Temporal Variations in L-band Reflectivity Profiles of a Boreal Forest During Growth and Drought

Artikel i vetenskaplig tidskrift, 2026

Forest water dynamics related to vitality, such as tree water stress and transpiration, modulates the forest's microwave reflectivity, but the connections have not yet been established, leaving unexplored potential for satellite missions. This study presents and models time series of L-band reflectivity and attenuation observed by a multi-polarization tomographic radar in a Picea abies L. Karst (Norway spruce) stand during a growth season subject to drought. Temporal phenomena in reflectivity are found, with surprising differences between polarizations and with characteristics shifting over time-scales of weeks. Diurnal cycles of 0.5 to 1 dB were often present in HH and VV canopy intensity (reflectivity above 4 m height), not necessarily simultaneously, with opposite phase. A model is formulated that describes the reflectivity profile as a combination of microwave scattering and extinction profiles. Model-estimated scattering is found more strongly linked to forest water dynamics than the observed canopy intensity. During the drought, coefficient of determination (R²) values of VV canopy scattering and intensity are 0.70 and 0.43 for diurnal variations with vapor pressure deficit, a driver of transpiration, and 0.92 and 0.75 for daily averages with soil moisture (SM), indicating tree water stress. The daily average HV ground scattering and intensity show R² values of 0.81 and 0.29 with SM over the growth period. This study shows that a combination of microwave extinction and scattering can explain the reflectivity phenomena, and can be decomposed by model-based estimation via radar tomography and attenuation measurements, producing more direct relations to tree water stress and transpiration.