E. Rignot et al., RADAR ESTIMATES OF ABOVEGROUND BIOMASS IN BOREAL FORESTS OF INTERIOR ALASKA, IEEE transactions on geoscience and remote sensing, 32(5), 1994, pp. 1117-1124
Airborne SAR data gathered by the NASA/JPL three-frequency, polarimetr
ic, radar system in winter, spring, and summer over the Bonanza Creek
Experimental Forest, near Fairbanks, AK, are compared to estimates of
whole-tree aboveground dry biomass from 21 forest stands and two clear
-cuts. While C-band radar backscatter shows little sensitivity to biom
ass, L- and P-band radar backscatter increase by more than 6 dB when b
iomass increases from 5 to 200 tons/ha. Using second-order polynomial
regressions, biomass values are predicted from the radar at L- and P-b
and and compared to actual biomass values. At P-band HV-polarization,
the error in predicted biomass is about 30% of the actual biomass. Whe
n HV- , HH- , and VV-polarization are used together in the regression,
the error in predicted biomass is about 20%. Errors obtained using L-
band data are a few percents larger. These errors are caused by uncert
ainties in actual stand biomass estimates, significant inner-stand spa
tial variations in biomass, unusual conditions of forest stands follow
ing natural disturbances, along with interactions of the radar signals
with a complex three-dimensional structure of the canopy. Multiple in
cidence angle data reveal that the incidence angle theta(i) of the rad
ar illumination is also a factor influencing the retrieval of biomass,
even at HV-polarization, when theta(i) > 50-degrees or theta(i) < 25-
degrees. Finally, the radar response of the forest-and thereby the reg
ression curves for biomas retrieval-are dependent on the seasonal and
environmental conditions.