L-band vegetation optical depth as an indicator of plant water potential in a temperate deciduous forest stand

Abstract

Abstract. Vegetation optical depth (VOD) retrieved from microwave radiometry correlates with the total amount of water in vegetation, based on theoretical and\nempirical evidence. Because the total amount of water in vegetation varies with relative water content (as well as with biomass), this correlation\nfurther suggests a possible relationship between VOD and plant water potential, a quantity that drives plant hydraulic behavior. Previous studies\nhave found evidence for that relationship on the scale of satellite pixels tens of kilometers across, but these comparisons suffer from significant\nscaling error. Here we used small-scale remote sensing to test the link between remotely sensed VOD and plant water potential. We placed an L-band\nradiometer on a tower above the canopy looking down at red oak forest stand during the 2019 growing season in central Massachusetts, United\nStates. We measured stem xylem and leaf water potentials of trees within the stand and retrieved VOD with a single-channel algorithm based on\ncontinuous radiometer measurements and measured soil moisture. VOD exhibited a diurnal cycle similar to that of leaf and stem water potential, with\na peak at approximately 05:00 eastern daylight time (UTC − 4). VOD was also positively correlated with both the measured dielectric constant and water potentials of stem xylem\nover the growing season. The presence of moisture on the leaves did not affect the observed relationship between VOD and stem water potential. We\nused our observed VOD–water-potential relationship to estimate stand-level values for a radiative transfer parameter and a plant hydraulic\nparameter, which compared well with the published literature. Our findings support the use of VOD for plant hydraulic studies in temperate forests.