The effect of fire on the carbon fluxes and productivity of Brazilian woodland savannas

The effects of altered fire regimes within open ecosystems are poorly understood and can have serious consequences on functioning and conservation across savanna ecoregions worldwide. In South American savannas like the Cerrado, there is a gap of knowledge relating to carbon cycling in the presence of fire, meaning the impacts of altered fire regimes on the carbon fluxes and budgets are virtually unknown. We thus investigate vegetation carbon flux dynamics within the Cerrado making use of an experimental fire and carbon monitoring research project at the Estação Ecológica da Serra das Araras, Brazil. We present a thorough carbon budget of woodland-type savannas (cerrado sensu stricto), and investigate how annual (every year), biennial (every two years), and triennial (every three years) controlled fire frequencies have influenced net primary productivity and respiration fluxes. Six years of experimental fire had noticeable effects on the vegetation structure and carbon dynamics, reducing woody cover and productivity in favour of grass-dominated carbon balances. Woody NPP increased by 35 % in the unburnt plots from 2017 to 2019 to 2021–2023, but decreased by 75 %, 33 % and 20 % in the triennial, biennial and annual fire frequencies. By 2023, burnt plots revealed around three times higher herbaceous NPP than unburnt plots. Fluxes corresponding to different ecosystem components (canopy, stems, roots, herbs) showed varying patterns of change across the gradient of experimental fire frequencies, indicating other fire regime properties distinctively affect each vegetation segment. Fire intensity and severity appear to be linked with patterns in woody stems and the herbaceous layer. Our results indicate periodically burnt cerrado sensu stricto vegetation experiences different carbon dynamics than unburnt vegetation. Burning is revealed as a strategy that can successfully limit woody encroachment and help conserve open ecosystem structure in the Cerrado. Our study highlights the importance of long-term monitoring efforts in investigating the effects of management interventions and environmental shifts on ecosystem functioning.