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 θiof the radar illumination is also a factor influencing the retrieval of biomass, even at HV-polarization, when θi > 50° or θi< 25°. Finally, the radar response of the forest-and thereby the regression curves for biomas retrieval-are dependent on the seasonal and environmental conditions. © 1994 IEE