The Herbaceous Plant Layer
When refering to the herbaceous layer in the dry savannah woodlands, most,
if not all, authors mean the grass component. While other herbaceous
plants occur, these have been neglected significantly in the literature.
The effect of rainfall on the development
of the herbaceous layer
While woody plants may flower and produce leaves before the onset of the
first rains, Knoop (1982) reports that
grasses only flush after a sufficient amount of rain has fallen (unless
an out-of-season flush has been stimulated by
fire).
The effect of fire on the development of the
herbaceous plant layer
Direct effects of fire
The direct effects of fire may have both positive and negative influence
on vegetation development. On one hand, fire will remove moribund
material that would have caused the plants to shade
themselves (Savory & Butterfield
1999).
On the other hand, however, grass plants may be stimulated to flush
early in the dry season. If the grasses flush too early in the dry
season, they will not be able to sustain themselves to the begining of
the next rains. This causes them to use up root reserves that would
otherwise have been used for initial growth at the begining of the rains.
Since the root reserves of grasses that flush out of season are easily
depleted, they are more suceptible to overgrazing than grasses flushing
at the begining of the wet season.
Indirect effects of fire
The indirect effects of fire primarily manifest themselves through their
effect on the woody vegetation. As the fire regime favours or opposes
the development of the woody vegetation, the herbaceous plants are disadvantaged
or advantaged by fire. When the woody vegetation is able to develoop,
there is increasing interception of the incoming light causing grass plants
to be shaded.
Light availability and the development of the
herbaceous layer
Plants need light for growth and development. The amount of light each
plant / plant species requires varies. Lawton
(1978) and Skarpe (1991) have shown,
for instance, that many grasses grow more slowly if they are shaded by
trees. Others, like Panicum maximum grow well when shaded
(Müller 1985).
Self-shading by grasses
Savory & Butterfield (1999, p.32
) indicate that grasses may shade themselves if the moribund above-ground
biomass remains. Grasses have their growth points close to the ground
(Tainton, 1999), an addaptation that
prevents these to be damaged by grazing animals. Standing, moribund material
will reduce the amount of light that will reach the growth points and young
leaves, causing the grasses to grow less vigorously. On the other
hand, if old growth had been removed by grazing or fire, perennial grasses
will be able to sprout more vigorously at the begining of the next growing
season.