Mineral Nutrition: Tropical Savannas

Abstract

Tropical savannas are ecosystems characterized by a continuous herbaceous cover of heliophilous bunch grasses and sedges, almost exclusively with the C4 photosynthetic pathway, that show a clear seasonality related to water stress (Sarmiento 1984; Cole 1986; Frost et al. 1986). Woody species (shrubs, trees, and palms) occur in varying density depending on soil depth and water availability (Cole 1986; Medina and Silva 1990). Legumes are commonly found in savanna vegetation as herbaceous, shrubby and tree species, and particularly in South American savannas, they can reach high specific diversity, although their biomass seldom surpasses 1% of the total aboveground biomass of the grass layer (Medina and Bilbao 1991). In contrast to the relative homogeneity in form and function of the grass layer, the woody components vary in their phenology and structure between Africa, Australia, and South America. While in Africa most woody components are drought deciduous (with the exception of a few species in western African savannas), most shrub and tree species in Australia and South America are evergreen, changing their leaves during the dry season. Large differences are also observable in the leaf structure. Most woody species in Australia and South America are scleromorphic due to the abundance of mechanical tissue and silica, while typical African tree species are mesophyllous in structure (Cole 1986).