How does fire exclusion affect the belowground biomass of tropical open ecosystems?

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Prepared by Juliana Teixeira, Lara Souza, Aline Bombo & Soizig Le Sradic

Post-fire differences at Cerrado open savanna in central Brazil. The left side of A and B represented the unburnt plot. The right side of A represents the immediate post-fire conditions; and the right side of B represents three months after the last fire. Plots from the Long-term fire experiment, coordinate by Dr Alessandra Fidelis (Project “How fire season affects Cerrado Vegetation?”). Photo credit: Juliana Teixeira.

Fire is key for the maintenance of open ecosystems and biodiversity worldwide. In some environments, it prevents the establishment of trees, particularly in tropical areas where climate conditions are expected to sustain evergreen forests. The Brazilian savanna ecosystem (Cerrado) is one of such open tropical ecosystems shaped by fire. In the Cerrado, fire maintains an open landscape dominated by grasses, with scattered shrubs and trees. To resist and persist following frequent disturbances, plants in savanna ecosystems have developed various strategies. For example, increasing biomass allocation towards belowground structures for nutrient storage and bud protection will promote rapid resprouting after fire. However, global fire regimes are changing, with some savanna regions experiencing less fire than expected often by policies implementation that aimed fire suppression. The reduction of fire activity in savanna ecosystems may alter vegetation community strategies related to biomass allocation and the composition of belowground storage structures, potentially impacting their resistance and resilience over time.

To address this question, we investigated the effects of different fire frequency/regime on belowground biomass allocation and plant organ composition across five distinct tropical open ecosystems in Brazil, ranging from an open savanna in the north to southeastern Brazil. In each study area, we sampled aboveground and belowground biomass (i.e., fine roots, coarse roots, and organs) and identified belowground storage organs in one site area subjected to frequent burning and in one site where the fire had been excluded for at least 12 years. We demonstrated that fire-excluded areas had a lower root-to-shoot ratio and lower total belowground to aboveground biomass allocation than areas frequently burnt. The total belowground biomass, as well as fine and coarse root biomass and belowground bud-bearing organ biomass remained unchanged with fire exclusion. The composition of belowground bud-bearing organs changed towards organs with lateral spread as woody and fleshy rhizomes when fire was excluded.

Belowground storage structures, found in open savanna ecosystems. A: Post-fire resprout from belowground storage structures that also storage buds. B: Belowground storage structure (xylopodium) from Ichthyothere hirsuta Gardner. C: Diversity of belowground storage structure sampled in a Cerrado open ecosystem in the centre of Brazil (Reserva Natural Serra do Tombador, RNST, Góias). D: Belowground storage structure (xylopodium) from Brosimum gaudichaudii Trécul. E: Fine roots sample from savanna open ecosystem located in the west centre of Cerrado region (Parque Nacional Chapada dos Guimarães, PNCG, Mato Grosso). A, B: photo credit Alessandra Fidelis; C, E: Juliana Teixeira. D: Aline Bombo.

Our findings also highlighted that the response of belowground biomass allocation and composition to fire was heterogeneous between study sites, likely influenced by environmental factors such as soil constraints and climate differences among sites. Finally, our findings emphasized that fire is an important management tool for open savanna ecosystems but should be used carefully to effectively conserve their biodiversity.

This is a plain language summary of the paper by Teixeira et al. published in the Journal of Vegetation Science (https://doi.org/10.1111/jvs.70027)