Assessment of linear relationships between TanDEM-X coherence and canopy height as well as aboveground biomass in tropical forests

摘要

Information about the forest structure attributes like canopy height and aboveground biomass (AGB) are crucial for forest monitoring systems and in the assessment of the global carbon cycle. The globally available TerraSAR-X add-on for Digital Elevation Measurement (TanDEM-X) data are a potential data source for estimating canopy height and AGB. Several empirical and semi-empirical investigations have indicated a linear relationship between X-band coherence and canopy height as well as AGB. The generality of this relationship is to date underexplored. In this study, the volume coherence of bistatic TanDEM-X acquisitions was retrieved and linearly related to canopy height and AGB in different tropical study areas to assess the generality of their relationship. Airborne light detection and ranging (LiDAR) data were also used to complement the estimation of canopy height and AGB with the X-band volume coherence. The results indicated that the linear function provides a statistically significant fit to the observed relationship between coherence and canopy height as well as AGB. The estimation of canopy height with this linear relationship resulted in a coefficient of determination R (2) of 0.72 and a root mean square error (RMSE) of 6.4 m (15.7%), where the AGB estimation had a decreased accuracy with an R (2) of 0.59 and RMSE of 88.2 t ha(-1) (21%). The estimation where the LiDAR (light detection and ranging)-based AGB relationship and coherence was combined revealed generally similar accuracies compared to the coherence alone. This confirms that the TanDEM-X coherence can support the consistent estimation of canopy height and AGB, where it provides as a minimum a source for a stratification to assess the spatial distribution of qualitative AGB classes.