Patterns of Specific and Phenological Diversity in the Grass Community of the Venezuelan Tropical Savannas

The coexistence of several species of perennial grasses seems to be a definite trait of most tropical savannas. To analyse this feature, trying to relate this co-occurrence to environmental and ecosystem characteristics, two samplings were designed, one involving the whole savanna area in Venezuela, the other restricted to a climatically more homogeneous area within the Venezuelan western ilanos. Through the use of indexes of specific and phenological diversity, it is shown that all Venezuelan seasonal savannas have a combination of perennial grasses showing relatively medium evenness values. This specific diversity was partly explained by the phenological diversification of the grass community that leads to a temporal division of the niche. An analysis of phenological diversity and the patterns of occurrence of each phenological group in terms of climatic and soil factors, leads to the conclusion that each group represents a definite species guild more or less favoured by distinctive habitat conditions. Moreover, the unpredictability of some environmental constraints, like the extension of the rainy season when soil water is available to grasses, as well as the precise annual timing of fires, seems to have contributed to the maintenance of both high specific and phenological diversities in the grass community of this type of tropical ecosystem.