Drought is projected to increase in frequency and severity in temperate regions under conditions of climate change. However, we still lack a thorough understanding of drought effects on plant community assembly that enables us to project consequences of global change in grasslands. We hypothesize that if drought deterministically shapes communities, (1) species with low drought sensitivity are more abundant in drier habitats and (2) community weighted means (CWM) of traits conferring drought sensitivity are higher in drier habitats.
To understand the role of drought in community assembly, we integrated data of species cover and soil moisture from Biodiversity Exploratories (three regions across Germany, 50 plots each, 2009 ~ 2015) with comparative whole plant drought sensitivity, measured for 41 common temperate grassland species across the common garden drought experiment in 2015. As traits conferring plant differential drought sensitivity, we integrated the experimentally assessed phenotypic plasticity of rooting depth in response to drought and the water potential at turgor loss (i.e. wilting point). CWMs of drought sensitivity and of traits were computed to take into account species relative abundances in a given community. To address differential habitat moisture across time and space we used minimum daily soil moisture (SMmin) during the growing season.
Community weighted means of whole plant drought sensitivity and of traits significantly differed across plots, but not across years. Drought sensitivity decreased with decreasing soil moisture (SMmin) in the relatively dry region, but not in the other two regions. Consistently, plasticity of rooting depth and turgor loss point both increased with decreasing soil moisture, again only in the dry region. These findings show that drought has an important role as an environmental filter in temperate grasslands, determining the distribution of plant species especially in dry areas.