Decrease of L-band SAR backscatter with biomass of dense forests

Synthetic aperture radar (SAR) is one of the most promising remote sensors to map forest carbon. The unique spaceborne and longwavelength SAR data currently available are L-band data, but their relationship with forest biomass is still controversial, particularly for high biomass values. While many studies assume a complete loss of sensitivity above a saturation point, typically around 100 t.ha−1, others assume a continuous positive correlation between SAR backscatter and biomass. The objective of this paper is to revisit the relationship between L-band SAR backscatter and dense tropical forest biomass for a large range of biomass values using both theoretical and experimental approaches. Both approaches revealed that after reaching a maximum value, SAR backscatter correlates negatively with forest biomass. This phenomenon is interpreted as a signal attenuation from the forest canopy as the canopy becomes denser. This result has strong Preprint submitted to Remote Sensing of Environment October 16, 2014 implications for L-band vegetation mapping because it can lead to a greaterthan-expected under-estimation of biomass. The consequences for L-band biomass mapping are illustrated, and solutions are proposed.