Microwave Radiometry for Remote Sensing of Plant Water Status Dynamics: Stand-Scale Evaluation in a Temperate Deciduous Forest

Vegetation optical depth (VOD) retrieved from microwave radiometry is known theoretically and empirically to correlate with the total amount of water in vegetation, but the details of this relationship vary with vegetation type. This total water amount is affected by both the total biomass of plants in an area and the relative water content of those plants. Relative water content, in turn, monotonically depends on plant hydraulic status (e.g. leaf and xylem water potential), suggesting that VOD could be sensitive to plant water potential, which would enable a range of ecosystem-scale plant hydraulic studies with microwave radiometry. However, the sensitivity of VOD to leaf and xylem water potential has not been explicitly tested at small scales. Although field-scale studies in agricultural landscapes exist, in forests most previous studies of VOD and plant water content have used satellite observations. Microwave satellites have pixels tens of kilometers wide, whose likely sub-pixel heterogeneity complicates any comparison with ground truth data. Here we present results from a field experiment that avoids scaling errors by combining tower-based radiometer observations with intensive ground observations.

At a plot in Harvard Forest (Massachusetts, USA) dominated by red oak, we placed an L-band radiometer on a tower above the canopy looking down at a 20 m x 25 m patch of forest during an entire growing season. We deployed an array of sensors within the patch to continuously measure soil moisture, tree circumference, and xylem permittivity. Additional observations of leaf and xylem water potential were also made over several days in early July 2019.

The water potentials measured in xylem and leaves agree with each other and with expected diurnal dynamics, and show a larger amplitude on hotter days. The average diurnal cycle of VOD retrieved from the radiometer is similar to that of the leaf water potential measurements. We further compare the dynamics of VOD against other in situ observations, with specific attention to the relative sensitivity of VOD to leaf and xylem water potential, and to alternative quantities reflecting plant water status.