Modelling and Linking Radar Backscatter and Stem Water Content in a Boreal Forest

Licentiatavhandling, 2025

In the context of climate change, forests serve as crucial mitigators but are also increasingly threatened by disturbances such as wildfires, insect outbreaks, and windthrows. Monitoring forest vitality is essential, with tree water status serving as an important indicator of forest health.
Spaceborne synthetic aperture radar (SAR), known for its sensitivity to vegetation water content (VWC), offers a promising tool for providing forest VWC on a global scale with high spatial resolution. However, the processes linking SAR backscatter to forest VWC are not fully understood. This thesis addresses this gap by establishing a link between radar backscatter and stem water content in boreal forests, using data from two tower-based radars, BorealScat and BorealScat-2, as an intermediary platform.
In Paper I, for the first time, the relevance of tower-based radar observations for spaceborne SAR was examined. This was done by comparison of time series of BorealScat radar backscatter and backscatter observed by Sentinel-1 and ALOS-2 PALSAR-2 at C- and L-band, respectively. A good agreement was found for a healthy forest, with an RMSE of 0.9 dB and 0.6 dB at C-band and  L-band, respectively. At C-band, tree death was visible in tomographic tower-based backscatter but not in Sentinel-1 or BorealScat full-forest backscatter, indicating the potential value of tomographic spaceborne SAR missions for forest vitality monitoring.
In Paper II, for the first time, forest backscatter was linked to high quality stem water content measurements. A modified version of the water cloud model was used to relate stem water content time series to canopy backscatter and attenuation time series observed at L-band by BorealScat-2. Modelled stem water content closely matched in situ stem water content with a relative RMSE of 4% and 2% for the inversion of the canopy backscatter model and the inversion of the attenuation model, respectively. These findings demonstrate the feasibility of using L-band SAR backscatter to monitor VWC and the value of tomography in future spaceborne SAR missions.