生理过程及物理环境对湿润区森林生态系统蒸散的贡献及其调控机制研究

Evaporation and transpiration are the two components of evapotranspiration，due to the complexity two variables, evapotranspiration is a very complicated continuous process that regulated by plant physiological processes such as photosynthesis and physical environment, e.g temperature rising, it shows a lot of uncertainties. Up to date, all these uncertainties and the mechanism are not so clear in context of global warming. Comparing to the studies on arid or semi-arid region, previous studies on evapotranspiration are very scare in humid regions. Our project focuses on typical forest ecosystems in a humid southern China, and set up experimental sites at the Dinghushan Forest Ecosystem Research Station, we have established the experimental platform of forest ecosystems transplanted down an elevation gradient, which accurately controls the hydrological process. This big well designed field warming experiment platform is very favorable to determine the uncertainty of evapotranspiration. The main contents of the research are: (1) to study canopy conductance and transpiration rate in different forest types and different artificial temperature controlling conditions along the elevation, (2) to study the response of transpiration and its capacity to the seasonal changes of the leaf area index at stand scale, (3) to quantify different forest canopy and the atmosphere exchange characteristics of different forest tree species in water use efficiency, and quantify the contribution to evapotranspiration in different growth stages, (4) to assess the water-vapour exchange and its response to evapotranspiration capacity comprehensively in soil-plant-atmosphere continuum in humid region in lower subtropical China, and reveal biological, physical control mechanisms on evapotranspiration at plant ecosystem level under temperature rising. After carrying out this project, we will try to reveal the mechanism of water resources and water vegetation change, and clarify the linkage between temperature rising and water hydrology at the ecosystem level.

蒸散包括了蒸发与蒸腾，是一个受生理过程和物理环境作用的复杂过程，具有很大不确定性，这种不确定性的原因及其机理并不完全清楚，特别在全球变暖情形下。过去的研究更少见于湿润区，本项目选择华南湿润区森林生态系统为研究对象，于鼎湖山国家野外生态定位研究站设计了严格控制水循环过程的森林生态系统移位实验平台，该大型增温平台为研究蒸散的不确定性提供了条件。本项目研究不同温度环境下生态系统优势树种冠层导度和蒸腾速率、林分尺度上蒸腾量及蒸腾对叶面积指数季节变化的响应、不同森林冠层与大气之间的水分交换特点、不同林型树种的水分利用效率特征，不同生长阶段森林蒸散差异。综合评估南亚热带湿润区森林生态系统土壤-植物-大气连续体的水分交换能力及其在蒸散能力上的响应特点，揭示气温上升情形生态系统蒸散的生物、物理调控机制。通过本项目的研究，我们试图在生态系统水平上弄清楚水资源与植被变化机理，阐明气温上升与水资源的水文学联系。

对于南亚热带湿润区，即生理过程及物理环境对森林生态系统蒸散的贡献及其调控机制的物理学、生物学、生态学联系有待发掘，即全球环境改变表现在物理环境的改变及生理过程发生改变情形下，从机理机制上对森林蒸散的趋势性变化的解释，目前是一重大科学问题。本研究以位于华南湿润区鼎湖山的森林生态系统为研究对象，运用严格控制水循环过程的森林生态系统移位实验平台用长期生态学定位监测平台，研究不同温度环境下生态系统优势树种冠层导度和蒸腾速率、林分尺度上蒸腾量及蒸腾对叶面积指数季节变化的响应、不同森林冠层与大气之间的水分交换特点、不同林型树种的水分利用效率特征，不同生长阶段森林蒸散差异。以探究增温情形下生态系统蒸散的生物、物理调控机制。主要结论为：（1）林木耗水的本身生理属性上，树木边材面积与胸径相关，这种相关性不受树种及空间位置的限制。（2）不同林分之间差异水分利用是显著的。对于同一林分，林木水分利用与胸径相关，而与树种无关。然而，这种功能性关系在林分之间是不同的，进而导致相应林分的蒸散估计不同。不同立地间的差异是由于水汽压亏缺、光合作用光量子通量密度和叶冠层导度的专一性的差异。（3）鼎湖山森林生态系统水分利用效率（WUE）呈现出降低的波动趋势，年尺度上，生态系统水分利用效率变化的主要驱动因子为有效光合辐射、水汽压亏缺及气温。气温、VPD对干季WUE影响显著，PAR对湿季影响显著。（4）植物生长对水文过程有显著影响，增温在生态系统水平促进了鼎湖山三个林型（山地林、混交林及季风林）的植物生长。山地林和混交林的水量平衡对移位模拟增温有显著响应，增温使水量更多地分配到蒸散过程。季风林在红外增温的条件下产生的响应表现为：增温条件下总径流系数降低。增温通过直接作用和间接作用（植物生长）影响生态系统水文过程。本项目的结论对阐述生态系统水平上水资源与植被变化机理、及阐明气温上升与水资源的水文学联系有重要科学意义。