Savanna Biome

The savanna biome represents the complex of vegetation made up of grassland with trees in varying densities and arrangements that occupies the transition zone between forests and grasslands. Whereas the lay observer could usually identify a forest or grassland, the savanna biome would provide a challenge, greatly influenced by the scale of observation, since it is characterized by high variability in density, arrangement, clumping, and structure of grassland and trees. The large-scale savannas of the world are quite different on the major continents of Africa, South America, and Australia, and they are distributed in smaller, highly variable arrangements and formations in North America and Eurasia. The nature and stability of the savanna biome has received increasing attention because of its perceived dependence on disturbance by fire and herbivory to maintain tree-grass balance and because some savannas are biodiversity hotspots. Evolution of savannas is thought to be associated with a lower CO2 world where tropical grasses gain advantage from highly efficient photosynthetic systems and fire and grazing control woody encroachment. As a result, there is great interest in the effect of higher atmospheric concentrations of CO2 on the evolution of the tree-grass balance in the savanna biome. The explorer botanists of the early twentieth century paid significant attention to the neotropical and peri-Amazonian savannas of South America with their extraordinary biodiversity. In the 1980s, the West African savannas became the terrestrial focus of the genesis of remote sensing of land systems and the development of the Normalized Difference Vegetation Index (NDVI) as a global monitoring tool. During the early to mid-twentieth century, many temperate savannas were heavily converted to agriculture in the New World, and a similar trend is now continuing and potentially accelerating in the tropical savannas of South America and Africa. The Australian tropical savanna has remained largely intact as it is generally too arid for agricultural conversion. As a result, it has become increasingly important to conduct ecological and process studies on tree-grass ecosystem function across spatial scales. Tropical tall grass-tree systems in Asia tend to have been extirpated by dense human activity but have also been treated differently in vegetation classifications and so do not clearly appear in global land cover maps. There has been limited attention paid to these systems in the literature. With global population and food demand potentially ballooning in the twenty-first century, accelerated conversion of savannas is likely to intensify both concerns about decline in ecosystem function and competition for ecosystem services that will necessitate a significant expansion in integrated, interdisciplinary research, sophisticated modeling and future scenario development, and research on restoration ecology and amelioration of land degradation. This article maintains a balance between historical articles and books that address biogeography, landscape ecology, and ecophysiology in savannas in traditional ways less commonly applied now and newer articles and books that reflect expanded understanding of climate, atmosphere, anthropogenic disturbance, and theories about factors that control the grassland-savanna-forest ecotone. These newer studies also reflect rapid expansion in capability to handle very large quantities of spatially explicit data and apply these data to modeling across scales.