Background and Aims Grasses in subfamily Pooideae live in some of the world’s harshest terrestrial environments, from frigid boreal zones to the arid wind-swept steppe. It is hypothesized that the climate distribution of species within this group is driven by differences in climatic tolerance, and that tolerance can be partially explained by variation in stomatal traits. Methods We determined aridity index (AI) and minimum temperature of the coldest month (MTCM) for 22 diverse Pooideae accessions and one outgroup, and used comparative methods to assess predicted relationships for climate traits versus fitness traits, stomatal diffusive conductance to water (gw), and speed of stomatal closure following drought and/or cold. Key Results Results demonstrate that AI and MTCM predict variation in survival/regreening following drought/cold, and gw under drought/cold is positively correlated with ẟ 13C-measured water use efficiency (WUE). However, the relationship between climate traits and fitness under drought/cold was not explained by gw or speed of stomatal closure. Conclusions These findings suggest that Pooideae distributions are at least partly determined by tolerance to aridity and above freezing cold, but that variation in tolerance is not uniformly explained by variation in stomatal traits.

With the rise of atmospheric CO 2 , current global average temperatures have increased by about 1°C compared to pre-industrial times (IPCC, 2021). As a result, we are experiencing increasingly severe drought and extreme weather events, the latter including unseasonal bouts of low and high temperatures that can adversely affect agricultural yield and lead to food insecurity (Cook et al., 2018;Craufurd