decreasedthe relative abundance of ephemeral plants by 42 ± 13%; and increased perennialforbs by 20 ± 7% and graminoids by 80 ± 31%. SEM confirmed that annuals andbiennials had negative correlations with the other three PFGs, with perennialbunchgrasses facilitated by perennial rhizome grass. Moreover, the contributions ofgrazing to community structure (i.e., canopy height) were 1.6-6.1 times those fromdrought, whereas drought effect on community species richness was 3.6 times ofthe grazing treatment. Lastly, the interactive effects of grazing and drought on BEFwere greater than either alone; particularly, drought escalated grazing damage onprimary production. Synthesis. The responses of PFGs, CS, and BEF to grazing anddrought were identity-dependent, suggesting that grazing and drought regulationof plant functional groups might be a way to shape ecosystem structure and function in grasslands.