Plant Population Structure
The structure of a plant population significantly affects the development
of the population in the future. Such a population structure should
not only viewed in terms of the age / size structure of a single species,
but should needs to be consiederd also in terms of species composition.
Age specific mortality, or fecundity rates will determine how fast a population
may increase or decrease under given environmental circumstances.
Some trees in the woodlands are difficult to age, either because of
a limited age / size relationship (van
Daalen et al 1992), or simply because no dendrological studies
have been conducted. This makes the study of population structures
Size / age specific mortality
Size or age specific mortality may result in groups of plants that are
of similar age. In the savannah woodlands, this is often caused by
Mortality specifically of the younger / smaller classes may therefore
result in the formation of a woodland with few young / smaller trees.
A predominance of older plants could result in a drastic decline in woody
cover when the plants reach their maximum lifespan. Size specific
harvesting or senescence would effectively be a clearfelling of the area.
Size / age class specific reproduction
Size or age specific reproduction has also been observed for savannah woodland
species. Schinziophyton rautanenii for instance requires between
15-25 years to reach maturity (Peters
1987) and before it will bear fruit (Lee,
1973). Pterocarpus angolensis only produces fruit after 20
years (Vermeulen 1990).
Stands that are comprised predominently of younger trees due to size
class specific mortality may then remain even aged unless seed is brought
in from other areas.
The recruitment of woody seedlings is not necessarily an annual occurrence.
At this stage little is known about detailed environmental requirements
for regeneration such as those found by Joubert (unpublished data) for
melifera in Namibia. Vermeulen
(1990) suggests that the water requirements of Pterocarpus angolensis
cause the species to produce a step-wise population structure as is produced
The site requirements may be of significant importance for seedling establishment.