Traits explain sorting of C4 grasses along a global precipitation gradient

Species distributions are closely associated with moisture availability, but the underlying mechanisms remain unresolved. Drought relations are especially important for plants such as C₄ grasses that dominate seasonally dry ecosystems. Here, we test the hypothesis that C₄ grass species sampled across global precipitation gradients show variation in survival under drought that can be explained by their traits. Our experiment subjected 18 C₄ grass species to a lethal drought under controlled environmental conditions. The number of days until death was measured, along with root traits, senescence and aspects of hydraulic function. We identified two strategies; Drought avoiding species that stayed green as the water potential declined, and drought tolerating species that senesced more quickly but could extend survival via drought tolerant meristems. Plants that stay-green for longer occupied drier habitats and had the longest survival under drought, facilitated by narrow root diameter and isohydric stomatal behaviour. Plants that senesced quickly had thicker roots, an anisohydric strategy, and occupied wetter habitats. Global distributions of C₄ grasses can be predicted by variation in rates of senescence, meristem survival, root traits and stomatal strategy, showing the value of these traits for understanding plant distributions in relation to climate.