Biotic and abiotic controls in determining exceedingly variable responses of ecosystem functions to extreme seasonal precipitation in a mesophytic alpine grassland

•Responses of a mesophytic Tibetan grassland to extreme precipitation were simulated.•Completely divergent responses for growing season, plant functional types and organs.•Dominating species shift from sedges to forbs exaggerated grassland degradation.•Synergistic effects of soil temperature and water content were key abiotic factors.•Fairly high resilience contributed to the grassland acclimation and stability.

As a result of global climate change, frequent and intensive extreme climate events are observed and predicted. These extreme climate events can change ecosystem functions through driving plant growth and mortality, as well as ecological and evolutionary processes into different directions. We simulated a 1-in-100-year extreme precipitation event in different plant growth seasons (i.e. spring, summer, or autumn) to investigate the functional responses of a mesophytic alpine grassland ecosystem to climate change in the Tibetan plateau. The results demonstrated that the seasonal distribution of extreme precipitation is a critical factor in determining soil microbial- and plant-productivity. The response of vegetation net primary productivity (NPP) to extreme precipitation depends on the growth seasons and plant types. Total NPP in the treatments experienced extreme precipitation in the early (spring), mid- (summer) and late- (autumn) plant growth seasons were significant lower, higher, and no difference, respectively, compared with the control. Soil temperature and moisture were the key abiotic factors that affected ecosystem functions. For example, in the early plant growth season, no changes of soil moisture and the decreased temperature in response to the extreme precipitation resulted in a substantial decline in NPP. By contrast, in the mid-plant growth season, higher temperature and soil moisture elicited positive synergistic effects on plant growth and soil microbial processes. The increased sensitivity of above-ground NPP and the shift of dominant species from sedges to less palatable forbs might inevitably exaggerate the degradation of this grassland. Nevertheless, a high resilience index in the related ecological processes could potentially contribute to the grassland acclimation and stability, as most parameters returned to the similar levels as in the control after the extreme events ceased. Therefore, several synergistic or antagonistic mechanisms are hypothesized to operate in parallel or at different levels of organization and timescales. Further studies involving a range of different potential scenarios and longer periods are needed to predict the future climate change impacts on mesophytic grassland.