Mean root trait more than root trait diversity determines drought resilience in native and cultivated Mediterranean grass mixtures

Grasslands provide numerous ecosystem services but their sustainability is threatened by climate change. As plant functional diversity is expected to stabilize ecosystem functions, we tested whether mixing species with contrasting root systems could improve the resilience of Mediterranean grasslands under increasing aridity. We hypothesized that root functional identity (RFI) and diversity (RFD) respectively determines and improves soil water uptake capacity, aboveground biomass (AGB) production and resilience after drought stress (=post-stress AGB/pre-stress AGB). Monocultures, two- and three species mixtures of two groups of perennial grasses (cultivars and native species) were compared in a twin 3-years field experiment under two levels of summer drought in southern France. RFI and RFD were assessed as the mean and variance of multiple root traits (rooting depth, deep root mass fraction, root tissue density, root diameter and specific root length) measured in species monocultures. AGB and resilience were assessed from annual harvests; total transpirable soil water (TTSW) and evapotranspiration in summer (ET_sum) were assessed through the monitoring of soil water content. For both groups of species, RFI was a major predictor of TTSW and resilience, but not of AGB or ET_sum. Greater water uptake, especially from deep soil layers, increased resilience. Rooting depth distribution determined the potential depth of water uptake while root morphology influenced the precision of water uptake along the soil profile. However, RFD only marginally improved AGB production and resilience, although long-term effects of RFD should be tested. Designing artificial plant communities under water-limited conditions should therefore prioritize the maximization of rooting depth and root distribution along the soil profile. Diversifying root morphological traits associated with resource acquisition could also have a positive impact. The similarity of results between cultivars and native species suggests that agro-ecological guidelines for species assembly can be based on advances of functional ecology in natural ecosystems.