Relating SAR image texture to the biomass of regenerating tropical forests

Abstract

An accurate global carbon budget requires information on terrestrial carbon sink strength. Regenerating tropical forests are known to be important terrestrial carbon sinks but information on their location, extent and biomass (from which carbon content can be estimated) is incomplete. The use of remotely sensed data in optical wavelengths has been of limited use due to both the weak relationship between optical radiation and forest biomass and near‐constant cloud cover in the tropics. L‐band Synthetic Aperture Radar (SAR) backscatter, however, is related positively to biomass (but only up to an asymptote of around 40–90 T ha⁻¹) and can be obtained independently of cloud cover.

Both canopy structure and biomass change over time as pioneer species are replaced by early and late regenerating species. These structural changes are related to an increase in (i) tree height, (ii) tree species richness and (iii) canopy thickness and influence the roughness of the canopy surface and consequently SAR image texture. Therefore, we investigated the degree to which textural information could be used to increase the correlation between image tone (backscatter) and biomass. Field data were used to estimate the biomass of 37 regenerating forests plots in Brazilian Amazonia. Texture measures derived from local statistics, the grey level co‐occurrence matrix (GLCM) and the variogram were evaluated using simulated images on the basis of their ability to identify significant differences in image texture independently of image contrast. The selected texture measures were applied to L‐band JERS‐1 (Japanese Earth Resources Satellite) SAR images and the correlation between backscatter and biomass was determined for regenerating tropical forests. A strong correlation was found for the texture measures and biomass. The r _a ² (adjusted coefficient of determination), measuring the correlation between backscatter and biomass, increased from 0.74 to 0.82 with the addition of GLCM‐derived contrast. The addition of image texture (GLCM‐derived contrast) to image tone (backscatter) potentially increases the accuracy with which JERS‐1 SAR data can be used to estimate biomass in tropical forests.

Acknowledgements

The Japanese Space Agency (NASDA) supplied the JERS‐1 SAR data as part of the Global Rain Forest Mapping (GRFM) Project. The authors would like to acknowledge C. Rennó (INPE, Brazil) for his support with the Texture software. A. Luckman (University of Wales, Swansea, UK), R. Lucas (University of Wales, Aberystwyth, UK) and teams collected inventory data at the study areas. This work was part of T.M.K.'s PhD thesis funded by Brazilian CAPES (Fundação Coordenação de Aperfeiçoamento de Pessoal de Nível Superior). R. O. Bierregaard Jr (University of North Carolina at Charlotte, USA) kindly provided figure 4.