TROPICAL GRASSLANDS (SAVANNAH)

Location;
       Mainly between latitudes 5 and 15 north and south of the Equator.
       Interior of continents, e.g. the Llanos (Venezuela), the Campos (Brazilian Highlands), most of Central Africa surrounding the Congo Basin, parts of Mexico and northern Australia.
Climate:
       Climatically determined savannahs (African) develop in regions with marked wet and dry seasons
       Rainfall ranges between 100 and 400 mm a year.
       Variation in vegetation according to rainfall;
a)   When the rainfall is 100-200 mm, generally only grasses can survive the dry season.
b)   When the rainfall reaches 300 mm the soil retains enough water to sustain shrubs through the dry season as well.
c)   When the rainfall exceeds 300 mm, enough water is left to support solitary trees.
d)   When rainfall exceeds 400 mm, enough moisture remains during the dry season to allow trees to grow and form a canopy, which shades out the grasses.
       In regions of higher rainfall (East Africa), savannah vegetation is maintained by periodic fires.
       Fires consume dry grass at the end of the wet season, burn back forest vegetation, check tree/shrub growth and stimulate new grass growth.
       Savannahs are also influenced by large grazing mammals, e.g. wildebeest/zebra.
       Animals reduce vegetation so that the grassy cover cannot carry a fire.
       Woody vegetation increases as a result, to be controlled by large browsers, e.g. elephants
       Temperatures remain high throughout the year, but there is a slightly cooler season when the sun in overhead the tropic in the opposite hemisphere.
       Annual range is also slightly greater (8C in Kano) due to the sun’s slightly reduced angle in the sky for part of the year, distance from the sea and less complete cloud and vegetation cover.
       Temperature may drop slightly at the start of the rainy season. Highest temperatures coincide with the summer rains.
       Cloud cover is limited for most of the year, allowing diurnal temperatures to exceed 25C
       Higher maximum temperature than tropical rainforests (30C), but lower minimum (12-14C).
       Rainfall in summer wet and winter dry season. Summer maximum created by convectional rainfall.

Causes of rainfall;
Wet season;
       Sun moves overhead in correspondence with the heat equator (ITCZ) and the equatorial low pressure belt.
       Heavy convectional storms can give 80% of the annual rainfall total in four or five months.
Dry season;
       ITCZ moves away, leaving the area with strong, steady “trade winds”.
       Dry because they are warming as they blow towards the equator and will have shed moisture on the distant eastern coasts.
       Places nearer to the desert margins tend to experience dry, stable (Mediterranean) conditions caused by the subtropical/Azores high caused by the migration of the descending limb of the Hadley cell.

Humidity is also low.
TROPICAL GRASSLAND VEGETATION

Grasses and deciduous trees dominate the savannah landscape, generally showing less prolific growth as latitude and distance from the sea increase. The rate at which vegetation grows is known as Net Primary Production (NPP), expressed in grams of dry organic matter per square metre per year. The NPP for arable land is 650g/m2/yr., and the tropical grasslands are estimated to have a mean NPP of 900g/m2/yr.

Vegetation Sub Zones;

1.   Savannah parkland; twisting, branching deciduous trees, e.g. baobobs, acacia, and eucalyptus, which grow fairly close to each other. Grasses grow beneath trees, adapted to seed and die back at the start of the dry season.
2.   Savannah grassland; trees are less dense, and often draw water from an underlying aquifer through a long root system. Grasses tend to be tall and coarse in areas of higher precipitation, more sparse in areas with less
recipitation.
3.   Savannah scrub; vegetation tends to be a discontinuous layer of bushes, e.g. sagebrush and thorns and sparse, dried up tussocks of grass. This zone has experienced the worst effects of advancing desertification.

Characteristics of Savannah Vegetation;
Trees;
       Deciduous - lose their leaves to reduce transpiration because of the winter drought.
       Xerophytic - drought resistant.
       Sclerophyllous - Leaves are small, waxy and thorn like.
       Roots are long and extended to reach deep underground.
       Tend to grow to 6-12m in height and contain “Y” shaped branches.
       Trunks are gnarled, and bark is thick to retain moisture.
       Example; The baobob tree has a trunk of up to 10m in diameter. Its root like branches hold only a minimum number of tiny leaves in order to restrict transpiration. Some are estimated to be thousands of years old. Their trunks are resistant to the local fires.
Shrubs/Grasses;
       Grasses grow in tufts and tend to have inward curving blades and silvery spikes.
       After the onset of the summer rains, they grow very quickly to over 3m in height. Elephant grass grows to 5m.
       They become yellow by sun drying.
       By early winter, the straw like grass has died down, leaving seeds dormant on the surface until next year’s rains.

In terms of biotic activity, over 40 different species of large herbivore graze on the grasslands, including wildebeest, zebra and antelope. It is also home to several carnivores - predatory, e.g. lions/leopards/cheetah or scavengers, e.g. hyenas. Termites and microbes are the major saprophytes. Fire is the main determinant of the savannah biome.

As more trees are removed for fuel, and over grazing reduced the productivity of grasslands, the heavy rains gullies and the wind blows away the surface soil. Where the savannah is not farmed, there are usually more trees, suggesting that grass may not be the natural climactic climax vegetation.