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Topsoil Substitutes and Sustainability of Reconstructed Native Forest in the Hunter Valley

Open Cut » Environment

Published: May 06Project Number: C12033

Get ReportAuthor: Mike Cole, Yvonne Nussbaumer, Camen Caster, Nigel Fisher | University of Newcastle

Three primary objectives were set for this project, these are:

  • To identify the best growth media for the establishment of native forest communities in spoil and pasture soil and to test secondary and tertiary treatments such as inoculation with soil microbes.
  • To set up a field research facility at Mount Owen mine and a verification site at Warkworth mine to study the establishment, growth and sustainability of a dry sclerophyll forest community that further develops current links between the industry and The University of Newcastle.
  • To provide a benchmark site for future forest rehabilitation and develop a process by which reliable measures of sustainability can be achieved.

Increasingly consent conditions in the Hunter Valley include reconstruction of native vegetation on parts of rehabilitation sites or in offset areas. To achieve this may require the spreading of forest or woodland soil, pasture soil, or the use of ameliorated spoil. Often soil, be it forest or pasture, is composed of badly degraded B-horizon material that had has lost its A-horizon due to land clearing and grazing activities that preceded mining. The soil that remains, especially pasture soil, has also been invaded in many cases by a broad range of weeds. Hence, reverting land use back to forest or woodland has the added difficulty of weed competition, as deep ripping the surface of the soil tends to promote weed germination and growth. This is particularly true if the soil has been stockpiled.

This project provided the opportunity to formally test the value of a number of alternate media for use in the rehabilitation of spoil placement areas at open cut coal mines. In addition, the establishment of a Field Research Facility at Mount Owen Mine together with a verification site at Warkworth Mine provides the opportunity for longer-term studies of species persistence in rehabilitation sites and the development of community structure and function.

The results we have shown indicate general consistency across all of the trials, but differences in the field and pot trials as anticipated. These differences largely relate to water supply and weed competition. We have also begun to develop a long-term measure of success based on real-life events relative to a reference site and demonstrated the outcomes using pot and field trials. The concept of ecological resistance, and the measure of success, relative conditional probability, will continue to be developed.

The successful completion of this study was despite three major rain events at Mount Owen and one at Warkworth that interfered with the integrity of the trial. Fortunately the power in the experimental design used is such that the rain events did not cause a loss of the trial. This has been demonstrated in the analysis presented in this report and ought to encourage the mining industry to utilize formal experimental science in its pursuit of knowledge to assist it achieve its desired outcomes.

On the basis of the results gained so far from the two sites, we recommend that:

  • Wherever available, and possible, direct-transfer forest topsoil should be used for rehabilitation, as it produces the highest native plant density and species richness.
  • The best alternative to forest topsoil was found to be a plastic subsoil. Subsoil and plastic pre-strip should therefore be stockpiled separately from spoil for future use where insufficient forest or woodland soil is available to meet rehabilitation demand.
  • As not all subsoils will be suitable for rehabilitation due to some being highly dispersive, they should be trialled in properly designed, scientific experiments prior to large-scale use.
  • If using other topsoil such as woodland topsoil, it should be assessed prior to large-scale use because Warkworth woodland topsoil, that had been heavily grazed, was no more successful than bare spoil.
  • Pasture topsoil with large weed and grass seed banks as found at Mount Owen and Warkworth, should not be used for native forest reconstruction, using seeding, without amelioration to combat the weeds and grasses. Preliminary results suggest that capping pasture topsoil with either chitter1 or pasture subsoil and without deep ripping, may be a way of reconstructing forest and woodland using seed on weed infested, stockpiled pasture topsoil.
  • A trial should be set up to test whether capping biosolids with a weed and grass free medium will overcome the negative competitive effects found in biosolids for the reconstruction of forest-woodland vegetation.
  • Secondary treatments such as fertilizer and worm casts showed the potential to increase plant growth in the secondary treatment pot trial. However, no response was observed in the field.
  • Dose response experiments should be conducted on the secondary treatments to verify or adjust the application rate appropriate for use with native vegetation.
  • The Ecological Resistance Model that is under development and utilizes relative conditional probability could be compared to alternatives such as “Ecosystem Function Analysis” (Ludwig et al. 2003) and “Habitat Hectares” (Parkes et al. 2003), to compare its usefulness as a tool in rehabilitation management.

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