Drought- and heat-induced shifts in vegetation composition impact biomass production and water use of Alpine grasslands

Abstract Future climate change scenarios predict more frequent and intense droughts for Alpine grasslands, primarily driven by altered precipitation regimes and increased evapotranspiration. The grassland community is expected to adjust to decreasing soil moisture and increasing potential evapotranspiration, optimising water use according to the individual strategies and competitive interactions between the present species. Here, we show the reaction of an intensive Alpine grassland to drought and heat at community and plant functional group levels by using small high-precision lysimeters, as well as how the community adapts by a shift from legumes to grasses after a severe drought. Drought and heating experiments revealed a decrease in evapotranspiration as well as in biomass production only at the highest drought intensity (i.e. soil water potential reaching the permanent wilting point). At plant functional group level, an upscaling from leaf porometer measurements revealed a similar decrease of transpiration for legumes and grasses, but differences between them in the rewetting phase. Legumes were strongly affected by drought and showed a low regrowth during the recovery, while grasses enhanced transpiration and even exceeded biomass productivity of the moist treatment. This imbalance between functional groups caused a shift from legumes to grasses in the grassland community. We conclude that drought-induced community re-assemblage of Alpine grasslands enhances water use efficiency.