On the retrieving of forest stem volume from VHFSAR data: Observation and modeling

The objective of this paper is to investigate the relationships between VHF data and forest biomass using data acquired by the Airborne Imaging Radar CARABAS over two different pine plantation forests in southern France. Data are analyzed using detailed ground truth measurements available on both si tes. The backscattering coefficient is strongly correlated to characteristi cs of the tree trunk. Signal saturation is not observed up to goo m(3)/ha. However, the sensitivity to the volume is high in the range of 0-500 m(3)/h a (e.g., 1 to 1.5 dB for 50 m(3)/ha), whereas it is reduced beyond 500 m(3) /ba (< 0.5 dB for 50 m(3)/ha). The experimental analysis is supported by theoretical modeling using a cohe rent backscatter model based on the distorted Born approximation coupled wi th a tree growth model giving a fine and precise description of the trees a t both sites. The modeling results show that the trunk is the main scattere r, but that, when the branch dimensions are not insignificant compared to t runk dimension, branch scattering needs to be accounted for. However, since the two species under study are both coniferous, branch dimensions are rel atively small compared to trunk dimension. This explains no significant dif ferences observed in the backscatter behavior between both sites, except fo r mature stands with low stem density. Finally, the effect of topography is investigated both experimentally, usin g a digital elevation model (DEM), and theoretically with the coherent mode l. The loss of sensitivity to stem volume due to slope is clearly demonstra ted and explained by the decrease of the dihedral trunk-ground interaction as the slope increases.