Backscattering characteristics of L-band synthetic aperture radar (SAR) polarization data over industrial plantation forests of the fast-growing Acacia mangium in Sumatra, Indonesia, were investigated by combined analysis of microwave and optical reflectance imagery acquired by the Advanced Land Observation Satellite (ALOS) and ground observation data of forest stand characteristics. Prior to the satellite data analysis, growth curves of the planted trees are analyzed using ground observation data. The results show that tree growth fits a sigmoidal curve with a maximum growth rate between 1 and 2 years of age, which gradually declines after that. Our statistical analyses using satellite and ground observation data provide the following findings: 1. The regression between SAR backscattering intensity and forest stand characteristics fits negative quadratic curves for the cross-polarization HV, which show higher correlation than those obtained for the co-polarizations (HH, VV); 2. A decreasing trend in optical normalized difference vegetation index (NDVI) values with age prevails for the trees 2 years and older; 3. A significant correlation is obtained between SAR backscattering and NDVI for the cross-polarization. These findings suggest that the L-band SAR cross-polarization is strongly affected by the acacia tree foliage.