Forest Trees Are Not Equally Affected by Hydrologic and Atmospheric Drought

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 (Y_L)(isohydric species), and at the other they allow Y_L to decline (anisohydric species). The former adaptation reduces water loss at the cost of C gain, while the latter adaptation risks increased water loss and xylem failure while sustaining C uptake. Differences in water use along this continuum are most notable during drought, often characterized by low θ and high D. However, θ and D are not necessarily temporally coupled, and uncertainty remains about the sensitivity of different water use strategies to these variables. To determine whether species were more sensitive to changes in θ or D, we quantified the effects of both θ and D on g_s among three canopy-dominant species along the iso-anisohydric spectrum growing in different moisture conditions.

We analyzed data from three sites in the eastern United States: a mesic forest in western North Carolina, and two xeric forests in southern Indiana and Missouri. Leaf-level stomatal responses to changes in D were obtained using a LI-6400. Tree-level stomatal conductance was estimated using sap flow. Climatic measurements were obtained from flux towers at each site. For anisohydric species (i.e. Quercus alba), increasing D resulted in a 33% and 18% reduction of g_s in a xeric and mesic site, respectively, with no reductions due to decreasing θ. Isohydric species demonstrated a reduction in g_s driven by declining θ (22% (xeric) and 16% (mesic)) and increasing D (21% and 16%) with a greater loss of g_s attributed to θ (58%) than D (18%) during driest conditions at both xeric and mesic sites. Our results suggest that species types have similar sensitivities to increasing D under well-watered conditions, but that only isohydric speices decreased g_s with low θ. Thus, sensitivity to hydraulic stress varies among species and as a result the source of the immediate stress (atmospheric or soil). These results offer insights to ecosystem responses to environmental change as communities shift in dominance towards more mesic- or more xeric-adapted species.