Quantification of selective logging in tropical forest with spaceborne SAR interferometry

Abstract

Tropical forest disturbance (such as selective logging and fire) along with deforestation have significant contributions to the carbon source due to land-use change and anthropogenic CO2 emissions, and thus envisioned by United Nation's REDD+ programme. In previous work, spaceborne single-pass InSAR phase-center height has been shown to have the capability of accurately monitoring the subtle height change due to forest growth and degradation (with meter or even sub-meter level RMSE about the regression curve fit to time). In this paper, a new approach using spaceborne SAR interferometry has been developed to detect and quantify selective logging events. In particular, a quantitative indicator of forest disturbance is first defined, namely disturbance index (DI; from 0 “no disturbance” to 1 “deforestation”). A numerical field data-based InSAR simulation is then performed to study the functional relationship between the field-measured DI and InSAR relative phase-center height change from a modeled perspective. A selective logging event (October 2015 through January 2016) over the Tapajos National Forest in Brazil is used for experimental validation. The InSAR-inverted DI estimates derived from DLR's TanDEM-X time-series data were compared with those measured from a field work over 32 quarter-hectare stands at Tapajos with relative RMSE of 30% for DI up to 0.3 and the disturbance epoch can be determined with an average accuracy of 13 days (constrained by the satellite repeat interval usually on the order of 2 weeks). As a comparison, the repeat-pass InSAR coherences from the concurrent JAXA's ALOS-2 data are shown to qualitatively correspond to the TanDEM-X results, confirming both the location and the epoch of the disturbance event. This new method is anticipated to contribute to the range of tools being developed for large-scale forest disturbance assessment and monitoring (for UN's REDD+ programme) through using spaceborne single-pass InSAR missions (e.g., DLR's TanDEM-X and in the future, TanDEM-L).