Recurrent drought amplifies drought impacts and increases seasonal synchrony in mountain grassland communities

Climate change increases the recurrence of drought events with strong repercussions on grassland ecosystems. While the effects of single drought events on ecosystem structure and functioning are well understood, it is largely unknown whether and how recurrent drought events modify ecosystem responses to subsequent drought. Here, we assessed how the increase in drought recurrence of severe, annual summer drought impacted grassland communities. We examined these effects in a species‐rich sub‐alpine mountain meadow with drought recurrence of one, three and 13 years, compared to ambient conditions. We found that greater annual drought recurrence increased plant growth synchrony within the growing season. This pattern was associated with a reduction in species richness, a shift in plant functional groups, a loss of early‐seasonal plant species, and a constrained establishment of seedlings throughout the growing season. Furthermore, we show that negative drought effects were enhanced with an increasing drought recurrence, and that negative drought effects on plant communities outweighed the weak adaptive effects of individual species. We conclude that single drought and low‐recurrence drought studies may not adequately predict longer‐term plant community changes in our rapidly shifting climate. Furthermore, with the ongoing increase in drought recurrence, we predict grassland plant communities to become more vulnerable to future disturbances, as increasing seasonal synchrony reduces temporal buffering within the community, making the community less resilient. Moreover, given the weak adaptive effects of individual plant species to long‐term recurrent drought, we conclude that plant communities are unlikely to be able to adapt to the rapid increase in annually recurrent drought events.