Sensitivity of P- and L-Band SAR Tomography to Above-Ground Biomass in a Hilly Temperate Forest

Tomographic synthetic aperture radar (TomoSAR) is a promising technique for the estimation of forest above-ground biomass (AGB), but knowledge gaps still remain concerning the effects of forest type and ground topography. This article presents new results at P- and L-bands based on data acquired during the TomoSense campaign. The study area is a temperate forest, predominantly beech and spruce, with ground slopes ranging up to 40°. Analysis of vertical reflectivity profiles shows distinct differences for spruce and beech. Three AGB retrieval methods are analyzed, i.e., total vertical backscatter
$I_{\text {tot}}$
, canopy backscatter from a height layer
$I_{c}$
, and the ratio
$I_{\text {cr}} = I_{c}/I_{\text {tot}}$
. All three methods show sensitivity to AGB for spruce, whereas for beech, this is only true for the two latter methods. For the P-band, a significant ground slope effect is observed, while less so for the L-band. The highest
$R^{2}$
is obtained for spruce with HV polarization,
$I_{c}$
and ground slopes less than 10°, i.e.,
$R^{2} = 0.86$
and RMSE =15.6% for P-band and
$R^{2} = 0.75$
and RMSE =12.5% for L-band. Corresponding results by including all forest types are
$R^{2} = 0.77$
and RMSE =11.4% for P-band and
$R^{2} = 0.54$
and RMSE =12.0% for the L-band. Moreover, the performance of using
$I_{\text {cr}}$
is similar to that of
$I_{c}$
. The ratio
$I_{\text {cr}}$
can be determined without absolute radiometric calibration which relaxes system requirements. This article reinforces the potential of TomoSAR for forest AGB estimation and draws attention to important effects of tree species and ground slope.