Amazonian caatinga and related communities at La Esmeralda, Venezuela: Forest structure, physiognomy and floristics, and control by soil factors

Tall and short Amazonian caatinga at La Esmeralda (rainfall 2600 mm yr(-1)) are described and compared with those at San Carlos de Rio Negro (3600 mm yr(-1)). The tall forests have much in common: thin-boled trees, many with scleromorphic mesophylls, lack of thick-stemmed climbers and of herbs, species paucity in all life-forms, domination by Eperua, and a high proportion of Bombacaceae, Clusiaceae and Euphorbiaceae. The short caatingas are also structurally and floristically similar but at La Esmeralda the notophyll scrubland species are lacking. At La Esmeralda, the common tree species are autochorous, but the majority of plants (particularly understorey species) are zoochorous. Relatively high transmission of light to the floor of the tall caatinga (1.5% PAR) may explain how trees with light-demanding architecture regenerate in the understorey. The presence of Cecropia and 'weedy' species in the caatinga only at San Carlos may be the result of greater human interference there. The soils at San Carlos and La Esmeralda are similar: thick humus overlying bleached sand, waterlogged most of the year. The water table at La Esmeralda drops to >1 m depth during the drier months, but tensiometers at 30 cm depth did not indicate significant water shortage. Soils are consistently wetter in the short caatinga than the tall. Shortage of nitrogen appears to determine the major structural and physiognomic features of the caatinga, and depth of aerated soil appears to determine its stature. Comparisons are made between tall caatinga, wallaba and swamp forest in the Guianas and heath forests of Malaysia.