On the retrieval of forest biomass from SAR data

The question of retrieving forest biomass from SAR data has been subject of numerous research works since the early 1990s. The first results using AIRSAR data showed that it was possible to invert P band SAR data into biomass maps. Using the currently available spaceborne SAR data, namely ERS, JERS and RADARSAT, the biomass retrieval appears limited in terms of biomass range and, in certain cases, site specific. To provide an overview on issues related to forest biomass retrieval, this paper will present a survey of the possibilities and limitations of SAR data at different frequencies, polarisations and incidence angles. First the definition of biomass and biomass parameters (age, height, stem volume, dbh) will be addressed, and the information requirements in forest management and in global environment studies for biomass information will be discussed. The information content of SAR data in terms of the retrieval of biomass parameters will be assessed, based on an understanding of the underlying scattering mechanisms, which in turn are derived from observations and modelling results. For this purpose, an analysis of data acquired by multiple frequency, incidence and polarisation systems and by systems with interferometric capabilities has been carried out. The results, which have shown that the sensitivity to biomass parameters differs strongly at different frequencies, polarisations and incidences, have been interpreted using theoretical models. The aim was to determine the scattering mechanisms involving different categories of tree elements such as trunks, branches and leaves. This understanding is necessary to explain the relationships between the radar measurements with the dielectric and geometrical properties of the constituent scatterers, which in turn are related to the required biomass parameters (e.g. above ground biomass or stem volume) via allometric equations. The theoretical models appeared useful to assess the validity domains of the observed relationships, when the effects of ground characteristics, tree species, mixture of species, topography need to be accounted for. The information content relevant to biomass retrieval of ERS, JERS, RADARSAT data, including interferometric measurements will be summarised.