Forest aboveground biomass estimation using polarization coherence tomography and PolSAR segmentation

Abstract

Forest aboveground biomass (AGB) is essential for monitoring the carbon cycle budget and climate change. This study proposes a method for the estimation of forest AGB based on polarization coherence tomography (PCT) and polarimetric synthetic aperture radar (PolSAR) segmentation. The data used are the single-baseline polarimetric SAR interferometry data acquired by the German Aerospace Center’s E-SAR sensor at the L-band over the city of Traunstein and its vicinity in 2003. First, vertical structure tomographic profiles (relative reflectivity distribution in vertical direction) were produced by PCT for each pixel and then averaged within each field-surveyed forest stand to obtain the mean tomographic profile. Next, the mean vertical tomographic profiles were parameterized by 10 shape parameters. Several models for biomass estimation were designed based on the relationships between the definition parameters and the in situ measurements using backward step-wise regression and the M-fold (10-fold) cross-validation method. The best model was chosen by evaluation criteria such as R, root mean square error (RMSE), R², etc. Forest polygons (objects) were produced by image segmentation using forest heights, (produced by coherence optimization representing double-bounce scattering mechanism), (produced by coherence optimization representing the volume scattering mechanism), and the volume scattering mechanism class (produced by Freeman–Durden decomposition and Whishart classification). Finally, the selected model was used to estimate the AGB of each forest polygon and was validated using ground-measured biomass with R² of 0.883 and RMSE of 39.98 tons ha⁻¹. The results show that the proposed method works well for the estimation of forest AGB. No saturation phenomena have been observed even for the forest stands with AGB larger than 500 tons ha⁻¹.

Acknowledgements

Thanks to ESA/DLR for provision of the Traunstein Pol-InSAR data sets and in situ forest height and AGB data for the MOST-ESA DRAGON Pol-InSAR project. We would like to thank the professors and workers who work for the Radar data and field inventory data collection and pre-processing. We also thank the two anonymous reviewers for many constructive comments on the manuscript.

Notes

1. The distance between two antennas in parallel flight lines; sometimes it is called the interferometric baseline. It is usually used to resolve the directional ambiguity.

2. The time difference between two passes; it means the time it takes an aircraft to turn and re-fly the scene, or for a satellite system with a long repeat period.

3. A strong disadvantage of (usable) wood volume is that trees and stands below 10 m height have virtually no volume or biomass, which leads to a poor representation of young regrowing forests.

Additional information

Funding

This work was partially supported by National Key Basic Research Development Programme (973 Programme) sub-project [Grant 2013CB733404]; High Technology Research Project [Contract number 2011AA120402]; National Natural Science Foundation of China [Grant 41401480].