温带典型树种木质部孔道结构对水分传输的影响机制

Water transport processes in trees are important parts of mass cycling and energy flow in forest ecosystem, which determine directly the forest ecological functions of soil and water conservation. Previous studies more concerned the boundary resistances of canopy transpiration or root water uptake, and thought the resistance of xylem network could be negligible. However, latest studies found that xylem network, especially the pit membrane structure, is very important for the water transport processes of trees, but the impact mechanism is not yet clear unfortunately. Hence, our project is planned to answer this scientific question. Based on the typical temperate coniferous and deciduous tree species in China, relationships between three-dimensional xylem structures of roots, trunks, branches and water transport resistance / conductance will be concluded by hydraulic experiments and micro-CT scanning electron microscopy observations. Xylem network hydraulic model will also be built, and will be evaluated by the sap flow observation data of different tree positons. Then, the impact mechanism of xylem structure network on water transport process in different temperate tree species will be explained, which can provide scientific basses for understanding accurately the water transport process and regulation of soil - plant - atmosphere continuum.

树木水分传输是森林生态系统物质循环和能量流动研究的重要内容，直接决定着森林涵养水源和水土保持等生态功能的发挥。传统研究多强调冠层蒸腾和根系吸水的界面阻力，视木质部孔道阻力忽略不计。最新研究发现，木质部孔道网络尤其是纹孔膜结构，对树木水分传输有重要的制约作用，但是影响机制尚未明晰。针对这一科学问题，本项目拟以我国温带典型针阔叶树种为研究对象，通过水力学模拟实验和微CT扫描电镜观察，分析各树种根、树干和枝条部位的孔道三维结构与水分传输阻力/导度的关系，构建基于木质部三维孔道网络的水力学模型，并利用树木不同部位液流观测结果进行模型验证，系统阐释不同树种木质部孔道结构对树木水分传输的影响机制，为准确理解土壤-植物-大气连续体水分传输及调控提供科学基础。

本项目以温带典型树种为研究对象，揭示了温带树种木质部孔道结构对其水分传输的影响机制，定量评价了不同树种木质部孔道结构对水分传输效率和安全以及固碳能力间的权衡作用。并基于上述研究结果，进一步研究了全球变化下树木木质部孔道结构改变对树种水分传输和固碳能力的影响机制。主要结论如下：..（1）不同温带树种具有明显的木质部孔道结构差异，这些差异与不同的木材类型、叶片形式和植物异速生长有关。环孔型树种与散孔材相比具有更宽的茎木质部导管直径，因而具有更高的水力传导性。复合叶树种比单叶树种的单叶面积更小，生长更快，更有利于木质部构件的快速生长，进而增强水力传导能力。..（2）温带树种的木质部孔道结构特征对于水分传输效率具有重要影响。研究发现木质部导管直径的改变提高了水力效率。然而，在水分充足的情况下，我们没有发现导水率和导水损失率有显著的相关性，表明多数温带树种木质部的水力效率和水力安全性之间只存在弱的权衡关系。..（3）基于氮添加和水分胁迫交互实验研究发现，短期氮添加对木质部孔道结构影响显著，但是没有改变干旱下的木质部水力结构和水力导度；长期施氮下木质部孔道结构的改变提高了水力效率，也导致了较高的木质部栓塞风险，但是碳水化合物的提高维持了木质部水力传输的完整性。