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

Authors: HOLTZMAN, N. - Stanford University; ANDEREGG, L.D. - Collaborator; KRAATZ, S. - University Of Massachusetts, Amherst; MAVROVIC, A. - University Of Quebec; SONNENTAG, O. - University Of Montreal; PAPPAS, C. - University Of Montreal; Cosh, Michael; LANGLOIS, A. - Universite De Sherbrooke; LAKHANKAR, T. - Collaborator; TESSER, D. - Collaborator; STEINER, N. - Collaborator; COLLIANDER, A. - Jet Propulsion Laboratory; ROY, A. - University Of Quebec; KONINGS, A. - Stanford University

Submitted to: Biogeosciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/1/2021
Publication Date: 2/1/2021
Citation: Holtzman, N., Anderegg, L.L., Kraatz, S., Mavrovic, A., Sonnentag, O., Pappas, C., Cosh, M.H., Langlois, A., Lakhankar, T., Tesser, D., Steiner, N., Colliander, A., Roy, A., Konings, A. 2021. L-band vegetation optical depth as an indicator of plant water potential in a temperate deciduous forest stand. Biogeosciences. 18(2):739-753. https://doi.org/10.5194/bg-18-739-2021.
DOI: https://doi.org/10.5194/bg-18-739-2021

Interpretive Summary: Vegetation is observable in the microwave spectrum because of the presence of water within the plant matter. Most microwave sensors are flown on aircraft or satellites, so it is challenging to calibrate algorithms for vegetation monitoring because of the scale mismatch. Therefore, a study was conducted using a tower-based microwave radiometer, to understand the water content of a tree stand at the Harvard Forest, in Petersham, Massachusetts. There was a good correlation between modeled water status and vegetation radiative transfer models. This result will be useful for monitoring of dense vegetation via remote sensing.

Technical Abstract: Vegetation optical depth (VOD) retrieved from microwave radiometry correlates with the total amount of water in vegetation, based on theoretical and empirical evidence. Because the total amount of water in vegetation varies with relative water content (as well as with biomass), this correlation further suggests a possible relationship between VOD and plant water potential, a quantity that drives plant hydraulic behavior. Previous studies have found evidence for that relationship on the scale of satellite pixels tens of kilometers across, but these comparisons suffer from significant scaling 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 radiometer on a tower above the canopy looking down at red oak forest stand during the 2019 growing season in central Massachusetts, United States. We measured stem xylem and leaf water potentials of trees within the stand and retrieved VOD with a single-channel algorithm based on continuous radiometer measurements and measured soil moisture. VOD exhibited a diurnal cycle similar to that of leaf and stem water potential, with a 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 over the growing season. The presence of moisture on the leaves did not affect the observed relationship between VOD and stem water potential. We used our observed VOD–water-potential relationship to estimate stand-level values for a radiative transfer parameter and a plant hydraulic parameter, which compared well with the published literature. Our findings support the use of VOD for plant hydraulic studies in temperate forests.