Interações entre floresta, chuva e solo em áreas com lençol freático superficial na Amazônia Central

Abstract

A central issue in ecology is the relationship between environmental gradients, species composition and forest structure. However, there is large environmental variability in Amazonia and these relationships have been little studied, especially in areas where the water table is shallow. I investigated the role of rainfall, water-table depth and soil physical properties on the variation in floristic composition, forest structure and biomass stocks in Central Amazonia. I evaluated if a new proxy for water table depth derived from remote sensing can be used as a predictor of floristic composition in different life forms (chapter 1), how forest structure components (stem density and the average individual mass) and biomass stocks are affected by rainfall gradients and soil properties (chapter 2), and if environmental gradients linked to soil water supply are limiting factors to canopy height (chapter 3). The first study was conducted in Reserva Ducke, north of Manaus, where the relief is dissected and the areas close to the streams have shallow water tables and sandy soils. The studies presented in chapters 2 and 3 were conducted in the Purus-Madeira interfluve, where the relief is relatively flat, the soil is predominately silty, and the water table is shallow even far from streams. Plant-species composition was strongly related to the proxy for water-table depth in the terra-firme forest north of Manaus, suggesting that water-table access differentially affects species establishment. Forests over shallow water tables had larger variation in species composition. This pattern may be associated with more dynamic forests in areas where the aerated soil volume for root development is limited by the shallow water table, anchorage is low due to sandy soils and individuals are more susceptible to death by uprooting. The areas where with most variation in species composition extend to hundreds of meters from the streams. Brazilian environmental law does not fully protect these areas of shallow water table because it considers only short horizontal distances from streams (30 m for small streams). Water table fluctuations are correlated with soil physical properties, such as the effective depth to which roots develop, and anoxic conditions. Shallower and more impeditive soils sustained forests with lower mean individual mass and higher stem density, supporting the hypothesis of a more dynamic forest over more restrictive soils. Shallow silty soils and superficial water table were also associated with lower canopy heights, suggesting that water excess and reduced space for root development limit forest growth. The positive effect of longer dry seasons on mean individual mass also indicates limitations to biomass accumulation related to water excess in areas of shallow water table. The results of the three studies suggest that forests over shallow water table and impeditive soils to root development are more rachitic in structure, and probably are more dynamic, with larger variation in species composition. Growth limitation by water excess seems to be an underappreciated mechanism affecting the structure and functioning of forests over shallow water table in central Amazonia, implying that current expectations of forest responses to droughts should be reconsidered.