RADAR ESTIMATES OF ABOVEGROUND BIOMASS IN BOREAL FORESTS OF INTERIOR ALASKA

Airborne SAR data gathered by the NASA/JPL three-frequency, polarimetric, 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 biomass, L- and P-band radar backscatter increase by more than 6 dB when biomass increases from 5 to 200 tons/ha. Using second-order polynomial regressions, biomass values are predicted from the radar at L- and P-band and compared to actual biomass values. At P-band HV-polarization, the error in predicted biomass is about 30% of the actual biomass. When 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 uncertainties in actual stand biomass estimates, significant inner-stand spatial variations in biomass, unusual conditions of forest stands following natural disturbances, along with interactions of the radar signals with a complex three-dimensional structure of the canopy. Multiple incidence angle data reveal that the incidence angle theta(i) of the radar 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 regression curves for biomas retrieval-are dependent on the seasonal and environmental conditions.