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Soils and Vegetation:
The soils of this savanna are porous, resulting in rapid drainage of water. Volcanic rock is often filled with spaces as a result of gases that were trapped in the molten lava as it was formed. When broken down soil texture is light. Unlike the grasslands of the prairie, little humus exists (organic portion of the soil created by partial decomposition of plant or animal matter),as soil nutrients and moisture imitations as well as heavy herbivory prevent much organic material to reenter the soil. This lack of humus which normally increases moisture holding capacity reinforces soil porosity to create still drier soils during the dry period.
Although some mark savannas as forests given they do support trees unlike the prairies, the low density of woody species would make deciduous or tropical forest ecologist cringe at that classification. As expected the dominant vegetation consists of grasses and forbs (small broad-leaved plants that grow with grasses). Drier savannas relative to those with higher precipitation sustain different grasses: dry Serengeti soils encourage Rhodes grass and red oat grass. Just as on the drier portions of the prairies, bunch grasses with little vertical stratification dominate These grasses as the title suggests form clumps unlike the continuous sod grasses we see on our lawns. Trees, like the acacia are usually thorned at least on the outer branches to inhibit herbivory though some of the herbivores like giraffes appear to have tongues which feel no pain. They are broader rather than taller, no more than 8 meters in height, as competition for light is nonexistent. As one might guess, root systems are wide ranging: both grasses & trees have either deep tap roots or horizontally extending lateral root systems. As in the deserts, even spacing of trees occurs with high competition for moisture and perhaps nutrients. Large herbivores seek the shade of these trees and 'reciprocate' by leaving a load of nutrients in the form of urine & dung before leaving. This humic addition also increases water holding capacity of the soil. Old termite mounds also help support tree growth in areas where this would not be possible otherwise. Accretion of organic materials by the termites increases local nutrient and moisture load; this results in clumps of trees in an otherwise open parklike area. Frequent fires and large grazing mammals kill the seedlings, keeping trees and shrubs density low. |
