MAPPING DEFORESTATION AND SECONDARY GROWTH IN RONDONIA, BRAZIL, USINGIMAGING RADAR AND THEMATIC MAPPER DATA

Excellent data on deforestation have been obtained in the tropics with the use of high-resolution optical sensors. Yet, several problems rem ain. Cloud cover creates data gaps that limit the possibility of compl ete and frequent assessments, and secondary growth is not well charact erized. Active microwave sensors could complement these sensors becaus e they operate independently of cloud cover and smoke and can detect d ifferences in woody biomass and forest structure associated with vario us stages of forest clearing and regrowth. An example of comparison an d synergy between the two techniques is discussed here. Polarimetric, C- (5.6 cm) and L-band (24 cm) frequency, radar data gathered in Octob er 1994 by NASA's Spaceborne Imaging Radar C, on a test site southeast of the city of Porto Velho, in the state of Rondonia, Brazil are anal yzed in conjunction with one 1993 Landsat Thematic Mapper (TM) scene, a 9-year time series of Satellite pour l'observation de la Terre (SPOT ) XS data, two Japan Earth Resources Satellite (JERS-1) radar images f rom 1994 and 1995, and a field visit conducted in 1995. The C-band rad ar data are found to be of limited utility for mapping deforestation. At L-band multiple polarizations are required to obtain a reliable cla ssification. The single polarization, L-band, single date, JERS-1 data underestimate the extent of deforestation, especially during the wet season. With multiple polarizations, sit classes of land cover includi ng one level of regrowth, are mapped with 90% accuracy, but intermedia te regrowth 5-8 years of age is not well separated from the forest. Th e Landsat TM data identify deforested areas better but provide less in formation on residual woody biomass levels. Combining the two classifi cations, seven classes of land cover including two levels of regrowth are mapped with 93% accuracy. The results show that the deforestation rate for 1994 was 1.7%. Large variations in residual woody biomass are detected among new clearings. Half of the total deforested land is in some stage of regrowth, but most of it is less than 5 years old. Seco ndary growth is therefore a significant form of land use that is recle ared quickly. (C) Elsevier Science Inc., 1997.