Coma/co Aluminium Limited operate an extensive open-cut bauxite mine covering a lease area of over 2590 km2 at Weipa north Queensland. In the course of mining operations and post-mining rehabilitation earthworks, there are major changes in the landscape, particularly in terms of altered hydrology and soil structural and nutrient quality.
The company carries out a wide ranging rehabilitation program, the major aim of which is to create a long-term, stable, maintenance-free ecosystem which comprises native flora and fauna. However, the quantitative data gathered to date on the nature and stability of the ecosystem currently being created has been limited.
In view of this, a major vegetation survey was conducted with two major aims; namely 1) to compare the treatments of fresh and stockpiled soil, ploughing and non-ploughing and various stripping times on the structure of the regenerated vegetation, and 2) from the resulting data to assess the current ecological condition of the ecosystem in terms of sustainability, stability and maintenance-free criteria and to assess the possible ecological directions of the rehabilitation.
A total of 45 sites, covering over 410 ha were surveyed for species richness, density, height, cover, basal area, grass and litter biomass. Data was variable as seed mixes used in the program have changed considerably over the last 14 years; additionally rehabilitation procedures have differed between ages of rehabilitation. At present, much of the post-mining vegetation appears to be ecologically unstable, especially in terms of fluctuations in both species richness and density between the company's routine six-month survey after establishment and that undertaken during the course of this project. However, the post-mining ecosystem is still in a young stage of development. This status is indicated by the relatively few seedlings in older age classes and a slow but active coloniser pool, a characteristic which possibly indicates that a self-regenerating community could develop in the future. Acacia species dominated the mine-site, and this poses a potential problem in terms of inhibiting the natural succession process. Eucaluptus species, although present, were markedly lower in terms of density and percentage composition of vegetation in comparison to Acacia. The future sustainability of this species, particularly in older regeneration, is questioned.
The ecosystems response to fire was also significant in terms of diversity and density change, with an overall decline in stem density. There was also a vigorous weed population evident in some of the older regeneration.
The rehabilitation program suffers from a lack of ecological focus, in that there is no basic ecological framework from within which a specific vegetation community can be constructed. While the use of fresh soil and ploughing is effective in terms of positively influencing vegetation parameters such as richness and density, these measures alone cannot guarantee the long-term success of the regeneration. The use of specific native reference areas and/or the formulation of vegetative success criteria need to be addressed for possible comparisons with the post-mining community being created. The introduction of more ecologically orientated management procedures such as various periods of fallow, appropriate species introduction and selection and a greater control of topsoil quality and seedbed conditions (such as the control of grass cover) would help to ensure the build-up of soil organic matter and seed propagule stores and contribute to the increased plant establishment and long-term survival of a native species community.