Mapping Forest Degradation with ALOS PALSAR: Case studies from Mexico and Ghana

Forest degradation is one of the least understood processes in the terrestrial biome. Information about both the extent and rate of degradation is limited; meaning that estimation of subsequent carbon emissions are poorly quantified. Countries are required to report emissions from forest degradation to the UNFCCC, therefore, increased efforts to improve mapping and monitoring of forest degradation are essential for national reporting. As forest degradation is related to more subtle changes within a forest, resulting from loss of trees, then earth observation technologies, which can quantify changes in Aboveground Biomass (AGB) offer great opportunities in mapping forest degradation.

 

We test a methodology for monitoring forest degradation, which combines time-series forest plot data with L-band Synthetic Aperture Radar (SAR) data (ALOS/ ALOS-2) to map changes in AGB over time. We test this method in two countries (Mexico and Ghana), to determine if a single methodology can be used to map forest degradation across the tropics. Our study-sites span two tropical continents, covering a range of precipitation, AGB values, forest types, and modes of degradation, from seasonally dry pine forests in Mexico to humid lowland forests in Ghana,

 

In lower biomass forest, we could map changes in AGB over time, including small AGB losses, associated with minor degradation events (<20% loss of AGB). These minor degradation events are far more widespread than major degradation events (>50% loss of AGB) and therefore are an important source of emissions from degradation. However, in high biomass forest there was some saturation of the radar backscatter signal (>200 Mg ha^-1).

 

The use of ground-based AGB change data is essential for calibration of SAR data, therefore well managed national forest plot networks, which include degraded forest, are essential for accurate monitoring of degradation. Additionally, using longer wavelength P-Band SAR data, alongside L-Band SAR, could help overcome some issues related to saturation in high biomass forest. If this method were adopted at the regional or national level, it would allow countries, particularly those with lower biomass forest, to quantify emissions from degradation more accurately.