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树叶流失的水量方面起着重要作用，这一过程被称为蒸腾作用。在植物蒸腾过程中，水分从树枝和茎中流失的速度比根部从土壤中吸收水分的速度要快。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，目前的陆地表面模型并不代表这些水力过程和树木内部的储水水平，而是直接根据土壤湿度预测蒸腾速率。这可以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学。FETCH2模型解析了流经树干的水流，以真实地模拟水力应力导致的蒸腾减少。来自大规模生态干扰实验的数据将用于验证该方法。将开发FETCH2，使其能够与其他生态系统和陆地表面模式相结合。我们将测试FETCH2的有效性，通过将几个生态系统模型的结果与干扰实验点和附近未受干扰的对照点的树木水分状况和蒸腾的观测结果进行比较。研究小组与俄亥俄州水资源中心（OWRC）合作，根据“WET”项目（教师水教育）起草的课程活动模板，开发了一个动手活动包，以说明多孔介质的流动，该项目将使用游戏和实验来说明小学水平的科学原理。树木的地上水分储存在调节森林冠层蒸腾作用中起着关键作用。植物从茎库中蒸发水分。当蒸腾速率高于土壤通过根、茎和枝的最大补给速率时，地上储存量减少，气孔关闭以限制蒸腾，以响应负木质部水势。这些水力限制控制了森林生态系统在潮湿和干燥条件下的蒸腾作用。目前的陆地表面模型不能表示树木的地上储存量。这些模型通过将气孔导度与土壤湿度直接联系起来，对蒸腾作用施加了水资源限制。由于土壤水分的日动态与树木木质部水分储存的动态有很大不同，目前的方法与观测到的蒸腾动态存在偏差。因此，陆地表面模式在模拟潜热通量时产生特征性的日内误差模式。本研究将建立一个框架来解决这样的树木流体动力学，该框架可以纳入水文，陆地表面和地球系统模型，以取代目前气孔导度和土壤湿度之间的经验联系。FETCH2模型解析了树干的水流和水势，将气孔导度与木质部的水势真实地联系起来，而土壤水分有效性为水动力系统提供了一个底部边界条件。来自密歇根州森林的大规模生态干扰实验的数据将用于验证这种方法。FETCH2模拟将与森林样地的树液通量和茎干储水量的观测结果以及与没有水动力模块的陆地表面模式模拟结果进行比较。在大气、水文和地球系统模型中对气孔导度和蒸腾模拟的改进将传播到诸如土壤湿度、地表能量预算和总初级生产力等相关变量。通过考虑森林冠层结构、树木储水量和水力策略对气孔导度的影响，本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.