A conservative resource use strategy in agricultural grasslands counteracts lower productivity and water use efficiency under drought conditions

Grassland response to changes in water availability is closely tied to the traits of the plant community which determine plant water uptake and resource use. Plants can adopt either moderate and efficient (conservative) or rapid and demanding (acquisitive) resource use strategies. These strategies combined with the plant interactions with microbes, such as arbuscular mycorrhiza fungi (AMF), determine the grassland productivity and efficiency. This study aims to compare the drought response of two agricultural grasslands that differ in their resource use strategies. In an experimental garden 12 small-scale lysimeters were installed with two different agricultural grassland types (conservative and acquisitive) and two different irrigation levels (wet and drought). We measured water fluxes, above- and belowground phytomass productivity and AMF spore productivity in these two grassland types throughout a drought of 64 days. Despite differing resource use strategies, both grassland types exhibited similar reductions in evapotranspiration and aboveground phytomass under drought. However, the conservative grassland showed higher water use efficiency (WUE) when considering only aboveground phytomass, and a less pronounced compositional shift towards greater grass phytomass. Furthermore, in acquisitive grasslands the root:shoot ratio of grasses and AMF spores abundance in the soil were greatly reduced than in conservative grasslands. We also identified differences in legume productivity, rooting system, and AMF spores community composition as key factors influencing WUE. In a changing climate with greater frequency and severity of droughts in the European Alps, opting for grassland mixtures with more species with conservative characteristics should be considered, as they i) reach the productivity of acquisitive grassland even under wetter conditions and ii) show higher efficiency and longer vitality under drought conditions.