Extreme heat events have increased in severity in recent decades and have caused widespread decline among forest trees. During heatwaves, plants often experience high heat and drought stress simultaneously, yet our understanding of how plant hydraulically adjust to the combined stresses is limited. Our aim was to characterize hydraulic adjustment in two declining boreal species, paper birch (Betula papyrifera) and white spruce (Picea glauca), in response to heat, drought, and heat+drought stress. We subjected greenhouse grown birch and spruce seedlings to two consecutive summers of heatwave stress where each summer, the seedlings were exposed to three 8-day long heatwaves at 10◦C above ambient with and without water restriction. We measured a series of leaf and stem hydraulic leaf traits including leaf water potentials (Ψmin, max), turgor loss point (TLP), stomatal density (Ds), specific leaf area (SLA), conduit lumen diameters (Vdia, Tdia), conduit wall reinforcement (t/bh2), sapwood-specific conductivity (Ks), and Ψ at 50% loss in stem conductivity (P50). We hypothesize that 1) the independent heat and drought stresses will have opposing effects on xylem traits and vulnerability to hydraulic failure, 2) heat+drought will result in unique responses from either independent stress, and 3) the two species will differ in their responses to the combination of heat and drought.
Results/Conclusions
The data show that when exposed to heat+drought, both the birch and spruce responded uniquely in several traits rather than more similarly to either independant stress. In birch, heat alone generally lowered the TLP, Ψmin, and P50 but increased Vdia. Heat increased vulnerability to embolism (larger Vdia) but also reduced vulnerability to conduit wall collapse (higher t/bh2). Drought alone reduced Ds, but did not reduce conduit density, Vdia or increase t/bh2. Heat+drought produced different responses than either heat or drought alone; e.g. TLP became less negative, Ds increased, and conduit density and t/bh2 increased. In spruce, heat alone mildly increased vulnerability to embolism for Ds and native Ks increased in the second season. Drought alone did not appear to affect vulnerability to embolism in spruce. Heat+drought also did not seem to have major impacts on spruce, these plants reduced Ψmax and Ψmin, and increased Ds in the second season. Overall, the data show that these species differ in their ability to hydraulically adjust to heat+drought stress, and that the combined effects of heat+drought produced some responses unique responses from either heat or drought.