CHARACTERS OF 3 UDULTS AND THEIR RELEVANCE TO THE COMPOSITION AND STRUCTURE OF VIRGIN RAIN-FOREST OF AMAZONIAN ECUADOR

Three well-drained soil profiles from Anangu, Amazonian Ecuador, are d escribed and their characteristics are discussed in relation to vegeta tion composition and structure within a small area of undisturbed terr a firme rain forest. The soils are clayey, kaolinitic, udic, isohypert hermic, and rich in aluminium and are classified as Typic Hapludult an d Typic Paleudults, with affinities to Inceptisols and Oxisols. The cl ay material of one profile includes expansible minerals which increase the CEC compared to that of the other profiles. The nutrient status i s 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 exch angeable 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. T hese differences correlate with soil concentration of aluminium, calci um, manganese, phosphorus, and clay. Many rare species occur on soils with a low concentration of calcium and high concentration of aluminiu m. A low basal area of trunks occurs on soil with high clay content an d low phosphorus concentration. The conclusions about tree-soil relati onships from Anangu are only tentative, but suggest that a high local species diversity in western Amazonia may be caused in part by local v ariations in soil characteristics.