Rooting depth and specific leaf area modify the impact of experimental drought duration on temperate grassland species

1. Due to climate change, temperate grasslands are being exposed to increasingly severe droughts. Concurrently, land-use intensification is altering grasslands' functional composition by promoting fast-growing, resource-acquisitive species with high specific leaf area (SLA).
2. How SLA affects the ability of grassland species to resist and recover from increasingly severe droughts and if deep roots improve their drought performance remains unclear. To investigate this, we established a common-garden field experiment including temperate grassland species with SLAs of 17.9–39.3 mm2 g−1 and maximal rooting depths of 16–252 cm. After 1.5 years, we simulated droughts for 0, 79, 134, 177 and 220 days.
3. Drought effects on plant performance increased with drought length, reducing the survival of green tissue and annual biomass by up to ~50% across all 32 species considered. As plant-available water remained in deep soil layers by the end of all treatments, deep roots mitigated the negative effect of increasing drought length on productivity in the later stage of drought and favoured productivity after a longer drought. The low-to-high SLA trait gradient among the 16 graminoid species seemed to represent alternative survival strategies ranging from dehydration tolerance to dehydration avoidance, rather than drought sensitivity.
Variable drought responses along the SLA gradient of forbs imply that multiple other traits are related to drought resistance across evolutionarily distant species.
4. Synthesis. Our results suggest that deep roots are beneficial for temperate grassland species subjected to longer periods without rainfall when plant-available water is lacking in shallow soil layers but remaining in deep soil layers. In the face of increasing drought severity, we thus recommend (1) fostering deep-rooted species in intensive grasslands on deep, productive soil and (2) directing further studies towards identifying management practices that support deep rooting in semi-natural grasslands.