中文摘要：

干旱是植物生长与存活最主要的限制因子之一。轻度或中度干旱胁迫会抑制树木的光合作用并降低森林生产力,而严重的干旱胁迫则可能造成森林死亡。全球气候变化背景下,干旱频率和强度的增加将对森林生态系统的碳水过程产生重要影响,甚至威胁森林的存活。极端干旱如何影响树木的碳水平衡存在很大争议,特别是干旱致死的生理机制仍不甚清楚。因此,探讨干旱导致树木死亡的生理机制对准确预测未来气候条件下森林的响应具有重要意义,是全球生态学和植物生理生态学的重要课题。目前备受关注的具有协同性的2个假说——＂水力失衡＂和＂碳饥饿＂可分别从水分关系与碳动态两方面解释树木干旱死亡的生理机制。近年来国际上相继有研究试图验证以上假说,但并无一致性结论,特别是非结构性碳水化合物的损耗在死亡过程中的作用仍不清楚。本文介绍干旱胁迫下的水分关系,对木质部栓塞以及水力安全范围进行探讨,解释＂水力失衡＂的基本原理。然后,讨论长期干旱胁迫对树木生长、光合作用和呼吸作用影响的差异,以及由此产生的非结构性碳水化合物动态,探讨＂碳饥饿＂潜在的基础,并总结干旱致死生理机制的研究进展。目前的研究表明,＂水力失衡＂是高强度干旱胁迫下树木死亡的主要原因,而＂碳饥饿＂是否发生有待在更长期的低强度干旱试验中验证。本文还对未来的研究方向提出建议：1）鉴于当前研究的地域和树种限制,以后的研究有待向中国的乡土树种拓展,并通过比较具有系统发生学差异的被子植物和裸子植物树种进一步阐明干旱胁迫对树木碳水关系的影响;2）探讨干旱胁迫下可溶性糖的组分变化以及植物激素的动态将从代谢方面丰富干旱胁迫影响树木的机制;3）为了更准确地预测森林对未来气候变化的响应规律,研究大?

英文摘要：

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 nonstructural carbohydrates for explaining the potential basics of＂carbon starvation＂. Furthermore,this paper reviewed recent progress in tree mortali