Speaker
Description
Approaches exploiting the complementary information on forest above-ground biomass contained in multi-frequency radar backscatter, considering the variability in the sensitivity of backscatter to biomass dependent on the imaging conditions, have hardly been explored. Based on a set of 225 air- and spaceborne backscatter images acquired at X-, C-, L-, and P-band by TerraSAR-X/Tandem-X, Sentinel-1, Radarsat-2, ERS-2, ALOS PALSAR, ALOS-2, FSAR, and SETHI for BioSAR and AfriSAR campaign sites in Sweden and Gabon, we analyzed i) the sensitivity of backscatter at different frequencies to above-ground biomass under varying environmental conditions, ii) the performance of a biomass retrieval using different frequencies separately or combined, and iii) the role of forest structure on the backscatter to biomass relationship. A semi-empirical modeling framework was implemented for retrieving biomass from X-, C-, and L-band backscatter; empirical models were used for P-band.
The results suggest that i) P-band is most sensitive to above-ground biomass under all imaging conditions in the tropics and the boreal zone, ii) C- and L-band backscatter, provided multi-temporal observations are available, may achieve comparable accuracies as (few) P-band observations up to biomass levels of 150 to 200 t/ha, and iii) the benefit of combining P-band with higher frequency backscatter is limited. With respect to the role of the imaging conditions, we find that i) wet season conditions are associated with reduced sensitivity to biomass at all frequencies in the tropics, ii) frozen conditions maximize the sensitivity to biomass at high frequencies in the boreal zone, unfrozen dry conditions maximize the sensitivity at L- band. For the test sites in Sweden, we also find that forest structural differences associated with different forest management affect the backscatter to biomass relationship and lead to biased biomass estimates when not considered in the modeling.
The results of this study reemphasize that P-band is a critical frequency for mapping biomass, in particular in the tropics. Nonetheless, multi-temporal C- and L-band may provide almost equivalent information in biomass ranges below 200 t/ha. Implications of the results with respect to large scale mapping applications will be discussed in the context of ongoing global mapping efforts (ESA GlobBiomass, CCI Biomass).
