PARENT SESSION
Poster Session #15: Vegetation Analysis.
Tuesday, August 7, 2001. Presentation from 10:30 AM to 12:00 PM. Exhibition Hall

Soil and vegetation changes along savanna-ecotone gradient in the Gran Sabana, Venezuela.

OSIO, ANAIS¹, BILBAO, BIBIANA¹, ¹

ABSTRACT- The Gran Sabana, Venezuela, is a mosaic of patches of tropical moist forests, shrubs and grass savannas, with a high occurrence of fire in the region. Most of these forest patches, located in extensive areas of savannas, are surrounded by belts of ferns communities ("helechal") dominated by Pteridium sp.. Three herbaceous savanna-helechal communities were studied to determine the change in species composition and their abundance, functional groups and soil characteristics along the gradient. Vegetation analysis and soil samples were taken in 41 quadrats located along three transects from savannas to "helechales" (100 m). The results indicate the existence of a physiological gradient characterized by a strong dominance of type C4 plants in the savanna that changes gradually to dominance type C3 plants in the fern communities (ferns and seedlings of woody species). According to the Analysis of Canonical Correspondence (the first two axes explain the 84% of the variance), the soil characteristics (pH, CEC, % Al saturation, organic matter, total N and P) determine the distribution of the 74 species found in the gradient. The higher soil nutrient availability, the great heterogeneity and the spatial arrangement of ferns that retain environmental humidity, produce an ideal microhabitat for the establishment of seedlings of forest species in the "helechal". These ecotone areas play an important role in the successional dynamics in gradients, acting as possible sites of forest regeneration.

KEY WORDS: Gradient, Ecotone, Fern, Savanna