Given the projected increase of extreme weather events in the future, the effects of drought events on ecosystem functions are of high interest. Grassland productivity follows a distinct seasonal pattern with high biomass productivity at the time of flowering and lower productivity during vegetative growth in early spring and after flowering in summer and fall. If the sensitivity of grassland productivity to drought varies throughout the growing season remains, however, unclear. To test the effects of drought events that occur at different times of the growing season on grassland productivity we installed a large field experiment where we simulated drought events in three seasons (spring, summer, fall) and determined the effects on productivity of different European grassland species. We designed our experiment with six harvests evenly distributed over the entire growing season allowing us to disentangle (i) immediate drought effects on the productivity and (ii) effects of seasonal droughts on the productivity over the entire growing season. By growing monocultures of different grassland species we were able to track species-specific responses of biomass productivity to seasonal droughts.
Results/Conclusions
Across all species, drought treatments had strong immediate effects on the biomass in every season (p<0.001). Drought treatments in spring and summer had the strongest immediate effects on biomass, while drought events occurring in fall reduced productivity less. Over the entire growing season, we found that annual net primary productivity (ANPP) was only affected by drought events occurring in summer (p<0.001), while spring and autumn drought events did not lead to a significant reduction of ANPP. This is caused by a significant increase of biomass production in the drought-following recovery phase to values exceeding those of the control. This increase is probably driven by accumulated soil nitrogen that becomes available after rewetting of the soil. Overcompensation of biomass production was also found for summer drought treatments, but to a much smaller extent. We did not find any significant inter-species differences of seasonal droughts neither on immediate productivity nor on ANPP. In summary, our results show that drought events occurring at different times of the growing season differ in their effects on grassland productivity. While immediate effects are detectable in every season and are strongest under drought events occurring in spring and summer, annual productivity is only affected by summer droughts.