These Botanic Gardens display a representative sample of the plant communities of the Darling Basin. Plants which do not occur naturally within the basin are not included in the collection. In nature, plants always grow in associations of several or many different species, never in populations of single species. An association of plants which grow naturally together is classed as a plant community.
Radiation, soil and water are the most dominant natural features influencing plants. Radiation is derived solely from the sun, soils are determined by the geology and past eons of climate, while water is delivered as either rain or flood. When the conditions are exactly right for it, you can expect to find a particular plant community. In this area, for instance, brigalow-belah woodland occurs on heavy grey clay soils, while right next to it on a sand ridge you are likely to find a cypress pine open forest. Radiation and water are the same, but the soil is different.
Knowledge of plant communities and the environmental factors that suit them makes it possible to judge the natural capacity of the countryside. Humans have used this knowledge in Australia for many thousands of years and continue to do so. It is well known that brigalow-belah grows on very fertile soils that are excellent for farming. By contrast, poplar box communities occur on less fertile soil, but are still useful for growing grain.
For the sake of consistency botanists adopt a classification system to describe plant communities. The system describes the structure of a community, and combines it with the name of the dominant, tallest most common plant species present. Structure is classified by:
Type of vegetation – trees, shrubs or herbs
Foliage cover of dominant plant
Thus plant communities have names like “Eucalyptus coolabah open woodland” or “Eucalyptus coolabah – Casuarina cristata open woodland”, both of which describe coolabah communities but with the second having a significant presence of belah. Both descriptions give a reasonable idea of the appearance of the community. They describe:
- The tallest, most common plants are dominant by height (coolabah and belah)
- A density that provides open sky between the trees (open rather than closed)
- Trees with a more open rather than dense canopy (woodland rather than forest)
“Astrebla wooded tussock grassland” or “Astrebla tussock grassland” describes two Mitchell Grass communities, of which the first has a scattering of trees and the second does not.
There will be many other plants in these communities, with the species present varying from place to place according to the specific environmental factors. Thus classification by plant community gives a general indication of what is there, but does not provide all of the details.