Pastures and Climate Extremes: Impacts of warming and drought on the productivity and resilience of key pasture species in a field experiment

Shifts in the timing and frequency of climate extremes, such as drought and heatwaves, can generate sustained shifts in ecosystem function with important ecological and economic impacts for rangelands and managed pastures. The Pastures and Climate Extremes experiment (PACE) in southeast Australia used a factorial combination of elevated temperature (ambient +3 {degrees}C) and winter/spring extreme drought (60% rainfall reduction) to evaluate the impacts of increased frequency of climate extremes on pasture productivity and subsequent summer/autumn recovery. The experiment included nine species comprising three plant functional groups (C3 grasses, C4 grasses, and legumes) in monoculture and three two-species mixtures. The winter/spring drought resulted in productivity declines of up to 73% (Digitaria eriantha) during the 6-month treatment period, with nine of the twelve plantings exhibiting significant yield reductions. Functional group identity was not an important predictor of yield response to drought. Many species recovered rapidly once the drought ended, although there were carry-over effects on warm season (summer/autumn) growth for four species/mixtures, spanning all functional groups. Cool season drought translated into significant reductions in annual biomass production for four species/mixtures, ranging from 33% (Medicago sativa) to 70% (Festuca arundinacea). Additionally, warming had neutral to negative effects on productivity during both winter/spring and summer/autumn periods, resulting in annual yield declines of up to 58%, driven at least partially by indirect effects on soil water content. The combination of winter/spring drought and year-round warming resulted in net yield reductions that were either additive or less-than-additive, compared to ambient plots. This study demonstrates that predicted extreme climate conditions will have substantial negative impacts on productivity of common pasture and rangeland species.