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 rather difficult.

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 fire.

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.

Cohorts

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this stage little is known about detailed environmental requirements for regeneration such as those found by Joubert (unpublished data) for Acacia melifera in Namibia.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 by cohorts.

Species Requirements

The site requirements may be of significant importance for seedling establishment.
 
See also:
The species composition of the woodland areas
The effect of fire on woodland development
Seedling establishment