Including tree water storage dynamics in modeling of stomatal conductance

储存在树木中的水在调节树木通过叶子失去多少水分方面起着重要作用，这一过程称为蒸腾作用。当植物蒸腾时，水分从枝和茎中流失的速度比根系从土壤中吸收水分补充的速度要快。为了防止过度干燥，植物通过降低蒸腾速率来应对这种“水力压力”。然而，目前的陆面模型并不能代表这些水力过程和树木的储水水平，而是直接根据土壤水分来预测蒸腾速率。这可能会在蒸腾模拟中产生典型的每日误差模式。本研究提出了一个模型来表示这些缺失的树内水动力。FETCH 2模型解析了通过树干的水流，以真实地模拟由于水力应力而导致的蒸腾作用的减少。一个大规模的生态干扰实验的数据将被用来验证这种方法。将开发FETCH 2，以便它能够与其他生态系统和陆地表面模型相结合。我们将测试FETCH 2的有效性，通过比较几个生态系统模型的结果，有和没有FETCH 2模拟与观测树木水分状况和蒸腾干扰实验地点和附近的未受干扰的控制网站。 研究团队与俄亥俄州水资源中心（OWRC）合作开发了一个实践活动包，以说明多孔介质流的基础上，项目“WET”（水教育教师）起草的课程活动模板，将使用游戏和实验来说明小学水平的科学原理。树木地下水贮量在调节森林冠层蒸腾作用中起着关键作用。植物从茎部的贮水器中蒸发水分。当蒸腾速率高于土壤通过根、茎和枝的最大补给速率时，地上部分的储存变得耗尽，气孔关闭以限制蒸腾，以响应负的木质部水势。这些水力限制控制着森林生态系统在干湿条件下的蒸腾作用。目前的陆面模型不能代表树木的地上储水量。这些模型通过直接将气孔导度与土壤湿度联系起来，对蒸腾作用施加了水资源限制。由于土壤水分的日内动态与树木木质部中的水储存动态有很大不同，目前的方法导致与所观察到的蒸腾动态的偏差。因此，陆面模式在潜热通量模拟中产生特征性的日内误差模式。这项研究将开发一个框架，以解决这种树的水动力学，可以纳入水文，地表和地球系统模型，以取代目前的气孔导度和土壤水分之间的经验联系。FETCH 2模型解决了树干中的水流和水势，并将气孔导度与木质部中的水势联系起来，而土壤中的水供应为水动力系统提供了底部边界条件。在密歇根州的森林大规模生态干扰实验的数据将被用来验证这种方法。FETCH 2模拟将与森林样地中的树液通量和树干水储存观测结果以及没有水动力学模块的陆面模型模拟结果进行比较。在大气、水文和地球系统模型中对气孔导度和蒸腾作用模拟的改进将传播到土壤湿度、表面能量收支和总初级生产力等相关变量。通过将森林冠层结构，树木储水，和水力策略对气孔导度的影响，这项研究可能会影响许多类型和类别的气候，水文和气象模型。

Water stored in trees has an important role in regulating how much water the trees lose through their leaves, a process called transpiration. As plants transpire, water is lost from branches and stems more quickly than it can be replenished by water uptake from the soil by the roots. In order to prevent excessive drying, plants respond to this "hydraulic stress" by reducing transpiration rates. However, current land-surface models do not represent these hydraulic processes and the level of water storage within trees, but predict the transpiration rates based directly on soil moisture. This can produce typical daily patterns of error in simulations of transpiration. This research proposes a model to represent these missing within-tree water dynamics. The FETCH2 model resolves water flow through the tree stem to simulate realistically reductions in transpiration due to hydraulic stresses. Data from a large scale ecological disturbance experiment will be used to validate this approach. FETCH2 will be developed so that it can be coupled with other ecosystem and land-surface models. We will test the effectivity of FETCH2 by comparing the results of several ecosystem models with and without FETCH2 simulations with observations of tree water status and transpiration made in the disturbance experiment site and in a nearby undisturbed control site. The research team has partnered with the Ohio Water Resources Center (OWRC) to develop a hands-on activity package to illustrate porous-media flow based on the curricular activities template drafted by Project 'WET' (Water Education for Teachers), which will use games and experiments to illustrate the scientific principles at the primary school level. Above-ground water storage in trees plays a key role in regulating transpiration in forest canopies. Plants transpire water from the stem storage. As transpiration rates are higher than the maximal recharge rate from the soil through the roots, stem, and branches, the above-ground storage becomes depleted and stomata close to restrict transpiration in response to the negative xylem water potential. These hydraulic limitations control transpiration in forest ecosystems under both wet and dry conditions. Current land-surface models do not represent the above-ground storage in trees. These models impose water resource limitations on transpiration by directly linking stomatal conductance to soil moisture. As the intra-daily dynamics of soil moisture are very different than the dynamics of water storage in the tree xylem, the current approach leads to deviations from the observed dynamics of transpiration. As a result, land surface models produce characteristic intra-daily patterns of errors in simulations of latent heat flux. This research will develop a framework to resolve such tree hydrodynamics that could be incorporated into hydrologic, land surface, and Earth System Models to replace the current empirical link between stomatal conductance and soil moisture. The FETCH2 model resolves the water flow and water potential in the tree stem and realistically links stomatal conductance to the water potential in the xylem, while water availability in the soil provides a bottom boundary condition for the hydrodynamic system. Data from a large scale ecological disturbance experiment at a forest in Michigan will be used to validate this approach. FETCH2 simulations will be compared to observations of sap flux and stem water storage in the forest plots and to land-surface model simulation results without the hydrodynamic module. Improvements to the simulation of stomatal conductance and transpiration in atmospheric, hydrologic, and Earth system models will propagate to connected variables such as soil moisture, the surface energy budget, and gross primary productivity. By incorporating the effects of forest canopy structure, tree water storage, and hydraulic strategy on stomatal conductance, this study may impact many types and classes of climate, hydrologic, and meteorological models.