The most abundant plant species form the three dimensional structure of a plant community. Typical plant communities range from forests and woodlands dominated by trees, to heaths dominated by shrubs, and grasslands dominated by grasses. Woodlands are communities characterised by an open or sparse layer of trees. Woodlands may be further defined by the nature of their shrub and groundcovers.
Our woodland at the Australian Botanic Garden, Mount Annan, a typical example of Cumberland Plain Woodland, has three vegetation layers or strata - a tree layer, a shrub layer and a groundcover layer.
The tree canopy layer
The tree stratum at the Australian Botanic Garden is dominated by three species of tree, the Grey Box Eucalyptus moluccana, the Forest Red Gum Eucalyptus tereticornis and the Narrow-leaved Ironbark Eucalyptus crebra. They are readily recognised by their different types of bark. Some of the trees are 20 to 25 m high with trunks up to 1 m in diameter. These trees may pehaps be several centuries old. There are also areas with smaller trees and patches of saplings which have grown up in the last 20 years.
The shrub layer
Beneath the trees may be a layer of shrubs sometimes up to 5 m high but mostly lower. The main plant species is the prickly shrub Blackthorn Bursaria spinosa. In many places the shrub layer is patchy or there is no shrub layer. This patchiness is important in small remnants such as our Mount Annan woodland and adds to the diversity of microhabitats there. Although we have 13 native shrub species listed in the table below most are rare in our woodland. For example there are only three plants of the interesting Native Cherry Exocarpus cupressiformis.
The groundcover layer
Below the shrub layer, and probably unnoticed by many people, is the ground layer. This layer has many different plant species and is the richest part of the woodland in terms of the number of different species.
In Cumberland Plain Woodland, most of the native plant biodiversity, often over 80% of the plant species on the site, is in the ground layer.
To see our research results as we have monitored woodland changes see Monitoring changes in the woodland 1988-2005.
Functional groups
In order to deal with the large number of plant species that may be present in a plant community, the species may be grouped into functional groups, that is, groups of species that may have similar functions or similar ecological behaviour. Initially we have divided our Australian Botanic Garden woodland flora into broad functional groups based on growth form, i.e. trees, shrubs, vines, groundlayer plants, with further subdivision of groundlayer plants by the main families, Asteraceae, Fabaceae and Poaceae. Other groupings that we use, which may cut across these groups, are native species versus exotics, or species resprouting after fire versus killed by fire. We may also use rare species versus common species. Functional groups depend on the circumstances and the ecological issues being considered.
Relative abundance of native plant species within our Australian Botanic Garden woodland
A few of the species, about 7%, in our Mount Annan woodland are widespread and common within the woodland, e.g. the shrub Bursaria spinosa, and herbs Brunoniella australis and Dichondra repens. However, 75% of the woodland species are rare in the woodland, with scattered or sporadic distributions. This has many implications for management. Our conservation aim is to ensure that all native species present in the woodland are maintained in the long term though their populations may fluctuate. Should we actively manage to increase the populations of some rare species? Maybe all rare species?
Many species occur only as localised patches
Pimelea spicata and Rhodanthe anthemoides are examples of species that occur only as localised patches that have not expanded significantly over 14 years. Other similar species include Daviesia ulicifolia, Linum marginale, Lomandra multiflora, Ranunculus lappaceus and Sorghum leiocladum. Some species may be localised because of narrow environmental constraints, others may be localised because of past historical events such as susceptibility to grazing, clearing of particular sites or loss of pollinators. Mostly we don't know very much about the reasons for rareness.