Date of Award

Winter 1992

Project Type

Dissertation

Program or Major

Natural Resources

Degree Name

Doctor of Philosophy

First Advisor

John D Aber

Abstract

A clear understanding of the role of the biota in the global carbon cycle is limited by an absence of accurate measurements of deforestation rates in the tropics. This study measures the rate and extent of deforestation in the Brazilian Amazon, a tropical forest biome approximately 5 $\times$ 10$\sp6$ km$\sp2$ in size and the largest extant tropical forest biome in the world. The study focuses on remote sensing measurements of deforestation rates and the area of secondary vegetation, but also utilizes tabular data to document deforestation when satellite data are not available. The analysis concludes: (1) Regression analysis of SPOT, TM, and AVHRR measurements suggests that the AVHRR will greatly overestimate deforestation and be highly variable; the use of a brightness temperature threshold is highly sensitive and unreliable. The upward bias of AVHRR is a function of the density of deforestation. (2) An accurate measurement of deforestation requires Landsat TM data, and can be accomplished using low cost visual interpretation of photographic products at 1:250,000 scale, with accuracies within 10% of that obtained using digital image processing techniques employing supervised statistical classifiers. (3) Secondary growth in the Brazilian Amazon represents a large fraction of the total deforested area, and the abandonment of agricultural land is an important land cover transition. Abandonment rates were 70-83% of clearing rates from primary forests. At any one point in time, approximately 30% of the deforested area is in some stage of abandonment, and quite likely nearly all deforested land becomes abandoned after approximately 5 years. (4) Previous estimates of the total area deforested in the Amazon, as well as the rate of deforestation, have been too high by as much as 4-fold. A complete assessment of the entire Legal Amazon using over 200 Landsat images measures 251 $\times$ 10$\sp3$ km$\sp2$ deforestation as of 1988, or approximately 6% of the closed forests of the region. The average annual rate of deforestation between 1978 and 1988 was 18 $\times$ 10$\sp3$ km$\sp2$ yr$\sp{-1}$. These findings are important to carbon cycle research. They suggest the estimates of carbon emissions from the Amazon for the late 1980s have been too high, since the area of regrowth is large and rates of deforestation are lower than previously believed.

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