Ecology Of Woody Plant Communities In Flooded Savannas (llanos) Of Central Venezuela, And The Role Of Copernicia Tectorum (palmae).

Abstract

Although many savannas are found under dry climates, the site of this study has a mean annual rainfall of 1629 mm, with most rain occurring in a 6- to 8-month wet season. Because of impeded vertical drainage (montmorillonitic soils) and negligible slope, moderate flooding develops 4-6 weeks after the start of the wet season and lasts for 5-6 months/year. The soil is saturated and anaerobic for most of that time. Clumps of woody vegetation of various sizes are dispersed throughout the savanna. The author conducted a 22-month field study focused on the development of the woody plant communities. The flora is depauperate, containing only 53 woody species in the 8480 m sampled. Phytosociological analysis revealed 4 distinct noda and 2 others that were closely interrelated. Flooding, soil type, and physiography were shown by analysis of variance to be highly significant factors in relation to the nodal designations and the distribution of the vegetation they represent, with flooding having the greatest influence. Nutrient levels were uniformly low and not a differential factor. A palm, Copernicia tectorum, is a key successional species since it is the most frequent invader of the open grassland and is followed by shrub species. A study of its autecology was a means to better understand the woody communities as a whole. Experiments involving fire and flooding provided data on how these abiotic factors control demography and distribution for this palm. Studies of root anatomy and physiology showed that C. tectorum continues to respire and function during the long periods of soil saturation because (1) O$\sb 2$ is passively transported to the roots from above-ground plant parts, (2) some of this O$\sb 2$ leaks out of the apex and branch root junctions into the rhizophere, and (3) this patchy aerobic microhabitat supports vesicular-arbuscular mycorrhizae. This apparent alteration of the soil environment may account for the establishment of the shrub species in the vicinity of the palm.