Divergent Water Use Strategies of Temperate Tree Species Estimated from Continuous Sap Flow Measurements

Due to a changing climate, droughts are becoming more frequent. As such, it is important to understand how trees will respond to droughts since forests play a key role in water and carbon cycling on Earth. Isohydricity is a continuum of tree hydraulic behavior used to describe a tree’s water use strategy. Trees with more isohydric tendency will close their stomata in order to avoid excessive water loss through transpiration. However, prolonged stomatal closure during drought might ultimately lead to these trees dying of carbon starvation due to the lack of photosynthesis. Meanwhile, trees with strong anisohydric tendency will leave their stomata open in order to assimilate more carbon, but lose water through transpiration. Anisohydric trees, which may run out of water, are subject to hydraulic failure and subsequent xylem cavitation. Therefore, it is important to understand species-specific response to water stress in order to understand the resilience of a forest ecosystem as a whole. To address this question, sap flow sensors were installed on four different tree species to monitor tree water uptake over the course of a growing season. The sap flow (transpiration) data for each tree was plotted with vapor pressure deficit (approximated using meteorological tower data) in order to quantitatively interpret isohydric or anisohydric tendencies of each tree and tree species across changing seasons and soil moisture conditions. For Acer rubrum, Quercus alba, and Pinus virginiana species, larger trees were more likely to exhibit isohydric behavior. It was unclear whether Fagus grandifolia trees were more likely to exhibit different isohydricity. The species exhibiting isohydric behavior the most was Pinus virginiana, followed by Acer rubrum. The species exhibiting anisohydric behavior the most was Quercus alba, followed by Fagus grandifolia. While many factors affect a tree’s water use strategy, this study provides an example of how sap flow analysis can be used to classify such strategies.