从气孔调控行为和木质部水力结构探索树木干旱适应策略--以华北石灰岩山地主要造林树种为例

Stomatal regulation and xylem embolism resistance are two important physiology mechanisms for trees to cope with drought stress, and they are closely related to hydraulic failure and carbon starvation which are the main causes of drought-induced tree mortality. The study on different machanisms in drought physiology of different tree species is helpful to the selection and collocation of tree species in afforestation in arid area. It is necessary to study different tree species in the same environment, so as to ensure the reliability of the difference among tree species. However, there is very rare study on the relationships among stomatal behavior, xylem vulnerability to cavitation and tree mortality rate under extreme drought condition for symbiotic multi-species. In this study, we are going to choose 20 tree species in one representative limestone afforestation site with largely different survival rates after afforestation in northern China. By measuring a series of key functional traits of plant leaves (turgor loss point, water potential at stomatal closure, leaf hydraulic conductivity, etc.) and xylem (xylem embolism resistance, xylem hydraulic conductivity, etc.) related to plant hydraulics, and analyzing coordination of stomatal behavior and xylem hydraulic architecture, this study aims to clarify the machanisms for the difference in stomatal behavior between species and reveal the hydraulic strategy and drought resistance mechanism of different tree species in arid environment of limestone mountain in northern China. We expect to propose a hypothesis, i.e., species with higher coordination ability of stomatal behavior and xylem embolism resistance tends to be more resistant to drought.

气孔调控和抗木质部栓塞是树木应对干旱胁迫的两个重要生理机制，与干旱引起的碳饥饿和水力失衡树木死亡机制密切相关。研究不同树种抗旱生理机制的差异有助于干旱立地造林时的树种选择和搭配，而在同一环境条件下对不同树种进行研究才能保证树种间差异的可靠性，但是目前缺乏同一干旱环境中共生的多个树种气孔行为、木质部水力结构和树种死亡率之间关系的研究。本研究以华北地区具有代表性的石灰岩造林后显示出极大存活率差异的20个树种为对象，通过测定叶片(失膨水势点、气孔关闭水势值、叶片导水率等)和木质部(木质部抗栓塞能力、枝条导水率等)的一系列水力功能性状，分析不同树种气孔行为和木质部水力结构的协同关系，阐明树种间气孔行为差异的调控机制，揭示不同树种在石灰岩干旱环境中的水力策略和抗旱性差异机制，并提出假说：气孔行为和抗木质部栓塞协同能力强的树种抗旱能力更强。

大量的证据表明气候变化背景下的干旱导致大范围的树木死亡、衰败，但是多树种共生群落的野外长期干旱试验较少，并且在野外条件下干旱导致树木死亡的机制研究不足。本项目在华北典型石灰岩多树种造林试验地，以造林多年后呈现出明显的死亡率差异梯度的20个温带阔叶造林树种为研究对象，围绕气孔调控和木质部栓塞抵抗，测定了系列的茎叶功能性状。项目完成了预期目标，取得的主要成果如下：（1）发现气孔关闭水势点显著相关于许多茎叶性状，显示茎叶解剖和生理性状的协同与权衡形成了沿isohydry至anisohydry的性状谱系。相比isohydric树种，更anisohydric的树种显示更强的胁迫抵抗，对茎叶的投资更大，在干旱条件下更能维持气孔开放。（2）栓塞抵抗能力正相关于木质部机械支撑性状，负相关于轴向薄壁组织比例；树种间存在一个木质部的储存能力和栓塞抵抗的权衡，推测温带阔叶树种可能存在一个从栓塞抵抗（更负的P50）到栓塞修复（更大的薄壁组织）的树种间的谱系。（3）栓塞抵抗能力和水力安全边际显著相关于树种存活率，表明水力失衡是树种在石灰岩干旱条件下死亡的主要生理学原因；气孔安全边际是比传统的栓塞抵抗和水力安全边际更好的预测树种存活率的指标。本项目研究结果拓展了植物水力学基础理论知识，表明气孔安全边际可以作为预测干旱条件下树木存活的指标，有利于干旱困难立地条件下人工造林树种的快速选择和预测气候变化背景下的森林动态。相关成果已发表论文5篇。

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