Efeito da alteração no ciclo hidrológico do Rio Uatumã sobre a regeneração de plântulas a jusante da barragem da Hidrelétrica de Balbina, AM

Abstract

In the Amazon, the flooded forests are affected by frequent dam constructions for hydroelectric power plants (UHEs), which have altered the natural flood cycle affecting mainly the forests of igapó (flooded by rivers of black water). The vegetation downstream the Balbina Dam, built in 1987 in the Uatumã River, presents modifications in the structure, diversity and demographics of trees in relation to igapó forests with a regular flood pulse. The scarce studies in the region were performed with saplings and adult trees, however the seedling phase is the one that best responds to an environmental disturbance. Thus, the aim of this study is to verify how seedling regeneration is affected along the topographic gradient by changing the flood pulse of the Uatumã River, in an igapó forest downstream of the Balbina dam, comparing it with one or two igapó forests without interference by hydroelectric dam. The areas were compared at different topographic levels along the flood gradient from the level closest to the river to the closest to the adjacent non-flooded forest. For sampling, they were installed in each igapó forest, transects of 1 x 25 meters in plots of 25 x 25 meters. Data collection was performed during the dry season of the flood pulse for three years (2015-2017). Along the topography in the seedling community of the sampled areas, phytosociology, richness, diversity, genera distribution and biomass data were evaluated. These data were related to the days and height of flooding per year according to the topographic level of each area. The igapó forest of the Uatumã River, after more than thirty years of construction of the dam, presents differences in the regeneration patterns of the seedling community throughout the. topography, which can be irreversible. Species diversity was reduced at the lowest levels of topography, and species increased in the highest areas. The seedlings of the Uatumã area formed three different communities along the topography. Each community has a dominant species: Pouteria elegans (Sapotaceae) and the palm trees Astrocaryum jauari and Attalea maripa. The change in flooding and floristic composition increase seedling biomass in the Uatumã area compared to the regular pulse area. Thus, we hope that the results of the present study can assist in long-term conservation and management decisions in flooded forests affected by hydroelectric dams in planning or installation process in the Amazon region.