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.
 
See also:
The importance of rooting strategies
soil conditions and 
effective precipitation