Environmental Engineering Reference
In-Depth Information
Many savanna trees are xerophytes . Their morphological and physiological resistance to
water loss, and their ability to maximize the uptake of water, allow them to survive dry
periods. They also have deep roots and flattened crowns. Some shed their leaves during
the dry season in order to reduce transpiration. Savanna trees are often stunted, and may
be overtopped by the tall grasses of this biome. Browsing by the savanna animals is a
major constraint on tree growth and survival, and overgrazing is one of the main causes
of savanna degradation.
Herbaceous savanna plants are dominated by a few species. African elephant grass is
sometimes abundant, and may reach heights of several metres. The density of trees
relative to grass is, to some extent, climatically controlled, and trees become scarcer in
the drier margins of the savanna. However, there are often subtle local variations in
vegetation related through topography and drainage to moisture availability (Figure 27.4),
particularly in the drier areas, where vegetation is attuned to the short growing season,
and herb-layer plants grow rapidly once the rains come. Although the soils are relatively
dry before the beginning of the wet season, there is no need for the rain to replenish soil
moisture before plants can extract
Figure 27.4 Environmental
relationships in a savanna area.
Source: After Collinson (1977).
the water efficiently. Instead the plants transpire at their full rate immediately, as much of
the rainfall seems to be absorbed by the plants before it can move into the finer pore
spaces in the soil.
The main determinants of vegetation in the savanna are probably related, first, to
moisture availability and, secondly, to nutrient supply. Fire and grazing are seen as
PLANT TYPES
new developments
Most savanna grass species belong to the C 4 group of photosynthetic plants in contrast to
the C 3 group, which dominate in temperate latitudes. In the C 4 group the rate of
photosynthesis increases with the increase in intensity of solar radiation rather than
reaching a plateau of CO
uptake
as happens in most C
species
This feature makes
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