Plant functional types drive differential responses of grassland ecosystem functions along a precipitation gradient

Abstract

Climatic variations can profoundly impact the functioning of grassland ecosystem, and understanding the drivers is critical for predicting ecosystem function under future climate. However, whether and how the responses of multiple grassland ecosystem functions to precipitation across spatial scales are controlled by the differential responses of the dominant plant functional types (PFT) remains unclear, mainly due to the scarcity of regional observational data. Here, we examined grassland ecosystem functions (aboveground plant biomass, belowground plant biomass, soil organic carbon, and total nitrogen) with different dominant plant functional types across 100 sites of four precipitation gradients in the Northeastern Qinghai-Tibetan Plateau by the moisture index. Across the broad precipitation gradient, higher mean annual precipitation significantly increased the plant species diversity and grassland ecosystem functions (aboveground plant biomass, soil organic C, and total N). From arid zone to humid zone, the above- and below-ground plant biomass allocation shifted to aboveground, and the trade-off between above- and below-ground plant biomass was the largest in semi-humid zone (0.13). Moreover, above- and below-ground plant biomass in arid, semi-arid, and semi-humid zones were driven by the grass-dominated communities (52%∼91%), while the humid zone was driven by the sedge- and forb- dominated communities (36%∼46%). The results of structural equational models further showed that the effects of precipitation on the ecosystem functions (soil organic carbon and total nitrogen) were mediated by either aboveground plant biomass (mainly in sedge-dominated communities) or species diversity (forb- and grass-dominated communities). These findings reveal that the asynchrony responses of different PFT-dominated communities to precipitation are altering grassland ecosystem functions, which provides the novel insight that plant functional group is particularly important for predicting ecosystem functioning under future climate conditions.