Savannas in the Orinoco Llanos are found on a variety of highly leached substrates, from Tertiary sediments to alluvial soils, with markedly seasonal rainfall, 800-2500 mm/yr. Physiognomic types range from tree-less grasslands to woodland type communities. Dominant tree species are evergreen and sclerophyllous being favoured by low soil fertility and fire against deciduous, mesophyllous trees. Water appears not to be a limiting factor for established trees since leaf flushing and flowering take place in the dry season. Tree/grass ratios increase with soil water availability during the dry season. Areas with high water table, or in which a large fraction of the previous rainfall is accessible to tree roots, have higher tree densities. Tree recruitment is dependent on their capability to withstand grass-root competition during early growth stages, and to reach deeper soil layers to guarantee water availability during dry periods. Tree seedling establishment appears associated with flush germination during sequences of humid years. Duration of the season with plant available moisture (PAM) in any year determines the productivity of the herbaceous layer, particularly the perennial grasses. Establishment of perennial grass seedlings depends on the length of PAM and is closely related to the phenological patttern of the species. Duration of PAM regulates specific and phenological diversity of the grass layer. Regular occurrence of fire determines low diversity of the tree layer and may affect its productivity if it occurs after leaf flushing and the initiation of flowering. Fire also appears to maintain vigour of the herbaceous layer, its exclusion leading to deleterious changes. The interplay of dry, fire-prone years, and wet, fireless years probably determines short-term changes in the composition of the herbaceous layer. Grass productivity is enhanced by fire during the middle of the rainy season, when there is still some water left in the upper soil layers, and the rainy season begins before water reserves are exhausted. -from Authors
