Abstract:

Tree mortality under drought stress is an important physiological and ecological process that alters the structure and function of forests and influences forest management decisions. Under drought stress conditions, the water and carbon metabolisms in trees have malfunctioned, which leads to tree mortality. However, the exact physiological mechanisms of tree mortality in drought-stressed environments and the physiological-ecological response of various organs in trees during the drought-rewatering process have not yet been elucidated, and this scientific question is currently at the forefront of research worldwide. There are at least three hypotheses for the mechanism of drought mortality in trees: hydraulic failure, carbon starvation and the biological attack hypothesis. The hydraulic failure hypothesis suggests that under drought stress, the xylem of a tree reaches a threshold of embolism and is unable to restore water conduction, thus causing the tree to die of dehydration and desiccation; the carbon starvation hypothesis suggests that under drought stress, stomata are tightened or even closed and the tree cannot photosynthesize to produce organic matter, but the tree is depleted of available carbon in order to maintain its normal growth and metabolism; the biological attack hypothesis suggests that drought stress affects the distribution of biological factors and growth rate, making the tree vulnerable to attack by insects and pathogens, thus causing the tree to die. All hypotheses have been shown to explain a proportion of tree mortality, but are insufficient to explain all tree mortality in drought-stressed environments, and in particular the role of non-structural carbohydrate depletion in the mortality process remains unclear. This paper introduces water relations under drought stress and provides an overall account of the specific physiological processes involved in xylem embolism in the hydraulic failure hypothesis. It then discusses the effects of long-term drought stress on tree growth and development, transpiration, and the resulting changes in non-structural carbohydrate dynamics, explores the physiological mechanisms of individual tree organs during drought mortality, and summarizes the progress of research on physiological and ecological changes in trees during the drought-rehydration process. The current study shows that the hydraulic failure hypothesis remains the dominant hypothesis for tree mortality under drought stress, while the contribution of changes in non-structural organic carbon in trees to tree mortality remains undetermined. Post-drought rehydration can restore plant physiological functions and can compensate to a certain extent for the damage caused by drought, which affects both the morphological and physiological characteristics of trees. This paper also suggests directions for future research on tree mortality, with the aim of providing recommendations for the rational planting and management of trees in the future.

Key words: tree mortality, drought stress, hydraulic failure, rehydration, carbon starvation