Compound Effects of Intensified Precipitation Patterns and Drought Occur Belowground in a Central US Grassland

Climate change is altering precipitation regimes globally, with expectations of intensified precipitation patterns (e.g., larger but fewer rainfall events) and more frequent and extreme drought. Both aspects of precipitation change can impact ecosystem function individually, but it is more likely that they will occur in combination. In a water-limited grassland in the central US, we imposed an extreme 2-yr drought (growing season precipitation reduced by 66%) on plots with a long-term history of exposure to either ambient or intensified precipitation patterns (average 3-fold increase in event size and 3-fold decrease in event number during the growing season, for 16 years). While this intensified pattern did not alter total precipitation amount, it generally led to ecosystem responses consistent with a drier environment (e.g., reduced soil moisture, aboveground net primary production (ANPP), and soil CO2 flux, but little evidence for altered root biomass). Surprisingly, this history of intensified precipitation patterns did not affect the response of ANPP to the subsequently-imposed extreme drought. In contrast, previous exposure to intensified precipitation patterns reduced root production (both mass and length) and muted soil CO2 flux responses to rainfall events during drought (though it did not alter growing season average soil CO2 flux). Reduced root production in plots experiencing compounded precipitation extremes was driven not by the dominant C4 grass species, Andropogon gerardii, but collectively by the subdominant species in the plant community. Overall, our results reveal that compound changes in precipitation pattern intensity and drought affected this dryland ecosystem in ways that were less apparent (i.e., belowground) than responses to either change individually, but the consequences for carbon cycling may be no less important.