Supplementary Material from Quantifying immediate carbon emissions from El Niño-mediated wildfires in humid tropical forests

Wildfires produce substantial CO₂ emissions in the humid tropics during El Niño-mediated extreme droughts, and these emissions are expected to increase in coming decades. Immediate carbon emissions from uncontrolled wildfires in human-modified tropical forests can be considerable due to high necromass fuel loads. Yet, data on necromass combustion during wildfires are severely lacking. Here, we evaluated necromass carbon stocks before and after the 2015–2016 El Niño in Amazonian forests along a gradient of prior human disturbance. We then used Landsat-derived burn scars to extrapolate regional immediate wildfire CO₂ emissions during the 2015–2016 El Niño. Before the El Niño, necromass stocks varied significantly with respect to prior disturbance and were the largest in undisturbed primary forests (24.6 ± 2.81 Mg ha⁻¹, mean ± s.e.) and smallest in secondary forests (12.1 ± 2.6). However, neither prior disturbance nor fire intensity explained necromass losses due to wildfires. Regionally, almost 1M ha of primary (disturbed and undisturbed) and 20 000 ha of secondary forest burned during the 2015–2016 El Niño. Covering less than 0.2% of Brazilian Amazonia, these wildfires resulted in expected immediate CO₂ emissions of 25 Tg, two to three times greater than previous regional estimates. Uncontrolled understorey wildfires in humid tropical forests during extreme droughts are a large and poorly quantified source of CO₂ emissions.This article is part of a discussion meeting issue ‘The impact of the 2015/2016 El Nino on the terrestrial tropical carbon cycle: patterns, mechanisms and implications'.