Tropical forest degradation by mega-fires in the northern Brazilian Amazon

► We inventoried burned forests in the “Arc of Fire” in the State of Roraima, Amazon. ► We found five classes of fire-disturbance, forming a complex mosaic in the region. ► Floristic and structural factors were useful to reveal a significant gradient of fire. ► Density of Cecropia spp. was positively related to forest degradation by fire. ► We found strong trends of “secondarization” of forests burned by recurrent fires. In this study, we examine the degradation of tropical forests caused by the forest fires of anthropogenic origin that have frequently and intensely affected a large portion of the State of Roraima (northern Brazilian Amazon), such as the one that occurred in 1998 and extended over 12,000km2. In a region located in the so-called “Arc of Fire” in the State, data from all individuals with DBH⩾10cm were collected in 50 sample plots (12.5ha). These plots were collected across five fire-degradation levels (classes), from unaffected areas to areas affected by one, two or three fires over 10years: 1998, 2003 and 2007. The floristic and phytosociological quantitative parameters in the analysis were as follows: species richness (defined classically and by rarefaction), evenness, Shannon diversity index, mixture coefficient, importance value (dominance, density and frequency), basal area, diameter, total height and volume. In the full set of plots, 5593 individuals were measured and identified in 177species/morphospecies and 53 families. We performed both univariate (ANOVA and Tukey’s test) and multivariate analyses (ordination methods), aiming to find patterns of degradation by fire. The results revealed that forests affected by one low-intensity fire (BF1L) showed slight evidence of alteration in comparison with forests that have not burned (UF), taking all indicators evaluated into account. In extremely degraded forest plots, the heavily impacted forests lose their primary-forest characteristics, showing significant floristic and structural changes and similarity to areas of young secondary succession. Some aspects of these disturbances were observed not only in twice or thrice-affected areas (BF2, BF3) but also in areas that suffered from one high-intensity fire (BF1H). Given the observed results, we ought to consider that an extensive “secondarization” process caused by fire could plausibly reach more forest areas in this part of the Amazon in the near future.