Studies are needed to evaluate the ability of present or future SAR data to extract forest attributes over various sites. This study focuses on large unmanaged pine plantations in a vast flat area of ca. 500,000 ha where the tree 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>=.8), the stand timber volume and the stand density. 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 statistical and semiempirical models, which take these effects into account. Inversion results show that forest biomass attributes can be estimated with relatively small errors commensurate with those achieved by field measurements. Best overall accuracy of ca. 21 m3/ha is reached with the semiempirical model. Error decomposition as a function of age classes shows that, for the same biomass range, errors are higher for old stands than for young stands. Finally, the results 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. In order to develop reliable biomass-retrieval schemes, future research should focus on examining in a more mechanistic way the relationship between canopy structure and radar signature.