Vegetation development in gold mine rehabilitation in relation to cattle grazing in the Northern Territory, Australia

Many gold mines have operated and been rehabilitated in savanna vegetation between Darwin and Katherine in the Northern Territory, Australia. A range of these sites were managed by Crocodile Gold Australian Operations and had been rehabilitated approximately 9 - 12 years previously to achieve the rehabilitation goal to become eucalypt-dominated savanna woodland. The aim of this research was to assess the success of the sites in terms of establishment of trees and ongoing tree recruitment. The effects of fire, grazing, weeds and a range of environmental parameters on the composition and density of mature trees, saplings and seedlings were also investigated. Assessment of established trees and of tree seedling and sapling recruitment was carried out over three years at 28 sites, including analogue sites in the natural woodland. Understorey vegetation cover, soil surface condition, soil chemical contents and grazing were measured. Fieldwork was supported by experimental studies into effects of media on seedling establishment undertaken in the shadehouse. Overall tree density and sapling densities at most rehabilitation sites were somewhat similar to natural woodland when analysed by General Linear Modelling. They differed markedly in tree seedling density, tree structure, species richness and composition. Ordination analyses revealed litter thickness and cover, cattle dung, soil calcium concentration and rainfall were the environmental variables which correlated to significant differences in tree densities across mine rehabilitation and natural woodland sites. Grass cover, aluminium concentration, sulphur concentration and soil electrical conductivity all had negative correlations with sapling and seedling density, while soil phosphorous, litter cover and litter thickness had positive correlations with sapling and seedling density. A shade house experiment investigated effects of litter cover on seedling germination of Acacia and Corymbia species and showed that an intermediate 609 g per m 2 of dry litter was a favourable litter thickness for the seedling emergence of Acacia and Corymbia species. Tree seedling growth in mine rehabilitation media was less than in natural woodland media. Grazing and fire affected established trees and tree recruitment in mine rehabilitation. Cattle grazing modified understorey vegetation and in doing so it benefited tree seedling establishment but was slightly detrimental to sapling and mature trees. The impact of cattle grazing on established trees and saplings was variable but generally saplings and established tree densities tended to be lower in the grazed mine rehabilitation sites. Fire killed tree seedlings and small trees with diameter less than 3 cm and thus affects tree sustainability in mine rehabilitation. Recruitment after fire tended to occur but was variable between sites. Rehabilitation sites on undesirable pathways will require appropriate management intervention to return them to the desired successional trajectory. A set of interventions has been developed for potential application by rehabilitation managers to improve rehabilitation success. These include interventions applicable in the initial rehabilitation as well as techniques applicable for ongoing management of the rehabilitation. Potential management interventions suggested as a result of this study include replanting, grass management, fire and grazing management and thinning of Acacia species. Further monitoring and research is needed to identify factors that cause low tree recruitment, to assess the impact of stocking rate and to assess effects of prescribed fires earlier during rehabilitation on tree establishment in savanna gold mine rehabilitation.