Including tree water storage dynamics in modeling of stomatal conductance

树木中储存的水分在调节树木通过树叶流失多少水分方面起着重要作用，这一过程被称为蒸腾作用。随着植物的蒸腾，水分从树枝和茎中流失的速度比根从土壤中吸收水分补充的速度更快。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，目前的地表模型并不代表这些水力过程和树木内的储水量，而是直接基于土壤水分来预测蒸腾速率。这会在蒸腾作用的模拟中产生典型的每日误差模式。这项研究提出了一个模型来描述这些缺失的树内水分动态。FETCH2模型解析了流过树干的水流，以逼真地模拟由于水力压力而导致的蒸腾减少。大规模生态干扰实验的数据将被用来验证该方法。将开发FETCH2，以便它可以与其他生态系统和陆地表面模型相结合。我们将通过比较几个生态系统模型在FETCH2模拟和没有FETCH2模拟的情况下的结果，以及在干扰试验场和附近未受干扰的控制点进行的树木水分状况和蒸腾作用的观测，来测试FETCH2的有效性。该研究小组与俄亥俄州水资源中心(OWRC)合作，开发了一个动手活动包，以说明渗透媒体流，其基础是由教师水教育项目起草的课程活动模板，该项目将使用游戏和实验来说明小学层面的科学原理。林木地上储水在调节林冠蒸腾中起着关键作用。植物从茎库中排出水分。当蒸腾速率高于土壤通过根、茎和枝条的最大补给速率时，地上部分的储藏变得枯竭，气孔接近限制蒸腾作用，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潮湿和干燥条件下的蒸腾作用。目前的地表模型并不能反映树木的地上储存情况。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。由于土壤水分的日内动态与树木木质部的水分储存动态有很大的不同，目前的方法导致了与观测到的蒸腾动力学的偏差。因此，陆地表面模式在模拟潜热通量时产生了特有的日内误差模式。这项研究将开发一种框架来解决这种树木水动力学问题，该框架可以被纳入水文、地表和地球系统模型，以取代目前气孔导度和土壤水分之间的经验联系。FETCH2模型解决了树干中的水流和水势，并真实地将气孔导度与木质部中的水势联系起来，而土壤中的水分有效性为水动力系统提供了一个底边界条件。来自密歇根州一片森林的大规模生态干扰实验的数据将被用来验证这种方法。FETCH2模拟将与林地树液通量和树干水储存的观测结果以及没有水动力模块的陆地表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相关变量，如土壤湿度、地表能量收支和总初级生产力。通过结合林冠结构、树木蓄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.