Tropical forests in ecotonal regions as a carbon source linked to anthropogenic fires: a 15-year study case in Atlantic forest? Cerrado transition zone

Abstract

Tropical forests are extremely important for biodiversity maintenance and ecosystem services’ provisions. However, such services have been largely threatened by anthropogenic pressures, with a major role from forest fires, particularly in transitional regions that encompass savanna ecosystems. To improve our understanding on the impacts of an anthropogenic fire on the carbon stock and uptake, we used a dataset with 38 tropical forest plots located in a forest-savanna transition region, to test the hypothesis that fire decreases forest's carbon stocks and ceases its ability to sink carbon, becoming a source of carbon for the atmosphere. Tree communities (diameter at the breast height ≥ 5 cm) were monitored for 10 years in the absence of fire (2001 to January 2011), until September 2011, when 24 plots burned (fire from adjacent farms). The two groups of plots (unburned and fire-affected in 2011) were remeasured in 2016, encompassing a 15-year monitoring period over 4 inventories (2001, 2006, 2011 and 2016). We specifically investigated the temporal trends of each group of plots in relation to its carbon stock and uptake and partitioned the contribution of the different component processes (gain, loss, mortality, recruitment, increment and decrement). We found that the trend of increasing forest carbon stock observed in the period before the fire (2001–2011) was abruptly interrupted by fires, decreasing carbon stocks due to negative productivity in the 2011–2016 interval. Carbon losses were mainly driven by an increase in tree mortality, especially in small-DBH trees. In turn, forests that played an important role in carbon sink became a source of carbon to the atmosphere. This result has an important impact on the ecosystem services provided by these forests, especially considering the context of increased fires in the forest-savanna transition regions and its possible interaction with climate change.