Fitossociologia do sub-bosque e estrutura populacional de Cenostigma tocantinum Ducke, em três fragmentos florestais no lago da hidrelétrica de Tucuruí

Abstract

The point of this research was to evaluate the understory phytossociology within three forest stands from the remaining islands of plateau, which appeared after the flood of the lake in the surrounding area of the HPS – Tucuruí, PA. The research was carried through in the reservoir of the HPS - Tucuruí, located at the district of Tucuruí – PA, with the geographic coordinates (3º43’ e 5º15’S; 49º12’ e 50º00’W). Within each stand of 1 ha, divided in 25 plots (20 x 20 m), thirteen subplots (10 x 10 m) were delimited in order to study the understory, selected randomly, to the sub sample S1, where all individuals were inventoried which were included in the DBH class of 5 cm ≤ DBH < 10 cm. Within the subplot of 10 x 10 m, thirteen subplots (2 x 1 m) were delimited to the second sub sample (S2), where all individuals were inventoried which had 2 cm ≤ DBH < 5 cm, and a third sub sample (S3), to count individuals with diameter < 2 cm e height ≥ 1 m. Phytossociological analysis was carried through using Mata Nativa software, calculating frequency, density, dominance, diversity, importance value and similarity. Floristic composition of the understory in Base 3, Base 4 and Germoplasm Island registered 375 individuals within 42 families, 79 genera and 139 species. Floristic similarity was greater between Base 4 and Germoplasm Island, with 12 species in common. Families Myrtaceae, Sapindaceae, Annonaceae, Bignoniaceae, Lecythidaceae, Meliaceae, Mimosaceae, Fabaceae and Caesalpiniaceae were the most representatives. Gustavia augusta L., Pseudima frutescens (Aubl.) Radlk. and Rinorea sp. were present in all three sub samples (S1, S2 and S3), being species of tree habit, important for the understory structure, obviously germinating under low light and reaching adult stages. For further studies a wider sampling area is needed, along with growth and mortality monitoring and recruitment within remaining forests.

The point of this research was to study the spatial distribution and the population structure of the species Cenostigma tocantinum in three forest stands (Base 3, Base 4 and Germoplasm Island) of the lake in the surrounding area of the HPS – Tucuruí, PA. The research was carried through in the reservoir of the HPS - Tucuruí, located at the district of Tucuruí – PA, with the geographic coordinates (3º43’ e 5º15’S; 49º12’ e 50º00’W). After locating the areas, plots of 1 ha were implanted (100 x 100 m), which were divided into 25 subplots of 20 x 20 m (400 m²). Floristic inventory was 100% carried through and, from that on, the species Cenostigma tocantinum Ducke was chosen, usually known as “pau-preto” (dark wood), belonging to Caesalpinaceae family. Data analysis was carried through using Mata Nativa software and, to study the spatial distribution, the graphic method was used, considering points of axles X and Y, acquired with the individuals mapping. Abundance parameters analyzed by ANOVA (n=25; F= 6,4512; (p)=0,003), presented significative discrepancies among remaining forests. The species Cenostigma tocantinum Ducke showed high abundance and medium importance value (IV%) around 20,03%, and achieved higher values of density, frequency and dominance when compared to other species form floristic inventories in the fragments of the Hydro-electric Power Station – Tucuruí, what could indicate that the remaining fragmented forest is not influencing ecological conditions for the development of the species. Cenostigma tocantinum high values for phytossociological parameters obtained from values of density, frequency and dominance may be indicating that the lake flood caused by men is not influencing the better adaptation and ecological conditions for the development of this species. All three populations presented structures with stability and population balance for the diametric distribution with a population considered to be balanced. According to the graphic method, Cenostigma tocantinum showed a distribution with tendency to grouping within the three fragments surrounding the lake area of the Hydro-electric Power Station of Tucuruí. Considering factors and the landscape modified by men thought to be irreversible due to the lake flood, protecting these remaining fragments and ecologically studying this species considered to be restrict to the region, and many other species found in higher abundance in the inventories, is of fundamental importance for this ecosystem conservation, although studying other long-term ecological factors, topography and edaphic variables of the forest become necessary for further comprehension of the ecological processes that occur within the fragment and that influence the population dynamics of this species.