There is a very good study by Lola Fatoyinbo (Estimation of mangrove structure and biomass from SAR and lidar remote sensing). Many papers are in this field such as ( Relating forest biomass to SAR data) and (
Dependence of radar backscatter on coniferous forest biomass). These works should answer your questions.
The mangrove vegetation in the study area is not that much dense and have an average height of 5-6 meters. What could be the best band combinations and polarization of SAR data for that very particular area.