Tropical secondary forest regrowth in the Amazon: age, area and change estimation with Thematic Mapper data

Abstract

The purpose of this study is to assess the potential of estimating the area, age and changes of tropical secondary forest regrowth from the Landsat Thematic Mapper (TM). Sites of mature forest, agriculture, pasture, pasture with remnant trees, and stands of secondary forest regrowth from 2 to 19 years of age were surveyed in two study areas near Manaus, Amazonas, Brazil, and mapped to the TM imagery. Considerable changes in spectral reflectance were observed over the first 19 years of regrowth, and these can be summarized by indices related to canopy brightness and greenness. The near-infrared reflectance, the difference index, Kauth-Thomas greenness, and percentage leaf cover all increase over the first 4 years after abandonment, peak from 4 to 8 years, and decrease from 8 to 13 years. The Normalized Difference Vegetation Index (NDVI) rapidly rises over the first 4 years, and displays no apparent relation to stand age thereafter. The brightness of regrowth canopies decreases from 8 to 13 years. Stands from 13 to 19 years-old are spectrally similar to mature forest. All regrowth age classes are spectrally distinct from agriculture and pasture, and all regrowth age classes younger than 14 years are spectrally distinct from mature forest. Spectral indices of canopy brightness are significantly correlated with regrowth stand age (r ²≤0·48, p<0·001). Based on field research in several areas of Amazonia, these spectral patterns can be explained in terms of temporal changes in canopy geometry and leaf area. The estimated rates of regrowth clearance and reversion to regrowth between 1988 and 1991 suggest the intensity of rotational agriculture practised by both communities. These results suggest that monitoring tropical secondary regrowth with TM imagery could substantially improve estimates of carbon sequestration subsequent to tropical deforestation.