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.