宁夏河东沙地典型防护林树种冠层蒸腾及其调控机理研究

Estimating tree water consumption by transpiration is an important step in assessing the effects of increasing vegetation cultivation on the hydrological cycle especially in sandy land, an arid region of northwestern China, which is characteristic of degradation in shelter-forest due to the increased contradiction between forest water use and water supply. In this study, water movement monitoring in Soil-Plant and Plant-Atmosphere system combined with water relations of leaves analyzing provide a reasonable option to study the physiological transpiration of typical shelter-forest tree species (Caragana intermedia and Ulmus pumila L.) response to environmental factors on a continuous basis in Hedong sandy land of Ningxia. With the guidance of SPAC theory, we analyze the dynamic changes of precipitation resource in plot, the soil water content in forest land, the physiological moisture characteristics of different trees and the content of undergrowth. Through studying the mechanism of soil moisture impacting on physiological characteristics of the tested tree species in pot experiment, the preferred soil water condition (as the range of soil water content being suitable for tree's growth) will be identified. In order to disclose the major control mechanisms of transpiration, leaf transpiration rate will be measured continuously and their internal controlling factors such as the water capacitance of tree tissues and the hydraulic conductivity will be measured respectively by water content potential method and flashing method. This study aims to provide a reference for analyzing the causes of vegetation degradation in Hedong sandy land of Ningxia. Meanwhile, it also could provide the scientific basis for optimizing forest structure allocation of regional vegetation restoration and reconstruction in this area.

树木蒸腾耗水量的准确测算是防护林体系优化配置、造林树种选择以及造林密度等林业生态工程建设技术中最关键、最核心的问题。本项目针对干旱沙区防护林建设中因林水矛盾加剧导致的沙地防护林植被退化的现实问题，以水分在"土壤－植物－大气连续体"不同界面的运移规律为基本研究思路，以宁夏河东沙地典型防护林树种柠条和白榆为研究对象，系统研究其水分生理特征及其蒸腾耗水节律，分析确定林木生长适宜的土壤水分区间；通过测定气孔导度对环境因子的响应、树体水力结构特征，探究林木蒸腾的调控机理。本项目旨在为剖析宁夏河东沙地植被退化问题提供参考，同时为该区域植被恢复与重建中林分结构的优化配置提供理论依据。

本研究借助大田定位观测和盆栽试验，重点研究了宁夏河东沙地典型防护林树种柠条和白榆的水分生理和蒸腾特性，并对林木生长适宜的土壤水分条件进行分析；通过设置不同水分梯度，研究干旱胁迫对苗木蒸腾作用的影响，着重分析了植物蒸腾对干旱环境的主要因子的响应过程，以探讨林木蒸腾的气孔调控机理。结果表明：（1）充分供水条件下的苗木蒸腾速率是土壤水分严重胁迫下的3倍左右，随着水分胁迫的加剧，叶部水分亏缺提早出现，使得蒸腾速率峰值前移，且日蒸腾过程在较为干旱时表现为双峰型，反映了其控制失水和维持体内水分平衡能力较强，是适应干旱的一种方式。（2）气孔的运动直接调控着蒸腾速率的变化，植株气孔导度与蒸腾速率具有相同的变化规律且二者成正比关系。（3）随着土壤含水量的逐渐降低，两种植物叶片蒸腾速率先增加后缓慢下降，过高的土壤含水量同样会抑制植物的蒸腾，该临界值分别为柠条：15.78%，白榆：15.41%，即当土壤质量含水量≤田间持水量的74.1%和72.3%时，柠条和白榆蒸腾速率随土壤供水量的增大而增大；高于此值，蒸腾速率降低。（4）低光强植物叶片蒸腾作用的光响应曲线差别不大，都表现出随着光合有效辐射强度增加而缓慢上升的趋势，但启动速度随着土壤含水量的减少而降低；在土壤水分受到限制时（土壤质量含水量≤40%田间持水量），进一步增强光合有效辐射量（≥1000 μmol•m-2•s-1）并没有使蒸腾速率提高，严重水分胁迫下，增加光辐射强度反而导致蒸腾速率降低。（5）选择维持较高叶片光合速率所需土壤含水量的低限值和维持最高叶片水分利用效率的土壤含水量临界值作为林木适宜的土壤水分范围。宁夏河东沙区典型树种柠条和白榆生长适宜的土壤水分区间分别为：10.22%-13.18%，10.78%-13.87%。（6）柠条和白榆蒸腾速率的主要影响因子偏相关分析表明：植物蒸腾速率对土壤水分胁迫的响应，通过光诱导气孔开放；在轻度水分胁迫下，光合有效辐射（PAR）是描述柠条和白榆蒸腾过程气孔行为的重要环境因子；随着土壤含水量的下降，气孔对PAR的敏感性降低，通过水汽压亏缺驱动气孔导度的变化，叶片内外水汽压亏缺成为调节柠条蒸腾的主导因素。（7）苗木叶水势随着土壤含水量的降低而下降，以便在植物水势和土壤水势之间形成较大的水势梯度，从而利于植物从土壤中吸收水分来满足蒸腾需求。研究成果可以为剖析宁夏河东沙地植被退化问题提供参考。