Detrended Canonical Correspondence Analysis (DCCA) was used to describe the relationship between savanna environments and vegetation community composition in the Orinoco Basin. TWINSPAN derived three major types of savanna vegetation, each with its own floristic and physiognomic features. The first group reflected a plant association with species of Trachypogon occurring in moderately infertile soils with low water availability and high bulk density. This group is distributed mostly on hilltops and dissected plains of the northern Orinoco savannas. The second group has the highest species diversity in habitats occurring in the northern Orinoco with extremely infertile soils. The third group, on the other hand, has a considerable number of species belonging to the Leguminosae and Cyperaceae families and linked to the habitats of the eastern Colombian savannas with the highest soil water availability, a shorter dry season and higher precipitation. The regional analysis of the Orinoco savannas suggests edaphic controls to be important. Changes in the underlying geology have affected topography and soil formation, which results in variation in water and nutrient status. The moisture regime and hydrological features are acting as the major regional determinants, while nutrient levels and distinctive surface soil properties provide the subregional determinants. The local boundary, at any given site, is strongly determined by nutrient shortage and acidity. Also, biogeographic and floristic considerations were taken into account to explain differences in species composition.
Aluminium concentration was analyzed in roots, stems and leaves of species growing in morichals with extreme flooded acid soils, deficient in K, Ca, Mg and P. Mean aluminium saturation was 73%. Results seem to indicate that species depending on seasonal changes in the A1 concentration of plant compartments, could be grouped into "perennial aluminium accumulators", "late aluminium accumulators" and "threshold aluminium accumulators".
We examined the carbon dynamic across the inland water-land palm ecotones (morichals) of the Orinoco lowlands to increase comprehension of their potential for accumulating global carbon. Ecotone biomass was quantified for ecotones representing the flat-floored valley type with sparse woody vegetation and the V-shaped valley type with closed woody vegetation. The soil organic carbon (SOC) was linearly ordinated in a double logarithmic plot by the rank-ordering technique. The plot showed that there was an inflection point corresponding to the two sets of ecotones in flat-floored and V-shaped valley type. On average, a smaller proportion of the SOC in the ecotones was stored in the V-shaped than in the flat-floored valley type. As a result, the fractal dimension for the two ecotone groups was 1.27 and 2.67, respectively. The seasonal trend of aboveand below-ground phytomass in the flat-floored and V-shaped valley type showed a temporal increase.However, the seasonal increment was only significant for the flat floored valley during the second annual course. The net primary productivity (NPP) of ecotone in flood-prone and stream bed areas in the flat-floored valley type was 340-837 and 2,438 g C m -2 year -1 , respectively. The NPP value for the stream bed in the flat-floored valley type exceeds the NPP of those reported for a wide range of ecotones. Depending on the annual hydrological dynamic, the ecotone of the flood-prone area ranged from carbon steady-state store-house to carbon sink.
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.
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