Excluding croplands, ecosystems with a grassy layer represent up to 40% of the terrestrial biosphere, and grasses further comprise much of the herbaceous layer in many other systems. Grasses exhibit broad trait variation across the multitude of environmental envelopes in which they grow, but represent a small proportion of global trait databases and grass-specific function is not well represented in earth system models. While existing over a large range of temperatures, all grasslands tend to exist in areas with seasonal precipitation inputs, often with high interannual variation. Therefore variation in both soil- and atmospheric-water availability is high, and understanding how grasses respond to such variation is important for expanding our knowledge of grassland ecophysiology and for updating models to better represent grass function.

Here we focus on two ecophysiological metrics for grass response to soil and atmospheric drought: stomatal responses to changing leaf-to-air vapor pressure deficit (VPD_L) and leaf turgor loss points. Where possible, we examine these results from both a lineage- and site-based framework along environmental gradients in North America at select NEON and LTER grassland sites. Historically, grass stomata are thought to be highly responsive to environmental variation, yet we show that grass stomatal responses are relatively insensitive to changes in VPD_L compared to other plant types. In woody plants, the leaf-turgor-loss point is generally reflective of plant distribution and drought tolerance, and has predictive association with reductions in hydraulic and stomatal conductance. We show that turgor loss points are consistent within grass lineages, but in contrast to woody plants, only mildly reflective of growth environment and poorly representative of the cessation of physiological activity. We describe potential reasons behind these observations that are grass-specific, and develop a cursory framework for implementing these responses into earth-system models.