Abstract
Secondary forests are increasing in the Brazilian Amazon and have been cited as an important mechanism for reducing net carbon emissions. However, our understanding of the contribution of secondary forests to the Amazonian carbon balance is incomplete, and it is unclear to what extent emissions from old-growth deforestation have been offset by secondary forest growth. Using MapBiomas 3.1 and recently refined IPCC carbon sequestration estimates, we mapped the age and extent of secondary forests in the Brazilian Amazon and estimated their role in offsetting old-growth deforestation emissions since 1985. We also assessed whether secondary forests in the Brazilian Amazon are growing in conditions favourable for carbon accumulation in relation to a suite of climatic, landscape and local factors. In 2017, the 129,361km2 of secondary forest in the Brazilian Amazon stored 0.33±0.05billionMg of above-ground carbon but had offset just 9.37% of old-growth emissions since 1985. However, we find that the majority of Brazilian secondary forests are situated in contexts that are less favourable for carbon accumulation than the biome average. Our results demonstrate that old-growth forest loss remains the most important factor determining the carbon balance in the Brazilian Amazon. Understanding the implications of these findings will be essential for improving estimates of secondary forest carbon sequestration potential. More accurate quantification of secondary forest carbon stocks will support the production of appropriate management proposals that can efficiently harness the potential of secondary forests as a low-cost, nature-based tool for mitigating climate change.
Highlights
Tropical forests are an enormous reservoir of carbon, storing upwards of 190 billion Mg of above-ground carbon (Saatchi et al, 2011)
Establishing the location of secondary forests will provide insights into whether they are growing in contexts that are more or less favourable to rapid carbon accumulation. 88 Here we address these knowledge gaps, using the MapBiomas 3.1 landcover dataset (1985-2017) and the Avitabile et al. (2016) pan-tropical biomass map to provide the first spatially explicit estimate of the role of secondary forests in offsetting deforestation emissions in the Brazilian Amazon
From 1995, these very young forests consistently represent almost half of total secondary forest extent (48.0±4.5%). 226 Old-growth deforestation emissions offset by secondary forest growth 227 Old-growth deforestation emissions: Between 1985 and 2017, MapBiomas detects the clearance of 512,473 km2 of old228 growth forest
Summary
Tropical forests are an enormous reservoir of carbon, storing upwards of 190 billion Mg of above-ground carbon (Saatchi et al, 2011). The Amazon basin is the largest remaining tropical carbon stock (Saatchi et al, 2011) It has the highest rates of forest clearance (Hansen et al, 2013), with carbon losses directly related to deforestation estimated to be 0.16–0.67 billion Mg C yr-1 (Achard et al, 2002; Loarie, Asner and Field, 2009). The trajectory and rate of secondary forest growth are influenced by numerous climatic, landscape and local factors, which contribute to a ten-fold difference in estimates of carbon sequestration rates across the tropics (Elias et al, 2019). (2016) pan-tropical biomass map to provide the first spatially explicit estimate of the role of secondary forests in offsetting deforestation emissions in the Brazilian Amazon. As a first step in identifying the potential for interacting effects, (3) how are these variables correlated spatially within the existing range of secondary forests? 97
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