Eastern US deciduous tree species respond dissimilarly to declining soil moisture but similarly to rising evaporative demand.

Hydraulic stress in plants occurs under conditions of low water availability (soil moisture; θ) and/or high atmospheric-demand for water (vapor pressure deficit; D). Different species are adapted to respond to hydraulic stress by functioning along a continuum where at one end they close stomata to maintain a constant leaf water potential (ΨL) (isohydric species), and at the other they allow ΨL to decline (anisohydric species). Differences in water use along this continuum are most notable during hydrologic stress, often characterized by low θ and high D; however, θ and D are often, but not necessarily coupled at timescales of weeks or longer, and uncertainty remains about the sensitivity of different water use strategies to these variables. We quantified the effects of both θ and D on canopy conductance (Gc) among widely-distributed canopy-dominant species along the iso-anisohydric spectrum growing along a hydroclimatological gradient. Tree-level Gc was estimated using hourly sap flow observations from three sites in the eastern United States: a mesic forest in western North Carolina, and two xeric forests in southern Indiana and Missouri. Each site experienced at least one year of substantial drought conditions. Our results suggest that sensitivity of Gc to θ varies across sites and species, with Gc sensitivity being greater in dry than in wet sites, and greater for isohydric compared to anisohydric species. However, once θ limitations are accounted for, sensitivity of Gc to D remains relatively constant across sites and species. While D limitations to Gc were similar across sites and species, ranging from 16-34% reductions, θ limitations to Gc ranged from 0-40%. The similarity in species sensitivity to D is encouraging from a modeling perspective, though it implies substantial reduction to Gc will be experienced by all species in a future characterized by higher D.