Heat-induced increases in xylem vulnerability: Experimental evidence and possible mechanisms

Hotter droughts driven by global warming are increasing tree mortality worldwide. Here, we propose a theoretical framework that explains how dissolved gas affects the stability of bubbles in xylem sap, and shows that in the presence of gas, sudden increases in temperature can accelerate embolism formation. We then provide experimental evidence that heat exposure can change xylem vulnerability to drought. Using data from seven Australian and European tree species, we assessed how elevated temperature (+15 and +20 {degrees}C above ambient) affects both xylem embolism resistance, measured with the Optical Vulnerability Technique, and VPD-corrected drying times. We found evidence for reduced embolism resistance and accelerated dehydration in heat-exposed plants. On average, trees reached the water potential at 50% loss of xylem function at 0.73 MPa less negative water potentials and 7.8 kPa h earlier, though responses were highly species-specific, and credibly different from zero only in the Australian species. Our results are consistent with the proposed framework and suggest that embolism resistance may not be a static trait, which has important repercussions for studies of plant vulnerability to drought. Further studies are necessary to assess the potential effects of heat-induced reductions in embolism resistance on tree mortality during heatwaves.