THE RATE AND EXTENT OF DEFORESTATION IN WATERSHEDS OF THE SOUTHWESTERN AMAZON BASIN: Fid-Bau Portal

THE RATE AND EXTENT OF DEFORESTATION IN WATERSHEDS OF THE SOUTHWESTERN AMAZON BASIN

Biggs, Trent W. / Dunne, Thomas / Roberts, Dar A. / Matricardi, E.

The rate and extent of deforestation determine the timing and magnitude of disturbance to both terrestrial and aquatic ecosystems. Rapid change can lead to transient impacts to hydrology and biogeochemistry, while complete and permanent conversion to other land uses can lead to chronic changes. A large population of watershed boundaries (N = 4788) and a time series of Landsat TM imagery (1975–1999) in the southwestern Amazon Basin showed that even small watersheds (2.5–15 km²) were deforested relatively slowly over 7–21 years. Less than 1% of all small watersheds were more than 50% cleared in a single year, and clearing rates averaged 5.6%/yr during active clearing. A large proportion (26%) of the small watersheds had a cumulative deforestation extent of more than 75%. The cumulative deforestation extent was highly spatially autocorrelated up to a 100–150 km lag due to the geometry of the agricultural zone and road network, so watersheds as large as ∼40 000 km² were more than 50% deforested by 1999. The rate of deforestation had minimal spatial autocorrelation beyond a lag of ∼30 km, and the mean rate decreased rapidly with increasing area. Approximately 85% of the cleared area remained in pasture, so deforestation in watersheds of Rondônia was a relatively slow, permanent, and complete transition to pasture, rather than a rapid, transient, and partial cutting with regrowth. Given the observed land‐cover transitions, the regional stream biogeochemical response is likely to resemble the chronic changes observed in streams draining established pastures, rather than a temporary pulse from slash‐and‐burn.