ABSTRACT

Four global mosaics of Advanced Land-Observing Satellite (ALOS) Phased Arrayed L-Band Synthetic Aperture Radar (SAR) HH and HV polarization data were generated at 25-m spatial resolution using data acquired annually from 2007 to 2010. Variability in L-band HH and HV gamma-naught (γ0) for forests was observed among regions, with this attributed to differences in forest structure and vegetation/surface moisture conditions. Region-specific backscatter thresholds were therefore applied to produce, from each annual mosaic, a global map of forest and non-forest cover from which maps of forest losses and gain were generated. The overall agreement with forest/non-forest assessments using the Degree Confluence Project, the Forest Resource Assessment, and Google Earth images was 85%, 91%, and 95%, respectively. Using 2007 as a baseline, decreases of 0.040 dB and 0.028 dB (with a 0.006 dB 99% confidence level) were observed in the HH and HV γ0, respectively, over the same areas. This suggests a decrease in forest area and/or increased smoothing of the global surface at the L-band radar observation over the four-year period. These maps provide a new global resource for documenting the changing extent of forests and offer opportunities for quantifying historical and future dynamics through comparison with historical (1992–1998) Japanese Earth Resources Satellite (JERS-1) SAR and the forthcoming (from 2014) ALOS-2 PALSAR-2 data. Four-year PALSAR mosaics and the forest/non-forest data, which were generated and analyzed in this paper, are open to the public for free downloading albeit with coarser resolutions (https://www.eorc.jaxa.jp/ALOS/en/palsar_fnf/fnf_index.htm). Future distribution of the higher (original) resolution data sets from PALSAR as well as from the ALOS-2/PALSAR-2 is planned.