Two SAR and biomass data sets of forests with different canopy architectures were examined for commonalties regarding backscatter/biomass saturation. The SAR data were collected using the NASA/JPL AIRSAR at incidence angles between 400 and 50 degrees for tropical broadleaf evergreen forests in Hawaii and coniferous forests in North America and Europe. Radar signal saturation limits with respect to biomass for both forest types were determined to be approximate to 100 tons/ha for P-band (0.44 GHz), approximate to 40 tons/ha for L-band (1.25 GHz), and approximate to 20 tons/ha for C-band (5.3 GHz). The effect of the saturation limits on making global biomass inventories with SAR sensors was assessed by comparing the biomass saturation limits to a global vegetation type and biomass data base. C-band can be used to measure biomass in biomes covering 25% of the world's total ice-free vegetated surface area accounting for 4% of Earth's store of terrestrial phytomass. L- and P-band can be used to measure biomass in biomes covering 37% and 62% of the total vegetated surface area accounting for 8% and 19% of Earth's pool of terrestrial phytomass respectively. Biomes occupying approximately 38% of Earth's vegetated surface area containing 81% of the estimated total terrestrial phytomass have biomass densities above the saturation limit of current SAR systems (>100 tons/ha for P-band). Since P-band radar systems cannot Currently operate effectively from orbit, the use of SAR sensors for biomass surveys may be limited even further to the L-band threshold. Emphasis should be shifted toward using SAR to characterize forest regeneration and development up to the saturation limits shown here rather than attempting to measure biomass directly in heavy forests. The development of new and innovative technologies for measuring biomass in high density vegetation is encouraged.
