Heterogeneity of the inland water-land palm ecotones (morichals) in the Orinoco lowlands, South America

We analyzed the distributional pattern of species and environmental gradients across inland water-land palm ecotones (morichals) of the Orinoco lowlands to increase comprehension of the nature of ecotone heterogeneity. A total of 91 species (53 genera and 45 families) with aboveground phytomass > 0.1 g dry mass were recorded. Detrented canonical correspondence analysis (DCCA) indicated that vegetation phytomass was related strongly to soil properties, including gradients of silt (14.5-4.7%), exchangeable Al (1.05-3.10 cmole kg⁻¹), K (0.03-0.30 cmole kg⁻¹), Na (0.01-0.08 cmole kg⁻¹), Mg (0.03-0.54 cmole kg⁻¹) concentrations, pH (3.7-5.0 units), and soil organic matter. Cluster analysis allowed the definition of four types of ecotones on the basis of hydrogeomorphic processes. The first major group (1) encompassing the sites from Venezuelan lowlands (i.e., 3V, 4V, 5V, 6V, and 7V) was related to less acidic soils with high organic matter content. The second group (II) from Eastern Colombian llanos (i.e., sites 8C, 9C, 11C, 12C, 13C, 14C, and 15C) was located in acidic soils (3.9-4.5 units) with high Mg concentration. The third group (III) (i.e., sites IV and 10C) was located in soils with high Na content, whereas the fourth group (IV) (i.e., site 2V) was characterized by species growing in soils with low exchangeable aluminum. The results evidenced the interactive role of valley constraint, landforms, hydrological regime, and soil feature in structuring the plant community. Biogeographic and floristic considerations were also taken into account to explain differences in species composition.