LAUSR

Abstract The monomodal flood pulse of major Amazonian rivers is a seasonal phenomenon that determines ecological and biogeochemical processes in adjacent floodplain forests. River damming transforms the pattern of downstream flood pulses and provides a natural disturbance to which the native biota might be poorly adapted. Severe modifications of the flood pulse were recorded in the Uatuma River after the installation of the Balbina dam, Central Amazonia. Flood pulse regulation increased mortality of flood-adapted species in the black-water floodplain (igapo) forest. No previous studies have investigated impacts of flood pulse regulation on the species composition and forest structure of igapo forests. Therefore, we examined species composition and forest structure of igapo forests along a regulated river in comparison to a pristine tributary, the Abacate River, evaluating soil texture characteristics and flood duration. In order to assess potential encroachment of species less sensitive to flood alteration, we also inventoried adjacent non-flooded upland forest in each river section. A quantitative inventory of all trees with diameter at breast height (DBH) ≥5 cm was carried out in low-igapo, high-igapo and adjacent upland forests, totaling one half-hectare in each river. In both rivers investigated, the clay fraction of the soil was significantly related to tree height. Flood duration was correlated to DBH and basal area, with the largest trees found in low-igapo forests which are exposed to long-term flooding. Species composition, richness and diversity significantly responded to flood duration. Species richness was highest in upland forests and lowest in low-igapo forest. In the pristine river, tree species composition exhibited a turnover of species along the flooding gradient. In the regulated river, flood intensification in the low-igapo forest increased dominance of a few flood-adapted species, which produced floristic dissimilarity to all forest types investigated. On the other hand, high-igapo forest showed higher floristic similarity with upland forest due to flood suppression that contributed to encroachment of species commonly described in secondary upland forests. Our results emphasize the urgent need for Brazilian environmental regulatory agencies to incorporate downstream impacts in the environmental assessments of dam projects in the Amazon Basin.