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; theta) 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, on one hand, they close stomata to maintain a constant leaf water potential (Psi(L)) (isohydric species), and on the other hand, they allow Psi(L) to decline (anisohydric species). Differences in water-use along this continuum are most notable during hydrologic stress, often characterized by low theta and high D; however, theta and D are often, but not necessarily, coupled at time scales of weeks or longer, and uncertainty remains about the sensitivity of different water-use strategies to these variables. We quantified the effects of both theta and D on canopy conductance (G(c)) among widely distributed canopy-dominant species along the isohydric-anisohydric spectrum growing along a hydroclimatological gradient. Tree-level G(c) 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 1 year of substantial drought conditions. Our results suggest that sensitivity of G(c) to theta varies across sites and species, with G(c) sensitivity being greater in dry than in wet sites, and greater for isohydric compared with anisohydric species. However, once theta limitations are accounted for, sensitivity of G(c) to D remains relatively constant across sites and species. While D limitations to G(c) were similar across sites and species, ranging from 16 to 34% reductions, theta limitations to G(c) ranged from 0 to 40%. The similarity in species sensitivity to D is encouraging from a modeling perspective, though it implies that substantial reduction to G(c) will be experienced by all species in a future characterized by higher D.