A alteração do regime hidrológico afeta a composição florística e estrutura de florestas de igapó? Um estudo comparativo entre um rio regulado e outro prístino na Reserva de Desenvolvimento Sustentável do Uatumã, Amazônia Central

Abstract

The monomodal flood pulse of major Amazonian rivers is seasonal phenomenon that determines ecological and biogeochemical processes in adjacent floodplain forests. Changes in the flood pulse transform the pattern of hydrological variation and natural disturbances to which the native biota may be poorly adapted. Severe modifications of the hydrological regime can be observed downstream of hydroelectric power plants, where the flood pulse is driven by energy demands. In the black-water floodplain forests (igapó) associated with the Uatumã river (Central Amazonia), hydrological changes and alterations in ecological processes of the associated river were recorded after the installation of the Balbina hydroelectric power plant. In this study, floristic and forests structure of igapó forests at the regulated river (Uatumã) were compared to a pristine river (Abacate), evaluating its edaphic characteristics and inundation period. We sampled adjacent non-flooded upland forests to evaluate behavior in relation to igapó forests. A quantitative inventory of all trees diameter breast height (DBH) ≥ 5 cm was carried in low igapó, high igapó and adjacent upland forests, totaling 1,5 ha in each river. In both rivers, the low igapó forests showed low relative tree density with DBH < 10 cm and high tree density with DBH ≥ 25 cm. High igapó forests of the Uatumã river presented higher height average than the Abacate river’s equivalent portion, being the only structural characteristic difference between pristine and regulated rivers. When we analyzed the rivers together, phosphorus content was related to species diversity and flood was determinant for species composition, richness, species diversity and structure variables (DBH and basal area). In Uatumã river, phosphorus amount was correlated to DBH and tree density. Additionally, tree density was associated with soil pH and clay proportion in the regulated river. Abacate river’s species composition followed flood gradients. In the regulated river, the species composition of high igapó forest was similar to upland forest and dissimilar in species composition to the low igapó forest. The pattern observed was understood as reflecting the flood period intensification in the lower portion of igapó forest and the invasion into high igapó forest of species commonly described in upland forests. The study results emphasize the need to include downstream effects in environmental impacts studies and reports of hydroelectric power plants in the Amazon region.