Forest-soil relationships of the western Llanos of Venezuela

Vita.Major woody plant communities of the western Llanos of Guarico, Venezuela, were studied in 1975-77 to determine community structure, soil types, relationships of plant distribution to topographic, drainage and edaphic variables, and long-term vegetation trends. Within the 925,000 ha study area 18 stands were sampled using the quadrat method along a drainage gradient ranging from seasonally flooded lowlands to dry upland sites. Ten soil pits were opened to a depth of 2 m in the major soils and extensive physical and chemical analyses were conducted by soil horizon. Soil orders encountered were Entisols, Inceptisols, Vertisols, Mollisols and Oxisols. One hundred and thirty-nine species of woody plants representing 105 genera and 46 families were identified. Major families included Leguminosae, Bignoniaceae, Rubiaceae, Boraginaceae and Sapindaceae. Six vegetation types and 2 transition zones were defined. Ordination procedures and interspecific correlations tended to support the community distinctions; however, both interspecific and species-environmental correlations were rare. Within the extremely complex Llanos ecosystem, a major control of plants is exerted by Man and the physical environment. Community structure tended to be best expressed as a continuous series of interlocking species population curves with each species having its own response to the environmental gradient.