Recurrent wildfires alter forest structure and reorganize community composition in terra firme Amazonia

Abstract Wildfires associated with land-use and climate have considered a key driver to the Amazon forest collapse. However, achieving a detailed understanding of how human-related disturbances impact forest successional trajectories needs comprehensive information spanning across forest strata. Here, we investigate the impact of recurrent wildfires on forest structure, species diversity, and composition, making a comprehensive assessment of the regenerating, understory, and mature tree communities in a sustainable use reserve in the eastern Amazon. Plant communities were described across 16 forest stands (old-growth, burned once and twice) into a protected area in the eastern Amazon region. A total of 3,620 individuals, 326 tree and palm species were recorded. Wildfires affected all attributes to relative forest structure across explaining up to 30% of their variation. Briefly, aboveground biomass decreased by 44% in forest burned once, but 71% in twice-burned forest stands. Forest canopy was the most affected layer after the second fire, with a 44%-decrease as compared to the unburned forest. The same pattern emerged for basal area, which decreased on 27.5% in average after the first fire and 53.8% elapsed the second fire event. Overall, plant communities experienced a 50%-species loss elapsed two fires, including both dominant and rare species. Finally, plant communities became more dissimilar as fire events accumulated, i.e. 58%-61% increment in species dissimilarity as elapsed two fires events. As wildfires continues to occur the old-growth forest of our focal landscape tend to be converted into a mosaic of regenerating forest stands dominated by local short-lived pioneers (i.e. low-biomass early-regenerating forest stands) and those few tree species less sensitive to fire. The extent to which single and recurrent forest fires influence the provision of ecosystem services, including forest products that support local livelihoods, is still to be examined.