Plant distribution patterns in the Australian Botanic Garden, Mount Annan woodland suggest that competition for light and water are important factors.
Competition for light - a place in the sun
Our observations in the woodland indicate that large established plants tend to crowd out smaller plants, particularly seedlings. As denser tree and shrub canopies (denser upper strata) develop, groundcover and lower strata composition may be altered. Where groundcover species are unable to cope with lower light conditions, there may be a change in species composition over time. Hypericum gramineum for example may disappear as canopy cover increases. Even in the groundlayer the growth of dense tussock grasses such as Themeda triandra may inhibit the establishment of other ground species.
However, this is not necessarily permanent as soil seedbanks of light-requiring species may remain, until impacting events such as periodic fire or drought remove competing canopies, albeit temporarily.
One species that has a significant effect on groundcover species is the exotic shrub African Olive *Olea europaea subsp. cuspidata which is able to tolerate a range of light conditions but forms a dense canopy that restricts the growth of native species (this may be in combination with root competition – we do not know). It is able to invade relatively undisturbed native woodland and form dense self-maintained thickets unless it is removed or poisoned (e.g. during bush regeneration).
Competition for water
The long-term competition for water is evidenced in the development of the local habitat and microhabitat variation that is seen in the woodland. For example particular species, such as the ferns Pellaea falcata and Asplenium flabellifolium, are restricted to slightly moister or more sheltered sites in the dry watercourse. Adjacent sheltered gully slopes may have Ranunculus lappaceus, Scutellaria humilis or Rhodanthe anthemoides.
Adults of most ground layer species are able to survive during drought as water availability declines, either by dying down to rootstocks completely (e.g. Mentha satureoides, Asperula conferta) or by allowing their leaves to turn brown (e.g. Themeda triandra). Shrubs and trees generally survive intact.
Ephemerals take advantage of rain following drought, growing quickly and maturing in the open areas before the perennial canopies are restored, though competition here is probably primarily for light rather than for moisture.
Asterisk * indicates exotic species naturalised at the Australian Botanic Garden.