Temperate grasslands facing heatwaves: species diversity buffers effects on shoot growth but not on leaf parameters

Current and future heatwaves threaten temperate grasslands. Plant temperature regulation, photosynthesis and therefore production are exposed. As with drought, species diversity could buffer the negative effects of heatwaves. Our main objective was to test the effect of diversity on grassland responses to heatwaves by comparing mixtures with monocultures. We conducted a greenhouse experiment to simulate four thermo-protocols: i) control ii) two mild heatwaves, iii) a severe heatwave, and iv) recurrent scenario with mild and severe heatwaves. We analyzed production at mowing time, resistance and recovery of species calculated from shoot growth, and foliar parameters sensitive to heatwaves such as leaf temperature and maximum quantum efficiency of Photosystem II. A severe heatwave induced negative effects on production and leaf parameters, whereas mild heatwaves not. Recurrent heatwaves induced negative cumulative effects on production and acclimation of Photosystem II. In this context, mixtures were more productive than monocultures average, as overyielding; certain species showed higher resistance or recovery in mixtures than in monocultures; but there was no diversity effect on leaf parameters. Our results suggest that leaf parameters alone cannot reflect the resilience of plants exposed to heat and that plant diversity can give temperate grasslands a higher capacity to cope with heatwaves.