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 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.

Validation of Soil Moisture Data Products from the NASA SMAP Mission

“…Thereafter, the objective was broadened to include areas outside of the original validation domain, including forests. The mission is currently engaged in exploring the improvement and validation of SMAP SM products over forested areas through field experiments [52], [156], an added focus on forested candidate CVS (see Table III) and other networks, such as the National Ecological Observatory Network (NEON) [53]. Furthermore, the effort to expand the validation domain includes accounting for the complex soil composition of the boreal and arctic regions.…”

Detecting forest response to droughts with global observations of vegetation water content

“…At such timescales, there is minimal influence from changes in phenology and forest structure on the total ecosystem‐scale VWC (Section 4). Microwave observations from RapidScat onboard the International Space Station (van Emmerik et al, 2017; Konings, Yu, et al, 2017) and from ground‐based tower systems (Holtzman et al, 2021; Monteith & Ulander, 2018; Schneebeli et al, 2011; Vermunt et al, 2020) have demonstrated the feasibility of quantifying the VWC dynamics throughout the day. Thus, the ability to monitor the diel signal of VWC is driven by the orbital choices of the existing sensors, not by limitations of the microwave observations’ intrinsic sensitivity.…”

Opportunities, challenges and pitfalls in characterizing plant water‐use strategies

“…Recent work has taken advantage of the sensitivity of remotely sensed microwave observations to vegetation water content, which can in turn be linked to leaf water potential (Holtzman et al, 2021;Konings et al, 2019). This approach enables the estimation of plant water-use strategies at canopy scales (Konings & Gentine, 2017;Liu et al, 2021) and has been linked to site-level functional traits and drought responses in eddy covariance data (Anderegg, Konings, et al, 2018).…”