Estimation of stem volume in boreal forests using ERS-1 C- and JERS-1 L-band SAR data

Multi-temporal ERS-1 C- and JERS-1 L-band Synthetic Aperture Radar (SAR) data were analysed to determine the relationship between radar backscattering and forest stem volume. The test site is located within the boreal conifer belt in northern Sweden with stem volume in the range of 0-300 m(3) ha(-1) The statistical analysis was carried out using the water cloud model and various linear regression models. It was confirmed that the L-band of JERS-1 shows a consistently higher sensitivity to stem volumes in boreal forests with a larger backscatter contrast between nonforested areas and areas having dense tree cover in comparison with the C-band of ERS-1. The saturation levels for the ERS-1 and JERS-1 SAR sensors were estimated at 64 m(3) ha(-1) and 143 m(3) ha(-1), respectively. Furthermore, the temporal analysis shows that the backscattering coefficient can differ by approximately three decibels, at most, between acquisitions for a given stem volume. However, a major finding is that the temporal variation could be modelled as an additive effect that is constant throughout the range of stem volume. For operational forestry purposes this would imply that stem volume can be predicted after calibration of the radar response using clear felled areas as reference targets. The multi-temporal JERS-1 dataset was used to develop a radar-based model of stem volume. The model was tested on a validation dataset resulting in a correlation coefficient of 0.78 between SAR estimated stem volume and ground data.