Characters of three Udults and their relevance to the composition and structure of virgin rain forest of Amazonian Ecuador

Abstract

Three well-drained soil profiles from Añangu, Amazonian Ecuador, are described and their characteristics are discussed in relation to vegetation composition and structure within a small area of undisturbed terra firme rain forest. The soils are clayey, kaolinitic, udic, isohyperthermic, and rich in aluminium and are classified as Typic Hapludult and Typic Paleudults, with affinities to Inceptisols and Oxisols. The clay material of one profile includes expansible minerals which increase the CEC compared to that of the other profiles. The nutrient status is low and the clay content and aluminium saturation are high compared to other soils in tropical America. The properties most restrictive to plant growth are likely to be low soil pH, low concentrations of exchangeable calcium and extractable phosphorus, high aluminium saturation, high content of clay and high concentration of extractable manganese. However, these properties differ among the profiles. The three plots differ with regard to species composition and basal area of trunks. These differences correlate with soil concentration of aluminium, calcium, manganese, phosphorus, and clay. Many rare species occur on soils with a low concentration of calcium and high concentration of aluminium. A low basal area of trunks occurs on soil with high clay content and low phosphorus concentration. The conclusions about tree-soil relationships from Añangu are only tentative, but suggest that a high local species diversity in western Amazonia may be caused in part by local variations in soil characteristics.