GNSS-T Monitoring of Canopy Water Dynamics in a European Beech Forest: Potentials and Caveats Across Diel to Seasonal Scales

European beech (Fagus sylvatica L.) is one of the most widespread and ecologically significant broadleaf tree species in Central Europe and is highly sensitive to drought and climatic extremes. Monitoring canopy water status is therefore critical for understanding plant hydraulic functioning, ecosystem resilience, and responses to environmental stress. Satellite-derived Vegetation Optical Depth (VOD) quantifies microwave signal attenuation by vegetation, serves as a proxy for vegetation water content, and provides valuable large-scale information for global vegetation monitoring and climate studies. However, well-characterized ground-based observations are required to resolve canopy-scale processes and hydraulic dynamics and to validate and improve satellite VOD retrievals.

To address this need, we employ Global Navigation Satellite System Transmissometry (GNSS-T) as an in situ remote sensing approach to observe canopy water content dynamics in a mature European beech forest in eastern Austria. GNSS-T exploits signals of opportunity from navigation satellites operating at L-band frequencies comparable to microwave radiometers by comparing simultaneous signal reception at paired open-sky reference and below-canopy receivers, with differences in received signal power attributable to vegetation. Using a stationary, multi-frequency, multi-constellation GNSS-T setup operated continuously for one and a half years, VOD was retrieved using a simplified tau–omega radiative transfer model as a function of GNSS system, frequency band, and ranging code type.

Retrieved VOD magnitudes are generally consistent across systems and frequency bands. GNSS-T-derived VOD resolves spatial canopy structure as well as pronounced diel and seasonal dynamics and shows sensitivity to meteorological drivers. Comparisons with camera-based vegetation proxies (in situ LAI- and NDVI-proxies) and satellite-derived vegetation indicators (AMSR2 X- and Ku-band VOD, MODIS LAI, Sentinel-1 cross-polarization ratio) support the physical interpretability of these observations. A systematic, azimuth-independent decrease of VOD toward the horizon might point to limitations of the implemented radiative transfer framework.

Overall, the results demonstrate the potential of GNSS-T to provide continuous, non-destructive in situ observations of canopy-scale hydraulic dynamics.