Evaluation of JERS-1 L-band SAR backscatter for stem volume retrieval in boreal forest

To assess the possibility of using L-band synthetic aperture radar (SAR) backscatter for forest stem volume mapping in the boreal zone, a comparative analysis was carried out at test sites located in Sweden, Finland and Central Siberia from which an extensive set of Japanese Earth Resources Satellite (JERS-1) images and ground data was available. The backscatter showed clear seasonal dynamics, increasing by 3-4 dB in dense forests and 1-2 dB in sparse forests when going from frozen to unfrozen conditions. To retrieve stem volume a simple L-band Water Cloud-related scattering model was used. Model training performed well in all cases, the model parameters estimates being affected by weather conditions at acquisition and test site-specific forest stand structure. At each test site the retrieval performed best under unfrozen conditions and worst under frozen conditions. The highest retrieval accuracy was achieved at the small and intensively managed test site of Kattbale in Sweden with a 25% relative RMS error. For the other test sites the much larger retrieval error could be explained as a consequence of heterogeneities in the forest stand structure, as well as differences in the ground dielectric properties and inaccuracy in the ground data.