Compound drought-heatwave events during establishment reinforce perennial ryegrass dominance in grassland mixtures

Abstract Aims As compound drought-heatwave (CDHW) events increasingly affect western European summers, concern is growing about the stability of managed grassland mixtures under stress, particularly during establishment—a critical window when stress can have lasting effects on productivity and ecosystem services. We therefore tested how young grassland communities respond to CDHW regarding physiological performance, biomass stability and species persistence. Methods In a controlled growth-chamber experiment, we established mesocosms of three functionally contrasting forage species ( Lolium perenne L., Cichorium intybus L., Trifolium repens L.) grown in monocultures and mixtures. Communities were exposed to ambient or CDHW conditions. Gross primary production (GPP), soil moisture, evapotranspiration and leaf water potential tracked stress and recovery dynamics, while biomass and species proportions were assessed at harvest. Results CDHW reduced GPP in most communities, with species-specific physiological strategies: T. repens showed the highest resistance, whereas C. intybus , despite a 24% decline in GPP, showed the strongest recovery. Mixture responses were intermediate, with 37%–47% GPP reductions, reflecting grass dominance rather than complementarity. At harvest, mixtures equalled but did not exceed the best monoculture, but generally showed greater biomass stability and enhanced net diversity effects under CDHW. Dominance of L. perenne increased under CDHW, from on average 72% biomass under control to 96%. Conclusion Grassland mixtures buffered biomass losses during CDHW events but lacked physiological facilitation. Biomass stability did not prevent reinforced grass dominance, highlighting a specific risk: should CDHWs strike during establishment, they can undermine successful grassland mixture formation, limiting long-term ecosystem service delivery under climate change.