Document: JEN-3-89-23

Determination of fire return intervals for the tropical savanna of central Brazil

RIGGAN, P.J.*, J.L.RECHEL, L.G.WOLDEN and R.G.TISSELL

USDA Forest Service-Pacific Southwest Research Station, Riverside, CA 92507-6099 USA ¹

Abstract:
To assess the extent of agricultural and wildland burning in the tropical forest and savanna of central Brazil, a multi-year research effort is underway. This research quantifies the area of tropical forest and cerrado (savanna) burned and the magnitude of fire emissions to the atmosphere. As part of this larger study, the areal estimate of cerrado burned was determined using Landsat 4 MSS (multi-spectral scanner) band 4 satellite images from 1986. The objective was to detect and classify ash layers present in a later date scene when compared to an earlier date scene. Ash pixels were classified with a supervised parallelpiped classification by selecting pixels with low digital numbers (DN). The ash layer classifications were then imported into Arc/Info geographic information system (GIS) and amount burned over time was calculated for 15 vegetation categories. Savanna-Estepe, parque, and gramineo-lenhosa vegetation types had the greatest area burned. TM (thematic mapper) images, with a resolution of 28.5m, were used to test the accuracy of the classification method. A one -km grid, the resolution of AVHRR (Advanced Very High Resolution Radiometer) images, was generated in the GIS to test for differences in scales. AVHRR images are commonly used to determine area burned, but at a less accurate, coarser resolution. Significant differences were found between the MSS images (80 m) and the AVHRR simulated maps (1 km) ability to detect ash layers, especially for fires less than 100 hectares. Our results show that the estimated fire return interval for the cerrado is 10 years and not two years, as previously published. This is the most comprehensive fire scar data set analyzed and represents the most accurate and current areal extent of burning for the cerrado. These fire area estimates contribute to the Brazilian government fire management plans to conserve reserves and national parks and to the development of a fire rating system to monitor and predict annual fire danger in central Brazil.

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