Hydraulic limitations are known to control transpiration in forest ecosystems when the soil is drying or when the vapor pressure deficit between the air and stomata (VPD) is very large, but they can also impact stomatal apertures under conditions of adequate soil moisture and lower evaporative demand. We calculated stomatal conductance from 2002-2010 using eddy-flux, meteorological and soil data at two forest sites in the University of Michigan Biological Station (UMBS). We compare two sites – the Forest-Accelerated-Succession site (FASET) and its control plot, at the footprint of the UMBS-Ameriflux tower. The FASET experiment simulates the transition from late-mid successional forest, dominated by tall and relatively uniform canopy of aspen (Populus grandidentata, Populus tremuloides) and birch (Betula papyifera), to a heterogeneous late-successional mixed deciduous forest of Quercus rubra, Acer rubrum and Pinus strobus. This was done by girdling all aspen and birch trees in a 33 hectare experimental plot. 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.
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 photosynthetically-equivalent costs and benefits of hydrological structural traits, FETCH modeling system enhanced our understanding of the role of hydraulic limitation on growth and for the tradeoffs between water and light in forest ecosystems.
Results/Conclusions
We parameterized a stomatal-conductance model that incorporates the effects of leaf density, PAR, light attenuation, temperature, VPD and soil moisture. We identified periods where stomatal conductance was lower than projected for a particular set of conditions. We hypothesize that these lower values arise due to hydrodynamic stresses related to the time lag of water movement through the tree system. 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. Despite having higher within-canopy VPD values, trees in the FASET plot are less hydrodynamically stressed, probably due to the more even distribution of leaf area in height and higher light penetration, which allows more photosynthesis at lower levels of the tree.
