Relaciones suelo-vegetación en una toposecuencia del Escudo Guayanés, Venezuela

The Guayana Shield is one of the oldest territories in the planet and has millions of years subjected to leaching and erosion processes that produce oligotrophic soils. This scarcity of nutrients has not avoided the development of high, diverse and magnificent forests, where plants have adaptations to survive these difficult conditions. The aim of this work was to characterize the physical and chemical parameters of the soils in different forest communities of the Imataca Forest Reserve (IFR), to establish some interpretations on their origin, and some relationships between the edaphic components and the inhabitant species. Terrestrial journeys were carried out and two pathways were traced for the interpretation of forest communities. In each community, three plots (100m x 100m) for the ecological survey were established. In each plot, all trees, palms, and lianas were measured (dbh≥10cm; dbh: diameter at breast height); four 10m x 10m understory sub-plots were evaluated, and a census of all the spermatophyta species (dbh<10cm) was made. Soils were evaluated by means of a trial pit in each community, and a bore in each one understory sub-plot. The obtained data were analyzed by both a detrended and a canonical correspondence analysis. Results from a total of 15 plots allowed us to identify three communities: a forest dominated by Mora excelsa located in a narrow valley with high content of silt; a swamp forest dominated by Catostemma commune in a sandy broad valley, and a forest in clay hills dominated by Alexa imperatricis. Most of the soils had a pH between 3.1 and 4.5. Nutrients were in very low concentrations (N: 0-0.2%; P: 0-14ppm; K: 0-0.3Cmol/kg) and Al in toxicity quantities. We observed that Ca/Al relationship greater than 1 meant a neutralization of the toxic effect of Al. Besides, basal area ranged from 20.4 to 32.3m2/ha; the highest level (top local forest development) corresponded with the C. commune community with a Ca/Al value of 2.5. This research found an adequate relationship between soil and vegetation, as it was able to identify three communities into three distinct soil conditions. Nevertheless, edaphic constraints indicated that high forest cover must be maintained as an essential element for the conservation of these communities.El Escudo Guayanés es uno de los territorios más antiguos del planeta; tiene millones de años sometido a procesos de lixiviación y erosión que dejan suelos oligotróficos. El objetivo de este trabajo fue hacer la caracterización de los parámetros físicos y químicos de los suelos, en diferentes comunidades boscosas de la Reserva Forestal Imataca, e identificar ciertas relaciones entre los componentes edáficos y las especies que allí se presentan. Se realizó un inventario de todos los individuos mayores a 10cm dap (diámetro a la altura del pecho) en 15 parcelas (1ha); en cada una de esas parcelas se evaluaron cuatro sub-parcelas de sotobosque (100m2) para contabilizar todas las espermatofitas; los suelos se evaluaron mediante una calicata por comunidad y un barreno en cada sub-parcela de sotobosque. La mayoría de los suelos tienen un pH muy ácido (3.1-4.5), los nutrientes están en concentraciones muy bajas (N: 0-0.2%; P: 0-14ppm; K: 0-0.3Cmol/kg) y el Al en concentraciones tóxicas. Esto coincide con otros estudios realizados en esta región, pero en este trabajo se encontró que la relación Ca/Al con valores superiores a 1, lo que implica una neutralización del efecto tóxico del Al y permite un mejor desarrollo de las comunidades boscosas