Plants and soil moisture
The total amount of water that becomes available to the plants is strongly
affected by their rooting strategy,
effective precipitation and the prevailing
soil
conditions. The root-systems presented by Timberlake
& Calvert (1983) show, that plants develop different rooting habits,
depending on the clay contents of the soil; shallow roots are associated
with more clay.
Access to Water
Walker & Noi-Meir (1982)
identify two soil layers for the savannah ecosystem, from which the herbaceous
and woody woody plant components obtain their water. Knoop
(1982) and Scholes & Walker (1993),
however, consider this division inappropriate for the woodland system.
While Knoop (1982) found that most
grass roots colonized the upper soil, experimental removal of grass had
little influence on woody plant growth for the duration of her study.
The woody species have diverse rooting habits, giving them different
ability to obtain their water from various parts of the soil. This
in turn indicates that the findings of Walker
& Noi-Meir (1982) and Knoop
(1982) should not be seen as mutually exclusive. Knoop's study extended
over only one growing period. While she did not find changes in growth
of adult trees, it is not clear if she was able to identify seedling establishment.
In addition, Knoop concentrated on diameter growth, rather than on height
growth.
Temporal Differences in Water Absorption
Scholes & Walker (1993) propose
a temporal division of the herbaceous and woody components of the system
in terms of water accessibility. Many of the woodland trees produce
leaves before the first rains (Childes
1989), while perennial grasses break dormancy later. Annual grasses
require sufficient moisture in the upper soil for germination and seedling
establishment. While the herbaceous component is weak, woody plants
benefit from water filtering into their root zone. According
Tainton
(1999, p.69), root development is hampered by low temperature and
water availability, as also found at the begining of the wet season.
Perennial grass plants will therefore only become competitive once these
basic restrictions are lifted.
The temporal division is to some extent supported by the findings of
Mitloehner (1997) who studied
the water potential of a number of woodland plants and soils in the Caprivi,
Namibia. Mitloehner showed that different species had different ability
to absorb water from the soil, with some species able to obtain water (Baikiaea
plurijuga) when others had already lost their leaves (Pterocarpus
angolensis). This not only effectively lengthens the growth period
of the species, but also provides it with a competitive advantage.