O processo sucessional na paisagem do município do Oiapoque, estado do Amapá.

Abstract

This study aimed to evaluate the process of secondary succession established in the northern Brazilian border, municipality of Oiapoque, in order to achieve the following specific objectives: To evaluate the floristic composition of pastures in order to verify that the regeneration of tree/shrub species is predominant, indicating the beginning of the forest succession process. We also tested the influence of the species of forage used, age of the pasture, type of pasture cleaning and chemical characteristics of the soil; evaluating if the floristic succession patterns are the same in different vegetation strata, and if the patterns are related to the same factors. The tested factors were age of pastures and sixteen physical and chemical soil factors. The study considered three main successional stages; pastures of Brachiaria brizantha and B. humidicola were considered as the initial stage of succession; secondary forests were considered as intermediate stages and preserved forests were the final stage. The floristic inventories were performed in areas of small family farmers. In twenty primary forests were installed plots of 20 x 50 m and plots of 10 x 50 m were installed in twenty secondary forests. To the whole extent of the plot were inventoried all individuals with DBH > 10 cm (upper stratum); and within these plots were allocated sub- plots of 10 x 50 m in primary forests and 5 x 50 m in secondary forests, to inventory the individuals with DBH < 10 cm and height > 2.0 m (middle stratum). Finally, to inventory individuals with 0.10 m < height < 2.0 m (lower stratum), a transect of 1 x 50 m was placed in the center of the plots in the forests and in 31 different pastures as well. In the same 71 plots of the floristic inventory were also collected soil samples for physical and chemical characterization. The analyzed soil factors were: pH, organic matter, P, K, Ca, Ca + Mg, Al, H + AL, Sum of bases, CTC, base saturation, saturation for Al and content of coarse sand, fine sand, clay and silt. Although in pastures most plants are herbaceous, often regarded as weeds, development of tree/shrub species indicate the installation of the successional process. In the pastures of B. humidicola, which contains more tree/shrub species and individuals, this process was more advanced than in the pastures of B. brizantha. The higher level of acidity in pastures of B. brizantha may be a result of grass architecture , their level of nutritional requirements combined with the frequent use of fire in pastures , leading to less development of tree / shrub species , which may delay the initial process forest regeneration.At Oiapoque, the age of the pastures did not influenced the floristic composition, as reported in the literature. Regardless of age, the pasture cleaning applied by the owners was more important; fact that led to different sets of dominant species that influenced differently the beginning of the successional process. Over the successional stages, the three strata revealed a floristic gradient, with increasing richness, equitability and diversity of Shannon; this gradient is associated to different environmental conditions that become limiting to species development. The middle and upper strata were influenced by the same soil factors, but the lower stratum seems to be more demanding in nutrients, especially MO, P and K. However, in the three analyzed strata, pH and Ca were associated with pastures and Al was associated with forests and secondary forests; and this can be considered as a similar pattern between vegetation strata.