中亚热带森林典型树种幼苗的碳水特征对干旱-复水的响应

Drought-induced xylem embolism can potentially lead to imbalance of water and carbon in trees. In the context of future climates characterized with more frequent extreme droughts, the ability of tree seedlings to resist and repair xylem embolism is therefore crucial for water and carbon balance, seedling growth and survival, potentially impacting forest regeneration. Yet, it remains unclear that how the detailed water and carbon processes of seedlings in response to drought stress vary among typical tree species in mid-subtropical China. Particularly, the possible correlation between species-specific embolism reversal efficiency and non-structural carbohydrates remains to be investigated. In this project, potted seedlings of five typical tree species in evergreen broad-leaved forests in mid-subtropical China will be subjected to drought and rewatering in a greenhouse polytunnel. Hydraulic traits and carbon dynamics (leaf gas exchange, growth and non-structural carbohydrates) will be determined. This project aims to examine how the water and carbon strategies in response to drought stress will vary among species and the mechanism underlying the species-specific xylem embolism reversal efficiency. Finally, this project will determine the species-specific strategies to cope with drought stress. This project hopes to elucidate eco-physiological mechanisms explaining how the water and carbon processes of tree seedlings will deal with drought stress, helping making sustainable policies on forest management, vegetation restoration and biodiversity conversation under future climate change.

干旱引起的木质部栓塞会导致树木碳水失衡。在未来干旱频率与强度增加的背景下，树木幼苗木质部耐栓塞化与修复栓塞的能力对其碳水平衡与生长存活具有重要意义，进而可能对森林更新产生重要影响。然而，中亚热带森林典型树种幼苗应对干旱胁迫的碳水机制差异仍不清楚，特别是旱后木质部栓塞修复能力的种间差异与非结构性碳水化合物(NSC)的相关性亟待实验探讨。本项目以中亚热带常绿阔叶林5种典型树种的幼苗为对象，通过在温室大棚内对盆栽进行干旱及复水处理，研究幼苗的水力特征和碳动态(叶片气体交换、生长和NSC)对干旱-复水的响应，揭示不同树种幼苗在干旱胁迫下的碳水利用策略，探讨旱后茎木质部栓塞修复能力的种间差异及其影响因子，并综合分析以上树种的抗旱策略。研究结果拟从碳水代谢角度阐明中亚热带森林典型树种幼苗应对干旱胁迫的生理生态机制，并为气候变化下中亚热带森林经营、植被恢复和生物多样性保护提供科学依据。

干旱引起的木质部栓塞会导致树木碳水失衡。在未来干旱频率与强度增加的背景下，树木幼苗木质部耐栓塞化与修复栓塞的能力对其碳水平衡与生长存活具有重要意义，进而可能对森林更新产生重要影响。然而，中亚热带森林典型树种幼苗应对干旱胁迫的碳水机制差异仍不清楚，特别是旱后木质部栓塞修复能力的种间差异及其影响因子亟待实验探讨。本项目以中亚热带常绿阔叶林5种优势树种(即樟树、木荷、青冈栎、苦槠和深山含笑)的幼苗为对象，通过在遮雨大棚对盆栽进行干旱处理，当每个树种的木质部水势达到ψ50(即茎木质部导水率下降一半时的水势)时进行复水，进而研究幼苗的水力特征和碳动态对干旱-复水的响应。结果表明，干旱条件下，苦槠木质部抗栓塞能力最差，水势下降最快，最不抗旱。木荷木质部抗栓塞能力最强，水势下降最慢，最抗旱。所有树种的叶片气体交换参数随着叶片水势下降而不断降低，体现了碳水耦合性。不同树种、不同器官中的NSC对干旱的响应具有差异。干旱临界点时，大部分树种茎与根的淀粉含量相对于对照处理均降低，而大部分树种的可溶性糖含量升高或者没有显著变化。复水过程中，绝大部分树种的水势在复水0.5天后均能恢复到对照水平，但是直到2周后部分树种的茎木质部导水率才具有一定程度的恢复。青冈栎的茎木质部导水率的恢复能力最强(2周后恢复到对照水平)，并具有较强的叶片气体交换恢复能力(2.5天后恢复到对照水平)。木荷和深山含笑的茎木质部导水率在旱后分别恢复了26%和11%，但是其叶片气体交换仅能恢复到对照的20%。然而，苦槠与樟树的茎木质部导水率未见恢复，但叶片气体交换1周后可完全恢复。研究进一步表明，茎木质部导水率的恢复能力与木材密度、茎水容或NSC均无明显的相关性。综上所述，本研究从干旱-恢复的角度揭示了以上5种中亚热带优势树种的抗旱策略，阐明了其应对干旱胁迫的生理生态机制，可为气候变化下亚热带森林经营、植被恢复和生物多样性保护提供科学依据。