Quantifying the impacts of wildfires on forest carbon stocks and CO2 emissions across Brazilian Amazonia

Silva, Camila and Barlow, Jos and Young, Paul (2021) Quantifying the impacts of wildfires on forest carbon stocks and CO2 emissions across Brazilian Amazonia. PhD thesis, Lancaster University.

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Abstract

Drought-induced wildfires are an increasing threat to tropical forests. More frequent and intense droughts combined with increasing anthropogenic disturbances are converting previously fire-resistant humid forests into fire-prone ecosystems. Understanding the impacts of wildfires in tropical forests is critical to maintain the vital role of tropical forests in regulating climate and supporting human wellbeing. However, the long-term effects of wildfires on forest carbon stocks and emissions are still poorly understood. This thesis addresses this knowledge gap by investigating fire-induced changes across 63 Amazonian forest plots and quantifying associated carbon emissions. I first assessed long-term changes in biomass stocks and dynamics among tree functional groups (chapter 2), showing that a 25% reduction in carbon stocks persists for at least 30 years after wildfires. Losses outweighed carbon gains in the short- term (1-8 years), but although the carbon balance returned to baseline levels over the long-term carbon stocks had not recovered to pre-fire levels, even after 30 years. In chapter 3, I quantified year-to-year net CO2 emissions from burned forests, based on changes in stem mortality, decomposition and vegetation growth. The models I proposed showed that following combustion emissions, a large pulse of carbon is released to the atmosphere through decomposition, peaking at c. 5 years after the fires, which was responsible for up to 73% of all fire-induced emissions over the 30-year period. Post-fire regrowth only offset 35% of all fire- induced carbon emissions. Finally, my spatio-temporal approach to scale-up immediate and long-term CO2 emissions from wildfires in chapter 4 showed that the greatest combustion and decomposition emissions occur in forests with the highest biomass. Overall, this thesis demonstrates that the effects of fire on forest carbon stocks persist for many years and that environmental policies should focus on tackling wildfires in the humid tropics, especially where forests are hyper carbon-rich.