Influência das secas severas na ocorrência de incêndios florestais e perdas de carbono no Sul da Amazônia, estudo de caso em terras indígenas

Abstract

The severe droughts that plague the Amazon have been associated with increased frequency and intensity of forest fires and agricultural burning, especially in the last decade, which has seen an increase in the intensity and frequency of drought events caused by warming of surface water in the Pacific (El Niño) and the North Atlantic (AMO - Atlantic Multidecadal Oscillation). Although ever more common throughout the Amazon, deforestation-frontier regions have shown increased susceptibility to the occurrence of fires in recent years when affected by severe drought because the fire has its greatest source of ignition from human activity. In the already consolidated agricultural areas in the triple-border region between the states of Rondônia, Mato Grosso and Amazonas, forests are concentrated in indigenous territories where, in the most extreme cases, these forests form barriers, with indigenous lands inhibiting deforestation and forest degradation because they have a relationship of control and land management that inhibits its advance. The strategy adopted by some indigenous groups in this region is the formation of blocks of contiguous indigenous lands to strengthen land management, where indigenous peoples are grouped into forest regions called “ethno- environmental corridors.” We analyzed two of these corridors located in the triple-border region under different intensities of human disturbance based on historical deforestation rates. We used satellite images from Landsat in time series covering the 1986-2015 period and examined the occurrence and recurrence of fire within the indigenous areas and in their surroundings. Finally, we performed a forest biomass inventory to assess the reduction in carbon stocks caused by recurrence of forest fires in the indigenous lands. Although significant, we found that the relationships of forest fires and agricultural burning with oceanic oscillations are weak. Themost influential was the AMO (r 2 = 0.209, p <0.05) in the southern portion of Amazonas state and El Niño (r 2 = 0.159; p <0.05),innortheastern Rondônia and in northwestern Mato Grosso. Driven by deforestation in the surrounding area, the corridor under greatest pressure had forest cover reduced 4.9 times more than in the corridor with lower anthropic pressure. As for the fire, the corridor under greater pressure had an area of 750,140 ha of forests burned, a value 6.8 times greater than the area burned in the corridor under less pressure. Indigenous lands had areas affected by fire 9.6 (r 2 = 0.629) and 4.6 (r 2 = 0.301) times smaller than in their respective surrounding areas in the corridors within greater and lesser human pressure, respectively. With the occurrence of severe drought, indigenous lands under greater pressure were most affected by fire, with an annual average area affected 52.8% higher than the area affected in neutral years, while in the surroundings of the indigenous lands the area affected by fires in years of severe drought increased by 30.0%. When we compare the areas affected by recurrent fires with adjacent unburned forests, we observed that there was a reduction in the stock of total biomass of 26.6% after the first fire, 46.0% after the second and 57.7% after the third fire (r 2 = 0.732; p <0.02). Although effective in curbing deforestation, the area of forests degraded by fire has increased considerably in these areas, as there is external pressure from the surrounding area that is able to radically change the culture of these people. The indigenous people tend to adopt the economic matrix of the surrounding areas, changing their farming practices and production within their territories, thus changing the landscape and the risk of future emissions in years with severe droughts in the Amazon.