The nature of savanna heterogeneity in the Orinoco Basin

Abstract

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.

Vegetation–environment relationships and classification of the seasonal savannas in Venezuela

“…This limits plant growth in the later savanna types (Baruch and Go´mez, 1996). Similar overriding influence of nutrient availability on the composition and structure of tropical savannas has been reported in other studies from Venezuela and Colombia (Beard, 1953;San Jose´et al, 1998;Susach, 1989) and from Guyana, Surinam and Brazil (Goodland, 1966;Goodland and Pollard, 1973;Goldsmith, 1974;Lopes and Cox, 1977;Miranda et al, 2003;van Donselaar, 1965). In addition, it has been proposed that fertility edaphic gradients were the main cause for the vegetation gradual transformation from grassland or treeless savanna to the typical woody savanna (Goodland and Pollard, 1973;Lopes and Cox, 1977;Sarmiento and Monasterio, 1969).…”

“…It separated the highland Type I sites, with 2-4 rainless months, from the other savanna types with 5-6 dry months each. San Jose´et al (1998) found that the length of the dry season was an important variable in the analysis of Venezuelan and Colombian lowland savannas. Soil texture was correlated with site ordination and defines the capacity of soils to retain nutrient and water.…”

“…The structure and composition of several local Trachypogon savannas in Venezuela have been described previously, at variable levels of detail and with widely contrasting methods (Blydenstein, 1962;Huber, 1995;Ramia, 1967;San Jose´et al, 1998;Sarmiento and Monasterio, 1969;Susach, 1989;Vareschi, 1970;Vela´s-quez, 1965). This study differs from the previous by: (a) it covers all major regions with Trachypogon savannas, including those south of the Orinoco and, (b) the sampling and analysis were based on uniform floristic, numerical and standardized methods based on measurements of species cover and density.…”

Impacts of climate change on the vegetation of Africa: an adaptive dynamic vegetation modelling approach

“…For instance, should we assume that only the 15 × 10 6 km 2 of Africa that Grace et al (2006) classified as savanna are influenced by fire and that 50 years of fire suppression could, on average increase the total carbon in trees by circa 6 t ha −1 (Higgins et al , 2007a) we would expect fire suppression in Africa to allow an additional carbon storage of about 9 Pg which is less than simulated by the model. San José et al (1998a, b) and Grace et al (2006) suggest that fire suppression in savannas could increase carbon storage by almost 1 t ha −1 yr −1 which would translate into an increase of 67.5 PgC in 50 years for Africa. Scholes (2004) estimates that values between 0.3 and 0.6 t ha −1 yr −1 would be more appropriate for savannas, which would translate into an increase between 14 and 27 PgC in for Africa, assuming that biomass accumulation can only occur for 30 years.…”

Conundrums in mixed woody–herbaceous plant systems

“…, 1997; Hanan et al. , 1998; San José et al. , 1998; Abbadie et al , in prep.), but they emphasize different suites of variables and do not include the full range of diversity of structural types and bioclimatic conditions found along the grassland–forest continuum.…”