Dynamics of savanna clearing and land degradation in the newest agricultural frontier in Brazil

摘要

We investigated the dynamics of savanna clearing and land degradation in the MATOPIBA region, the newest agricultural frontier over the Cerrado in Brazil. For geospatial analysis of the "big data," we used algorithms available on the Google Earth Engine (GEE) platform along with different satellite products. For the detection of vegetation clearing in the 1985-2018 period, we applied the LandTrendr algorithm to the normalized difference vegetation index (NDVI) obtained from Landsat satellite instruments. In the evaluation of land degradation (2001-2015), we used the Trends.Earth algorithm to analyze NDVI data calculated from the Moderate Resolution Imaging Spectroradiometer (MODIS). By combining different datasets into the analysis, we discussed the relationships among savanna clearings, land degradation, topographic attributes, and fire frequency. The results showed that the largest proportions of savanna clearing in the 1985-1990 period were detected in the south of MATOPIBA (mesoregions Ocidental do Tocantins and Extremo Oeste Baiano). The savanna clearing progressed toward the mesoregions Oriental do Tocantins, Sudoeste Piauiense, and Sul Maranhense between 1991 and 2005. Variations over time in cleared areas ranged from 0.086 Mha to 2.146 Mha per year, showing well-defined peaks in 1986, 1991, and 2012. The clearing events followed the soybean expansion, especially over flat terrains at high elevations. In addition to the spillover effects of the Amazon Soy Moratorium on the savannas, other political and economic factors promoted changes in cleared areas of MATOPIBA. Compared to the decade preceding the signature of the Amazon Soy Moratorium in 2006, the cleared area of MATOPIBA increased 41% in the 10-year post-moratorium period. In the other analysis, our Trends.Earth results showed that the largest proportions of land degradation areas, as proxied by the MODIS NDVI, were the Sudoeste Piauiense and Centro Maranhense. These mesoregions presented distinct patterns of precipitation and soil composition. The number of fire events increased with vegetation clearing. However, the amount of cleared areas correlated moderately with that of burned areas due to the multi-use of fire in the region. Finally, we observed that the amount of land-degraded areas increased with fire frequency over agricultural areas, but remained relatively stable over native vegetated areas. Results highlight the importance of satellite sensors and cloud-computing platforms to inspect long-term trajectories of land occupation and degradation over broad savanna areas.