Drought and heat waves are thought to be the main cause of increasing rates of tree die-back in several biomes. Three possible and not mutually exclusive mechanisms have been proposed to be the drivers of this phenomenon: hydraulic failure caused by massive xylem cavitation and leading to plant desiccation, carbon starvation caused by prolonged stomatal closure and leading to impairment of primary and secondary metabolism, and finally biotic attacks. The different mechanisms have been reported to have different relevance in the different species. On the basis of the analysis of seasonal changes of water relations, xylem sap isotopic composition, and concentration of non-structural carbohydrates in different woody species during a summer drought, we propose that rooting depth is a key trait connecting water and carbon plant metabolism, thus mediating the likelihood of hydraulic failure vs carbon starvation in drought-stressed trees.
