Definition and Occurrence

Savanna ecosystems predominantly occur in the seasonal tropics and are a unique mix of coexisting trees, shrubs, and grasses (Figure 1). Debate surrounds the use and definition of the term savanna, reflecting the range of tree:grass ratios found in these ecosystems. Savanna ecosystems feature a range of structures, from near treeless grasslands to woody dominant open-forest/woodlands of up to 80% woody cover. A widely used definition describes a savanna ecosystem as one consisting of a continuous or near continuous C4 grass dominated understorey, with a discontinuous woody overstorey. Woody components can be a mix of trees and shrubs of evergreen or deciduous phenology, broad or needle leafed. The grass-dominated understory can consist of a mix of species with either annual or perennial habit (often >1min height). Ecosystems that fit this definition have ambiguously been termed woodlands, rangelands, grasslands, wooded grasslands, shrublands, open-forests, or parklands.

Figure 1 Savanna ecosystems of the world, featuring the coexistence of a discontinuous woody overstorey with a continuous herbaceous understorey. Plates (a) and (b) are of a north Australian savanna site that receives approximately 1100 mm rainfall and is dominated by evergreen trees (Eucalyptus sp.) and tall C4 tropical grasses (Sarga spp.). Canopy fullness and grass growth are significantly differently in the wet (a) and dry (b) seasons. Tower-mounted instrumentation in plate (a) is monitoring ecosystem productivity and water use over wet and dry seasons. Plate (c) African savanna of the Kalahari Gemsbok National Park, Botswana. (a, b) Photo courtesy of Joerg Melzheimer.

Figure 1 Savanna ecosystems of the world, featuring the coexistence of a discontinuous woody overstorey with a continuous herbaceous understorey. Plates (a) and (b) are of a north Australian savanna site that receives approximately 1100 mm rainfall and is dominated by evergreen trees (Eucalyptus sp.) and tall C4 tropical grasses (Sarga spp.). Canopy fullness and grass growth are significantly differently in the wet (a) and dry (b) seasons. Tower-mounted instrumentation in plate (a) is monitoring ecosystem productivity and water use over wet and dry seasons. Plate (c) African savanna of the Kalahari Gemsbok National Park, Botswana. (a, b) Photo courtesy of Joerg Melzheimer.

Figure 2 The distribution of the world's savannas. Temperature and monthly rainfall data for a range of savannas are also given, with highly seasonal rainfall clearly evident.

Figure 2 The distribution of the world's savannas. Temperature and monthly rainfall data for a range of savannas are also given, with highly seasonal rainfall clearly evident.

Savanna formations occur on all continents of the world (Figure 2), with the largest extent found in the wet-dry tropical regions of Africa, South America, and Australia. Smaller areas occur in Asia, including Sri Lanka, Thailand, Vietnam, and Papua New Guinea. Savanna also occurs in India, although these tree and grass systems tend to be derived from dry deciduous forest and subhumid deciduous forest due to land-use changes and population pressure. Tropical savanna occupies an area of approximately 27.6 million km2 including the Asian savanna regions. Tree-grass mixtures also occur in temperate regions, in North America (Florida, Texas), Mediterranean Europe, and Russia, although these temperate savannas are far smaller in extent at approximately 5 million km2. In total, the savanna biome occupies one-fifth of the global land area and supports a large and growing population.

The existence of a dry season is a defining feature of savannas; rainfall is seasonal and ranges from 300 to 2000 mm, with a dry season lasting between 2 and 9 months of the year. There can be a single, extended dry season or several shorter dry periods. Inter-annual variation of rainfall is typically high, as is the commencement and cessation of the wet season and growing season length, making cropping in savanna lands difficult. Indeed, historical rainfall plays an important role in determining the vegetation structure of a savanna. Seasonally available moisture dramatically influences plant productivity, which in turn determines the timing of available resources for savanna animals.

Given their wide biogeographic range, savannas occur on a number of soils types, typically oxisols, ultisols, entisols, and alfisols (using US soil taxonomy). In general, these soils are ancient and highly weathered, low in organic matter and cation exchange capacity (CEC). Oxisols occur in tropical savanna regions of South America and central and eastern African savanna and consist of highly weathered, transported, and deposited material occurring on fluvial terraces. Extensive weathering of primary minerals has occurred and they are dominated by clay minerals such as kaolinite and gibbsite which have low CEC. Also present in the soil are acidic Fe and Al sesqui-oxides, which limits nutrient availability, especially phosphorus. Savanna soils tend to be sands to sandy loams, deep and well drained but with low soil moisture-holding capacity. Entisols that occur in Australian savanna also feature the occurrence of ferruginous gravels, further reducing water- and nutrient-holding capacity. Bioturbation by earthworms and termites are critical in the cycling of nutrients through the poor soil systems. Termites essentially act as primary consumers and in savannas that lack a significant herbivore biomass (e.g., Australian and some South American savannas), they have an ecological function similar to that of herbivorous mammals.

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