Modeling hydrodynamic stress limitations on transpiration

Evapotranspiration is one of the major forcing functions of Earth's climate, providing the link for the soil-plant-water continuum. Current models for transpiration assume a coupling between stomatal conductance and soil moisture through empirical relationships that do not resolve the hydrodynamic process of water movement from the soil to the leaves. This approach does not take advantage of recent advances in our understanding of water flow and storage in the trees, or of tree and canopy structure. It has been suggested that stomata respond to water potential in the leaf and branch, and that this hydrodynamic response is a mechanism for hydraulic limitation of stomatal conductance. Hydraulic limitations in forest ecosystems are common and are known to control transpiration when the soil is drying or when vapor pressure deficit is very large. Hydraulic limitation can also impact stomatal apertures under conditions of adequate soil moisture and lower evaporative demand. Hydrodynamic stresses at the tree level act at several time scales, including the fast, minute-hour scale. These dynamics are faster than the time scales of hours to days at which drying soil will affect stomata conductance. The Finite-Elements Tree-Crown Hydrodynamics model (FETCH), simulates water flow through the tree as a simplified system of porous media conduits. It explicitly resolves spatiotemporal hydraulic stresses throughout the tree's hydraulic system that cannot be easily represented using other stomatal-conductance models. By enabling mechanistic simulation of the effects of hydrological structural traits on stomata conductance, FETCH modeling system enhanced our understanding of the role of forest structure, growth and disturbance history in determining the tradeoffs between water and light in forest ecosystems. Though FETCH can simulate highly resolved individual tree structures, to the branch level, a simplified version of FETCH can rapidly represent a full forest patch using a small number of representative size/type trees. This type of simulation can be readily incorporated in a land-surface model, to dynamically resolve the effects of hydrodynamic stresses at short (minutes-hours) and long (days-seasons) time scales. We compare two sites – the Forest-Accelerated-Succession site (FASET) and its control plot, at the footprint of an Ameriflux tower. Soil moisture, eddy flux and sap flux observations where available throughout the growing season in both sites. The FASET experiment simulates the transition from late-mid successional forest, dominated by tall and relatively uniform canopy of aspen and birch, to a heterogeneous late-successional mixed deciduous forest. The treatment occurred in 2008, and canopy structure has been drastically modified since then. The FASET canopy is, on average, shorter, more open and more heterogeneous. We conducted FETCH simulations using simplified branch-scale tree structures of 3 contrasting trees of different sizes and species. By comparing the simulation results with eddy-flux observations we identified periods where stomatal conductance was lower than projected for a particular set of conditions. These periods were identified at two different time scales – short-term, intra-daily stresses that are typical for early afternoons in high light conditions and longer stress periods where the soil is dryer than usual. We showcase the potential application of this modeling approach as a component in a land-surface model.