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Biogeosciences An interactive open-access journal of the European Geosciences Union
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© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  16 Oct 2020

16 Oct 2020

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This preprint is currently under review for the journal BG.

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

Nataniel Holtzman1, Leander D. L. Anderegg2, Simon Kraatz3, Alex Mavrovic4, Oliver Sonnentag5, Christoforos Pappas5, Michael H. Cosh6, Alexandre Langlois7, Tarendra Lakhankar8, Derek Tesser8, Nicholas Steiner9, Andreas Colliander10, Alexandre Roy4, and Alexandra G. Konings1 Nataniel Holtzman et al.
  • 1Department of Earth System Science, Stanford University, Stanford, CA 94305, United States of America
  • 2Department of Integrative Biology, University of California Berkeley, Berkeley, CA; Department of Ecology, Evolution and Marine Biology, University of California Santa Barbara, Santa Barbara, CA
  • 3Department of Electrical and Computer Engineering, University of Massachusetts, Amherst, MA, USA
  • 4Département des Sciences de l'Environnement, Université du Québec à Trois-Rivières (UQTR), Trois-Rivières, Québec, Canada
  • 5Département de géographie, Université de Montréal, Montréal, Québec, H2V 2B8, Canada
  • 6USDA ARS Hydrology and Remote Sensing Laboratory, Beltsville, MD, USA
  • 7Département de Géomatique Appliquée, Université de Sherbrooke, Sherbrooke, Québec, Canada
  • 8NOAA-CESSRST, The City College of New York, City University of New York, New York, NY, USA
  • 9Department of Earth and Atmospheric Sciences, City College of New York, City University of New York, New York, NY, USA
  • 10Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA

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 5 AM. 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.

Nataniel Holtzman et al.

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Nataniel Holtzman et al.

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Publications Copernicus
Short summary
Microwave radiation coming from Earth's land surface is affected by both soil moisture and the water in plants that cover the soil. We measured such radiation with a sensor elevated above a forest canopy, while repeatedly measuring the amount of water stored in trees at the same location. Changes in the microwave signal over time were closely related to tree water storage changes. Satellites with similar sensors could thus be used to monitor how trees in an entire region respond to drought.
Microwave radiation coming from Earth's land surface is affected by both soil moisture and the...