Deforestation and pasture establishment in watersheds: Implications for stream biogeochemistry in the Amazon Basin

The ecological response of landscapes to land cover change depends on the rate and extent of conversion. Rapid clearing can cause transient pulses of nutrients to streams, while slow but extensive change can result in chronic changes. The measurement of the rate and extent of change also need to be made in ecologically relevant landscape units. A time-series of Landsat TM imagery are combined with a large population of watersheds (N=4788) to quantify the rate and extent of deforestation in a region of the Amazon Basin. The time series shows that clearing was slow (5-6 percent per year during active clearing) but proceeded to more than 75 percent of the watershed area for more than 25 percent of all watersheds draining less than 10 km2. Geostatistical methods suggest that the extent of deforestation is highly autocorrelated, while the rate is a random variable. The autocorrelation causes even large watersheds to be heavily deforested. A small fraction of the cleared area reverted to forest, suggesting that clearing was both extensive and permanent. Clearing of forest followed by pasture establishment in a wide range of watershed sizes is likely to lead to chronic changes in stream biogeochemistry, rather than transient pulses from cutting and burning of forest biomass.