Phenotypic plasticity of water-related traits reveals boundaries to the adaptive capacity of a dominant European grass species under increased drought

The intensification of droughts due to climate change is a global concern, and many plant species face increasing water deficits. Understanding the role of phenotypic plasticity in plant adaptation to these changing conditions is crucial. This research focuses on Bromopsis erecta, a dominant perennial grass in European and Mediterranean grasslands, to predict its potential adaptation to climate change. We assessed plants from shallow and deep soils (i.e., with contrasting water reserves) of a Mediterranean rangeland in southern France, and tested the effect of six years of experimentally increased summer drought compared to the ambient conditions on plant traits, survival and abundance. In both field and common garden experiments, we measured water-related traits, including static traits under non-limiting water conditions, and dynamic traits, such as rates of trait variation during drought. Trait plasticity was determined as a reaction norm to increasing soil water stress and was tested against changes in B. erecta abundance over the past decade, including the study period. Trait plasticity was detected only for leaf dry matter content (LDMC), revealing that the resource strategy of B. erecta became more conservative over less than a decade with higher LDMC and leaf thickness according to the plant economic spectrum. No plasticity was found for osmotic potential or specific leaf area. The variability of other traits was ascribed to the possible lagging effect of previous water stress and was associated more with soil depth than with previous summer drought intensity. The abundance decline of B. erecta, which dropped from 20 % to around 5 % in shallow soils, was not associated with the plasticity of LDMC but was positively correlated with variations in leaf base membrane damage, meaning unexpectedly, that plants exposed to the most severe summer drought also had the most sensitive leaf base membranes, a possible sign of maladaptive trait plasticity in the population. This key trait response reveals boundaries to the adaptive capacity of this perennial grass to survive pluri-annual drought.