Abstract: Drought is a major environmental constraint that adversely affects plant growth and survival. Mild or moderate drought stress would inhibit tree photosynthesis and reduce forest productivity, while extreme drought stress may lead to forest decline. Increases in droughts frequency and intensity accompanied with global climate change will have great impacts on the water and carbon balance of forest ecosystem, with the potential to generate tree mortality. However, it remains in debate that how the water and carbon balance of trees will respond to extreme droughts. Particularly, the physiological mechanisms underlying drought-induced mortality are poorly understood. Therefore, unraveling the mechanisms of drought-induced tree mortality is of great importance to accurately predicting forest responses to future climates and is a key scientific question in global ecology and plant ecophysiology. Two non-exclusive hypotheses-"hydraulic failure" and "carbon starvation", explaining the water and carbon processes during drought-induced tree mortality, have recently received much attention. Many studies worldwide have been conducted to test the two hypotheses. However, results are inconsistent and the role of carbon depletion remains to be determined. Due to the scant research on drought-induced mortality in China, this review paper aims to help Chinese readers to improve the understanding of water and carbon processes during drought-induced tree mortality, by reviewing the international progress in this field. Firstly, this paper gave an introduction in water relations of trees during drought stress, discussed xylem embolism and hydraulic safety margin, and explained the basic principle of "hydraulic failure". This paper then discussed the differential impacts of prolonged drought on rates of tree growth, photosynthesis and respiration, as well as the resulting dynamics of non-structural carbohydrates for explaining the potential basics of "carbon starvation". Furthermore, this paper reviewed recent progress in tree mortality researches. According to current results, "hydraulic failure" plays the predominant role in causing tree death during highly intense drought, while whether "carbon starvation" occurs during tree mortality remains to be tested in longer (multi-year) but less intense drought. Finally, future research directions were also proposed as following points. In view of the geographical and tree species limitation of the current study future research needs to be done in native tree species in China and the understanding of how drought would impact water relations and carbon processes of trees can be improved by comparing species with phylogenetic differences such as angiosperms and gymnosperms. Determining the drought impacts on components of soluble sugars and dynamics of plant hormones can provide metabolic basis for understanding mechanisms of drought mortality. To accurately predict forest responses in future climates, it is a new research direction to examine the interactive effects of elevated CO₂ and elevated temperature on drought-induced tree mortality.

Key words: carbon starvation, drought-induced mortality, extreme drought, global climate change, hydraulic failure