Como a seca artificial causada pela instalação da barragem hidrelétrica de Balbina modificou a composição florística dos igapós do Rio Uatumã na Amazônia Central?

The Amazon rainforest is the largest tropical rainforest and the largest river basin in the world, with approximately 30% of its area covered by wetlands of various types. Among these are the flooded igapó forests, characterized by vegetation adapted to the flood pulse developing morphological, anatomical, physiological, phenological and biochemical adaptations. However, changes in the flood pulse can severely impact the vegetation's adaptive capacity. An example of this impact occurred during the implementation of the Balbina hydroelectric plant at the Uatumã River (1983-1989), specifically during the period from October 1987 to February 1989, when the river was dammed to fill the gigantic reservoir. This scenario induced an artificial drought in the igapós downstream of the dam, caused by the reduction in the river's flow. The hypothesis is that the artificial drought generated and the consequent alteration of the hydrological cycle caused excessive mortality of the igapó forests. However, there is no ground-based information available on the extent of this impact and how the vegetation has changed with distance from the dam. In order to fill this gap, we investigated the extent of the impact caused by artificial drought downstream of the Balbina dam by analyzing the floristic composition, diversity, structure and biomass of the igapó forests of the Uatumã River. The results are based on floristic inventories conducted in 3.75 hectares of forest with plots distributed every 10 km from the dam. All trees with DBH ≥10 cm were marked and identified. Allometric equations were applied to estimate biomass. We sampled 2559 individuals, belonging to 76 tree species and 31 families. The five most abundant families accounted for 79.5% of the total number of individuals, with Fabaceae leading the way with 827 individuals. Our results suggest amortization of the disturbance from a distance of 120 km, given the transformation of the forest structure, reduction in the dominance of species indicative of disturbance, by separating the floristic composition into groups as a result of the ordination analysis (NMDS). Using simple linear regressions, we found significant relationships for the increase in richness, basal area and decrease in trees with regrowth as a function of distance from the river, indicating buffering of the disturbance. We emphasize that the maintenance of natural hydrological patterns is essential for the conservation of Amazonian biodiversity and the livelihoods of local communities, and mitigation solutions are urgently needed.