Further studies are needed to evaluate the ability of actual or incoming SAR data to extract forest attributes over various sites such as large unmanaged pine plantations. In this study, we focused on a forest cover, which is a vast flat area of ca. 500,000 ha where the biomass ranges from 0 to 200 m³/ha corresponding to different forest canopy structures. Results show a good correlation between the backscattering coefficient sigma0 (with a 6-dB dynamic range and R² upper or equal to 0.8), the stand timber volume and the stand density. In fact, the trend is mainly driven by stand density and different relationships are observed according to age class, which explicitly points out the effects of canopy structure on the backscattering level. Stem volume is derived from the inversion of a semi-empirical model by taking into account these effects. Inversion results show a global accuracy of ca. 21 m³/ha. Hence, the result indicate that 1) JERS-1 data can be used in an operational way for estimating the biomass of such plantations and 2) it is necessary to take forest stand structure into account.