Aspectos pedogeomorfológicos e evolutivos em ambiente de veredas, no norte da Amazônia

Abstract

The origin and dynamics of palm swamp in open areas in the extreme north of the Amazon is a topic that has been gradually debated. In this context, the objective of this thesis is the study of the formation of the palm swamp in the north of the Amazon, considering the geomorphological conditions and the different pedoclimatic contexts, for which two hypotheses guided this thesis: the first is that the palm swamp represent environments that have a variability of the accumulation pulses, with alternation of allochthonous and autochthonous materials, whose formation depended from the erosion of the slopes to processes resulting from the climatic and pedological stability in the course of the Holocene and that allowed the bioconstruction of peatlands. The second is that the attributes of distinct soil cover result from the contribution of different current climate gradients. The research that supported this thesis was based on: 1) cabinet steps with bibliographic, cartographic and remote sensor images; 2) field work to survey the arboreal component of palm swamp and collection of deformed and undisturbed soil samples and; 3) laboratory analysis to determine the chronology (LOE/SAR), mineralogy, granulometry, morphoscopy and soil chemistry. In addition, a micromorphological description of the soils was carried out. From the arboreal composition of palm swamp, data on aboveground biomass, richness and abundance of individuals were calculated. To help in the evaluation of the effect of climatic variables and of the physicochemical properties of the soils in the configuration of the vegetation of palm swamp, data of precipitation, temperature and local seasonality were used. The study accomplished in four soil profiles of palm swamp in the central-eastern region of Roraima proved that the increase in humidity conditions that occurred during the Late Holocene contributed to the establishment of the palm swamp, however, they developed in different phases in response to the new conditions of the spatial arrangement imposed by the relief, the ascent of these palm swamp occurred between LOE/SAR=2,160 (±275) years BP and LOE/SAR=520 (±80) years BP. The integration of field data, laboratory data, remote sensors and specific literature indicated that the palm swamp occur mainly in the accumulation modeled, where exudation zones from the water table occur conditioned by morphostructural and hydrological arrangements. The soils of the palm swamp are predominantly sandy, derived from parental material composed of allochthonous sediments of quartz-kaolinitic nature, for this reason, they are dystrophic and have high acidity. In the palm swamp under the influence of a higher rainfall regime and positioned in lowered sectors of the landscape, the soils are under greater hydromorphism conditions and, consequently, are more organic. Such factors reflect in the diversity of the vegetation of these palm swamp, which are much more robust intercropped with arboreal components. Regarding the palm swamp under the influence of greater seasonality, the soils are more sandy, with a decrease in clay, presenting horizons with simple grain structure and its vegetation complex associated with species adapted to water deficit. Thus, despite the palm swamp being located on the same source material, the modeled and climatic gradient proved to be an important environmental filter for edaphic and vegetational variables. The analyzed soils - Organosoil, Gleysoils and Neosoils - showed the domain of microstructure in blocks, pores in channels and cavities, with prevalence of porphyric c/f distribution. The redoxmorphic features observed in the thin sheets point to an alternation of humidity and gleization processes typical of a humid tropical environment.