宁夏河东沙地防护林冠层气孔导度和水力导度协同调节冠层蒸腾的机理研究

Estimating plant water consumption by transpiration is an important step in assessing the effects of increasing vegetation cultivation on the hydrological cycle especially in arid region. It is difficult to describe the regulation of water flux accurately by monitoring the variation of stomatal conductance solely, because the forest transpiration efficiency is related to the interaction among canopy stomatal conductance, leaf water potential, hydraulic conductance and environmental factors. Canopy transpiration reflects the relationship between plant water status and environmental conditions. It is necessary to dig further into the underlying mechanism that controlling the canopy transpiration by taking the coordination of stomatal conductance with whole-tree hydraulic conductance into account. In this study, water movement monitoring in Soil-Plant and Plant-Atmosphere system combined with sap flow techniques provide a low-cost option to study the canopy physiological transpiration of typical shelter-forest tree species (Populus bolleana, Caragana intermedia, Pinus sylvestris L. va r . mongolica Litv. and Ulmus pumila L.) response to environmental factors on a continuous basis in Hedong sandy land of Ningxia. To clarify the underlying mechanism, we integrate the measurements of gas exchange and sap flow as well as simultaneous recodes of micro-meteorological and other environmental factors．Such integrations will provide the basis for investigating the decoupling coefficients of canopy and atmosphere to disclose the stomatal control mechanisms of canopy transpiration, as well as figuring out the response of canopy stomatal conductance to vapor pressure deficit by analyzing the hydraulic conductance calculated from sap flow and leaf-soil water potential gradient. Meanwhile, the internal controlling factors such as the water capacitance of tree tissues and the hydraulic conductivity will be measured respectively to investigate the relationships between effective water storage and whole-tree transpiration, canopy stomatal conductance, as well as hydraulic conductance. We also study the effect of water storage on the water long-distance transport and water deficit expecting to reveal the mechanism of partial compensation for hydraulic limitation by water storage. It is expected that the elucidation of the coordinated regulation between canopy stomatal conductance and hydraulic conductance will help to accurately estimate water use of forest and will provide the scientific basis for optimizing forest structure allocation of regional vegetation restoration and reconstruction in arid region.

树木冠层蒸腾反映植物的水分状况及与环境的关系，蒸腾效率是一个涉及气孔导度、叶片水势和水力导度与环境因子相互作用的集合特征，单纯以气孔导度的变化难以完整描述水分通量的调节规律。冠层气孔导度与整树水力导度协同控制冠层蒸腾的潜在机理研究，对准确测算森林蒸腾耗水对生态系统水量平衡的潜在效应具有重要意义。本项目以宁夏河东沙地典型防护林树种为研究对象，运用热扩散式茎流计监测树干液流动态，观测林木冠层与大气的水汽交换特征，分析不同树种冠层气孔导度与大气的脱耦联系数和变异范围，阐明树木冠层蒸腾的气孔调节机理；研究树干液流和冠层蒸腾间的时滞，结合叶片/土壤水势梯度计算的水力导度，分析水力导度影响冠层气孔导度响应水汽压亏缺的敏感性，探讨树木储存水、根系与冠层间水分传输路径的水力导度对冠层蒸腾的影响，揭示水力导度和冠层气孔导度联合调节森林蒸腾的机理，为干旱半干旱区植被恢复与重建及林分结构的优化配置提供科学依据。

本项目以宁夏河东沙地典型防护林树种柠条、白榆、侧柏为研究对象，观测气孔导度变化、树干液流动态及其与大气的水汽交换特征，分析树木冠层蒸腾的气孔调节机理；研究树干液流和冠层蒸腾间的时滞，揭示水力导度和冠层气孔导度联合调节森林蒸腾的机理。结果表明：（1）太阳辐射（Rs）是描述蒸腾过程气孔行为的重要环境因子，随着土壤含水量（SWC）的下降，气孔对Rs敏感性降低，饱和水汽压亏缺（VPD）成为调节叶片蒸腾的主导因素。植物叶-土壤水势不平衡，黎明前叶水势比土壤水势低的多，林木叶片水容对蒸腾的调控表现为植物叶水容随SWC下降而降低。（2）太阳辐射是液流变化最直接的驱动力，但随着气温（Ta）上升，VPD增加到一定值后（1.35 kPa）植物的气孔调节作用启动，使得林木在第二阶段（11:00-17:00 h）虽然有较强的蒸腾拉力需求，但液流变化缓慢。在较高VPD下，植物通过生理策略限制气孔开度和气孔蒸腾来避免组织水分胁迫。冠层气孔导度（gc）与VPD、Ta的关系曲线上升阶段和下降阶段路径不同，呈现“磁滞回环”的特性，且不同时段表现出不同的响应规律。Jarvis-Stewart模型较好地模拟了gc对环境变量Rs、VPD、Ta的响应过程。（3）典型晴天下树干液流（Js）滞后于Rs 0.75 h，提前VPD、Ta 0.75 h和0.25 h。采用错位对比法得出典型晴天和雨后Js与冠层蒸腾（Ec）的实际时滞，结果表明：典型晴天下，白榆和侧柏Js与Ec变化同步；雨后，白榆Js滞后于Ec15 min，而侧柏Js较Ec提前15 min。时滞原因与其本身吸水潜力和木质部组织自适应能力有关，如侧柏输水组织为管胞，输水阻力较大，需要消耗掉较多的冠层储存水才能积累足够的蒸腾拉力；白榆时滞则是由于树干与叶片间运输距离的延滞。（4）侧柏和白榆gc则处于较低水平（1.053~2.853 mm•s-1），脱偶联系数Ω小于0.1（0.014~0.017），说明林木冠层与大气高度耦合。边界线分析得出引起气孔调节的太阳辐射阈值为247 W m-2，即当Rs>247 W m-2时，林木冠层蒸腾对Rs的响应敏感性降低；对VPD的响应阈值为1.35 kPa，对Ta的响应分为两个阶段一是>20.1 ℃时，Ec随Ta升高迅速增大，二是>25.9 ℃时，Ec随Ta的升高而降低。以上研究成果可为区域植被恢复及林分结构优化配置提供依据。