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Biomass estimates of primary and different ages of secondary vegetation are reported for a tropical forest region in Rondônia, Western Brazilian Amazon. The estimates are based on published allometric equations, and on vegetation composition and allometric data collected in areas of primary forest and secondary vegetation of ages 2, 3, 5, 9, 11, 16 and 18 years. Primary forest biomass estimates varied from 290 to 495 t ha–1. Secondary vegetation biomass estimates accounted for 40–60% of the primary forest biomass after 18 years of abandonment. Secondary growth rates in lightly used areas are estimated to have varied from 6.6 to 8.7 t ha–1 y–1 between the third and the eighteenth years after abandonment. CO2 sequestration by regrowing vegetation is discussed for two scenarios of land abandonment.

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Abstract. An analysis of landscape chanizes in a region of pioneer settlements in central Rondônia, western Brazilian Amazon, was derived from Landsat TM data. Total deforested area increased from 206 x 10 3 ha in 1977, to 565 x 103 ha in 1985 and to 1210 x 10 3 ha, or 35.5% of the region, in 1995. Eighty-one per cent of the total 1995 deforestation had occurred in regions within 12.5 km from arcas of pioneer colonization deforested by 1977. Deforested area exceeded 79% in regions within 12.5 km from the region's first road.

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