Extreme climatic events are expected to increase in frequency and magnitude as a consequence of global warming. Grasslands cover a large proportion of the European continent and contribute to both agricultural production and ecosystem services through inter and intraspecific genetic variability. This study analysed the effects of summer droughts and heat waves on the persistence and production of perennial forage grasses. Mediterranean and temperate populations of Dactylis glomerata L. and Festuca arundinacea (Schreb.) were compared at both Mediterranean and temperate sites in France. By manipulating canopy temperatures and water availability, grass swards in the field were subjected to cumulative summer and spring water deficits (CSSWD) ranging from 329 to 707 mm to test different projected climatic conditions and extreme summer events. Under controlled summer heat waves (6–21 days at a mean daily canopy temperature higher than 30–35 °C), there was no increase in membrane damage to surviving aerial tissues. Plant stress was thus mainly generated through greater soil water deficit. Under the greatest CSSWD, annual biomass production was reduced on average by 60% and 30% with temperate and Mediterranean populations, respectively. Thresholds for a significant increase in summer tiller mortality were seen at CSSWD higher than 450 mm for temperate populations and 550 mm for Mediterranean populations. The latter displayed lower predawn leaf water potentials in summer and recovered through intense tillering in the subsequent seasons. Under the most extreme CSSWD, fewer than 20% of tillers of temperate populations survived and their nitrogen uptake ability was drastically altered. The higher potential productivity of Mediterranean populations in winter was associated with greater frost sensitivity. The identification of thresholds for vulnerability and the determination of the role of genetic diversity will improve the management of plant resilience and the design of new plant material to cope with climate change.