Characterizing stomatal sensitivity to rising atmospheric aridity is critical for parameterizing Earth systems models of regional carbon and water budgets. Nowhere in the United States are impacts of increasing atmospheric aridity more profound than in the Southwest, where episodic droughts and heat waves are dramatically altering hydrological cycling and ecosystem processes. Riparian ecosystems of the Southwest are among the most sensitive to the effects of increased aridity due to the relative drought intolerance of riparian tree taxa. Yet, we still lack a unifying understanding of stomatal behaviors in riparian taxa, and how stomatal sensitivity to rising atmospheric aridity shapes ecohydrological processes of riparian ecosystems. Although considerable intra- and inter-specific variation in stomatal sensitivity to vapor pressure deficit (D) exists both within and across ecosystems, we predict that stomatal sensitivity to D is largely uniform in riparian taxa across broad phylogenetic groups. We conducted a meta-analysis of canopy conductance sensitivity to D for ten dominant riparian forest species from western US riparian ecosystems spanning a large elevation gradient. Canopy conductance was calculated from sap flow data collected over multiple growing seasons and sites, where comparison of species was evaluated by normalizing canopy conductance data by species to D = 1 kPa.
Results/Conclusions
In general, our expectation of functional convergence in sensitivity to D across taxa was not supported. However, several significant patterns were observed. First, we found significant differences between native and non-native species within a single site, where non-native species showed greater sensitivity to D. Second, we found differences in sensitivity to D within a single genera spanning the entire range of the elevation gradient, where sensitivity to D was highest at the high elevation site compared to species of the same genera at mid- and lower elevations. And third, we found significant differences between low versus high elevation sites, although the opposite pattern was true, where all species combined for lower elevation sites had higher sensitivity to D compared to high elevation sites. We expect this difference is likely attributable to inter-specific variability within each site as well as markedly different conditions in mean maximum daytime D across sites. Our results show significant variation in sensitivity to D across species and sites and have implications for predicting changes in the population dynamics and distribution of riparian taxa in western riparian forests with expected changes in climate.