Environmental Engineering Reference
In-Depth Information
et al., 2001; Stanhill and Cohen, 2001; Roderick and Farquhar, 2002).
Lower surface solar radiation reduces the energy available for evaporation
leading to reduced rainfall and disruption of tropical circulation patterns
(Ramanathan et al., 2001).
The climate of semi-arid regions and its variability has a strong inl u-
ence on the vegetation found in semi-arid regions. Vegetation distribu-
tion, diversity and productivity are all inl uenced in the i rst order by
climate, but also by soils, competition, i re and grazing (O'Brien, 1993;
HilleRisLambers et al., 2001). Two large phytogeographical regions are
recognized to cover most of the semi-arid region of Tanzania, the Somalia-
Maasai region and the Zambezian region as described by White (1983)
(see Figure 9.1). The Somalia-Maasai region is the driest vegetation type
found predominantly in north and central Tanzania and is comprised of
a range of grass-dominated savanna habitats. Savannas vary from com-
pletely grass-dominated grass savanna through bushland, thicket and
shrubland, to tree savannas and wooded savannas (Jacobs et al., 1999).
Trees and shrubs can comprise up to 50 per cent of species composition
but the canopy remains open (Trapnell and Langdale-Brown, 1972).
Included within the Somalia-Maasai region are the edaphic savanna
grasslands found on the Serengeti plains (see Figure 9.1). These grasslands
occur on soils derived from volcanic ash, which tend to be alkaline and
often have a hard calcareous pan created by the leaching of carbonates
through the soil by rainfall (White, 1983). The calcareous pan creates a
barrier to root growth, preventing the growth of woody vegetation, and
is also impermeable to water so that these areas can be prone to l ooding
during the rainy season. Savannas are believed to have existed in East
Africa for 20 million years and were, in part, created by the development
of seasonal rainfall patterns that restricted the distribution of tropical
rainforests (Pratt et al., 1966). Today a mixture of climate, soils, natural
i res, grazing and anthropogenic inl uences maintain the open nature of
savanna ecosystems.
The Zambezian region consists of miombo woodland dominated by
trees from the genus
Brachystegia
(Kingdon, 1971; White, 1983; Lovett
et al., 2001). The region forms a crescent in the south of Tanzania around
the more arid Somalia-Maasai region (see Figure 9.1). It is less recognized
as a semi-arid ecosystem even though it can occur in areas with rainfall
as low as 500mm per year. Drier miombo woodland tends to be less
diverse while wetter miombo woodland vegetation has to be adapted to
periods of l ooding and waterlogging followed by long dry seasons with
little available water and frequent i res. It has been argued that in some
areas miombo forests may be the result of long-term human disturbance
(Hamilton, 1982; White, 1983). Certainly, in areas with deeper soils,
