Where topsoil contains a viable native seed source, it should be conserved for reuse following mining. This not only provides a cheap source of plants, it helps ensure that they establish in amounts that reflect pre-mining densities and it promotes establishment of species whose seed may be hard to obtain or difficult to germinate.
The bauxite mine rehabilitation program conducted by Alcoa World Alumina Australia in the jarrah forest of southwestern Australia demonstrates how conservation of the soil seed bank can significantly enhance the botanical diversity of the post-mining vegetation community. After vegetation is cleared, the top 150 millimetres of soil, which contains most of the soil seed bank and nutrients, is stripped prior to mining and then directly returned to a pit about to be rehabilitated, wherever possible. Research has shown that the majority of native plant species (72 per cent) on rehabilitated areas comes from seed stored in topsoil. The importance of directly returning fresh topsoil has been demonstrated by trials comparing this technique with stockpiling. These have shown that disturbance associated with direct return of topsoil results in loss of less than 50 per cent of the seed contained in the pre-mining forest seed store; in contrast, stockpiling results in losses of 80–90 per cent.
Other aspects, such as the depth of re-spreading topsoil, the season when the soil is handled and the timing of seeding, are also important. Seed will not survive if buried too deep, and it persists better when the soil is moved during the dry season. Also, plant establishment from seeding is greater when the seed is applied to a freshly disturbed surface. Together, the combined use of fresh topsoil return, seeding, and planting of resistant plants have now resulted in numbers of plant species at 15 months of age equal to those recorded in equivalent-sized plots in unmined forest. For further information, see www.alcoa.com.au