Landscape dynamics and equilibrium in areas of slash-and-burn agriculture with short and long fallow period (Bragantina region, NE Brazilian Amazon)

Fallow periods used in slash-and-burn agriculture in the Bragantina region, the oldest agricultural frontier in the Brazilian Amazon, are being reduced. The objective of this study was to analyze the effects of a shortened fallow period on the Bragantina landscape dynamics and equilibrium. Dynamics were characterized by landscape structural changes, particularly in the spatial distribution of secondary forests, and by transition matrix. Equilibrium was defined by temporal and spatial parameters, and by the increment of agricultural areas from 1985 to 1996, analyzed with 6 LANDSAT-TM images. I worked with 6 areas of 250 ha each, 3 with short fallow periods (2-4 years) and 3 with long fallow periods (about 10 years). Results showed that short fallow period areas did not present an equilibrium situation. In these areas, developed secondary vegetation tended to disappear and agricultural areas were being expanded at an average rate of 3% per year. Landscape structure changes pointed out that a reduction in fallow period was occurring in already short fallow period areas. Long fallow period areas presented a shifting mosaic steady-state condition, where punctual changes due to agricultural uses were compensated by field abandonment rate. Both agricultural uses and field abandonment rates were lower in long fallow period areas when compared with short ones. Comparisons with indigenous traditional cropping-fallow cycles indicate that sustainable conditions could be maintained with 11 years of fallow for each cropping year, while shorter cycles would break down the system if agricultural improvements are not implemented.