Radar remote sensing of forest and wetland ecosystems in the Central American tropics

Abstract

We analyzed airborne synthetic aperture radar (AIRSAR) imagery of forest, wetland, and agricultural ecosystems in northern Belize, Central America. Our analyses are based upon four biophysical indices derived from the fully polarimetric SAR data: the volume scattering index (VSI), canopy structure index (CSI), biomass index (BMI), calculated from the backscatter magnitude data, and the interaction type index (ITI), calculated from the backscatter phase data. We developed a four-level landscape hierarchy based upon clustering analyses of the 12 index parameters (four indices each for P, L, and C band) from two test site images. Statistical analyses were used to examine the relative importance of the 12 parameters for discriminating ecosystem characteristics at various landscape scales. We found that ITI was the most important index (primarily C band = CITI) for level, vegetated terrain at all levels of the hierarchy. BMI was most important for differentiating between vegetated and nonvegetated areas and between sloping and level terrain. These findings indicate that upper canopy spatial characteristics and flooding in marshlands (reflected in the CITI) are more important than biomass in differentiating many tropical ecosystems with radar data. The relative importance of the indices varied with vegetation type; for example, PVSI was the most important for distinguishing between upland forests and regrowth, and PCSI was the most important for differentiating swamp forest types. Finally, we evaluated the potential of present and future spaceborne SARs for tropical ecosystem studies based on our results. Most of these SARs are single channel systems and will provide limited capability for characterizing biomass and structure of tropical vegetation. This is especially true for C band systems, which produce data similar to our CBMI parameter, which was one of the least important in our analyses. The SIR-C/X-SAR and proposed EOS SAR are future spaceborne multifrequency fully polarimetric SAR systems, and they will provide a significant contribution to tropical ecosystem studies.