Grazing’s effects on arid and semi-arid grasslands can be constrained by aridity. However, plant functional groups (PFGs) are the most basic component of community structure (CS) and ecosystem function (EF) and have been suggested as identity-dependent in response to grazing intensity and drought severity. Thus, it is necessary to examine how the relationships among PFGs, CS, EF, and grazing intensity are driven by climatic drought through experimental and quantitative analyses. We conducted a manipulative experiment with three grazing intensities in 2012 (non-drought year) and 2013 (drought year). We classified 62 herbaceous plants into four functional groups based on their life forms. We used the relative species abundance of PFGs to quantify the effects of grazing and drought, and to explore the mechanisms for the pathway correlations using structural equation models (SEM) among PFGs, CS, and EF directly or indirectly.
Results/Conclusions
Grazers consistently favored the most palatable or nutritious plants (i.e. perennial forbs), which decreased the relative abundance by23% – 38%. Drought decreased the relative abundance of ephemeral plants by 42±13% but increased perennial forbs by 20±7% and graminoids by 80±31%. Consequently, SEM suggested that annuals and biennials had competition (r < 0, p < 0.001) with the other three PFGs, but perennial bunchgrasses facilitated (r = 0.51, p < 0.001) perennial rhizome grass. Moreover, the contributions of grazing to community structural architectures were 1.6 – 6.1 times those from drought, whereas drought’s effect on community composition physiognomy was 3.6 times that of grazing treatment. Lastly, the interactive effects on EF were relatively greater than either grazing or drought effect; particularly, drought escalated grazing damage on primary production. The responses of PFGs, CS, and EF to grazing and drought were identity-dependent, suggesting that grazing and drought regulation of the changes in plant functional groups might be a way to shape ecosystem structure and function in grasslands.