Effects of fire on above-ground forest biomass in the northern Brazilian Amazon

Fires can significantly affect the structure, floristic composition and biomass content of tropical forests, which are not adapted to this disturbance. To assess the impact of understorey fires on above-ground biomass, this study was conducted in the northern Brazilian Amazon (Roraima state), where uncontrolled forest fires are recurrent. Fifty plots (0.25 ha each) distributed across five fire disturbance classes were inventoried. Losses in biomass stocks were significant (-57% and -63%) for forests that suffered from recurrent fires and progressively occurred until some point between 3 and 7 y after the last fire, as deduced from previous studies. Twelve years after a fire event, biomass stocks were reconstituted, although differences in the floristic composition were associated with greater fire severity, particularly driven by the dominance of the Cecropia spp. In thrice-burned forests, live biomass reached levels of secondary forests of the same region. For large trees (> 50 cm dbh), the reduction (-54%) in biomass was significant in thrice-burned areas. These findings highlight the threat that fires represent for tropical rain forests and emphasise the need for long-term surveys in order to distinguish between the effects of fire severity, fire frequency and time-since-fire artefact.