Recurrent heatwaves reduce production but reveal species mixture‑mediated resistance and recovery in grasslands.

Current and future heatwaves threaten temperate grasslands as they impair plant temperature regulation, photosynthesis, and eventually lead to production losses. Species mixture could mitigate the negative effects of heatwaves as documented for drought. Our main objective was to test the effect of species mixture on grassland responses to heatwaves by comparing mixtures with monocultures. We conducted a greenhouse experiment to simulate four heatwave treatments: i) control ii) two mild heatwaves, iii) a severe heatwave, and iv) recurrent scenario combining the mild and severe heatwaves. We analyzed production at mowing time, species resilience (as resistance and recovery) based on shoot growth, and foliar parameters known to be sensitive to heatwaves such as leaf temperature and maximum quantum efficiency of Photosystem II. Grassland responses were jointly shaped by the culture type and the heatwave treatment. While an isolated severe heatwave reduced production and negatively affected leaf physiological parameters, mild heatwaves had no detectable effects. Recurrent heatwaves induced cumulative reductions in production while promoting acclimation of Photosystem II. As for the type of culture, plants grown in mixture were more productive than when grown in monoculture, indicating overyielding. Certain species showed higher resistance or recovery in mixtures than in monocultures. Nevertheless, no mixture effect was significant on the studied leaf parameters. Our results suggest that the commonly used leaf parameters alone cannot reflect the resilience of plants exposed to heat, and that plant diversity improves the capacity to cope with heatwaves of temperate grasslands.