Droughts, heat waves and plant hydraulics: impacts and legacies

Extreme drought events accompanied by heat waves are posing a serious threat to tree survival in several forest biomes. Drought-induced tree mortality is thought to be driven by both hydraulic failure caused by extensive xylem embolism, and carbon starvation induced by prolonged stomatal closure to prevent or in response to massive xylem cavitation. Biotic attacks frequently accompany the terminal phases of tree decline and death. In this paper, we discuss the potential legacies of extreme droughts on plants’ vulnerability to successive stress events. By revising the current literature and presenting data recently obtained on Fraxinus ornus L., we argue that extreme droughts imply an hydraulic legacy in terms of increased vulnerability to cavitation and decreased hydraulic efficiency, possibly leading to long-term reduction of photosynthetic rates and further depletion of non-structural carbohydrates (NSC) pools. In turn, the ‘energetic’ legacy of reduced NSC availability is hypothesized to impair mechanisms underlying embolism repair, thus potentially exacerbating the hydraulic legacies. Future studies are called to elucidate the feedback and loops underlying progressive degradation of the hydraulic and metabolic networks of plants.