Open Cut » Environment
In recent years an increasing interest has been placed on mining operations to rehabilitate their post-mined lands to a standard that achieves a certain level of ecological recognisability and function. In some instances, there have been requirements for rehabilitation to meet the diagnostic characteristics of certain vegetation types, threatened and non-threatened. Additionally, legislation in New South Wales currently provides a pathway for ecological mine rehabilitation to contribute to the biodiversity offsets for new mining projects due to their potential contribution to conservation outcomes in a locality and through their assumed long-term persistence. Recent progression in environmental legislation, along with accompanying policy and guidelines, is evidence that there is a move within the New South Wales, Queensland and Commonwealth governments, and likely elsewhere, to consider the use of mine rehabilitation as a biodiversity offset. However, this has been hindered by a lack of knowledge and adequate research about the likely success of such ecological mine rehabilitation in relation to self-sustainability and recognisability. This project, undertaken in collaboration with New South Wales Department of Planning, Industry, and Environment (DPIE), focused on determining if mine rehabilitation can form a self-sustainable, recognised vegetation community using the Hunter Valley as a case study.
A desktop analysis was undertaken of seven operational mines in New South Wales and two in Queensland, to determine if they are required to establish specific vegetation types as part of their mining approvals. Existing data were collated to determine whether there is any evidence that these mines have been able to achieve, or are on a trajectory toward achieving, recognisable and self-sustainable ecological communities in their rehabilitation areas. The review indicated that the rehabilitation located at the New South Wales mines could be developing towards recognisable plant communities, while the two Queensland mines demonstrated little evidence that the mine rehabilitation will develop toward recognisable plant communities in the absence of management intervention. The review confirmed that threatened fauna species are utilising the habitats present in rehabilitation at several Hunter Valley coal mines.
Field surveys to obtain site-specific data (93 sites) for this study were conducted at five open cut coal mines and various remnant vegetation sites during March-May 2019 in the Hunter Valley. The data collected covered attributes that measure composition, structure and function of an ecological community. The recognisability of rehabilitated sites was assessed in terms of composition and structure, as well as at the levels of plant community type (PCT) and threatened ecological community (TEC) and tested the application of the New South Wales Government's draft PCT Assignment Tool. Assessments of self-sustainability, or those that focused on function, were undertaken by DPIE. A probabilistic determination was undertaken to determine whether any rehabilitation sites were likely to be self-sustaining or approaching self-sustainability.
94% of reference sites were assessed as very strongly recognisable as the 'best fit' PCT compared with 33% of rehabilitation sites, and a further six percent of reference sites and 20% of rehabilitation sites were assessed as strongly recognisable. There was found to be no correlation between the age of rehabilitation and the level of compositional recognisability.
The PCT Assignment Tool was found to be a very effective means of assessing floristic recognisability. The results from the structural recognisability analysis at the PCT level indicate that ecological mine rehabilitation can achieve vegetation structure comparable to intact vegetation when each attribute is assessed individually. However, only one site was identified as strongly recognisable for all nine structural attributes considered.
Substantial differences to assessing the presence of TECs listed under Commonwealth and New South Wales legislation exist, as demonstrated by this project. The way TECs are legally defined and described largely determine the criteria and attributes of the community that can be used to assess recognisability at this level. A total of 32 reference sites and 18 rehabilitation sites were assessed as being recognisable as Central Hunter Valley Eucalypt Forest and Woodland critically endangered ecological community (CEEC). Additional rehabilitation sites are likely to be recognisable as the CEEC as the woodland structure of the community develops and additional sites could also be recognisable through the selective removal of contra-indicative species.
The self-sustainability analyses identified that two rehabilitation sites (both 21 years old) were likely to be self-sustaining and a third site (12 years old) was assessed as approaching self-sustainability (Oliver and Dorrough 2019). Nine variables were identified by Oliver and Dorrough (2019) as potential performance indicators for measuring self-sustainability at rehabilitation sites.
Several performance measures were identified as suitable in assessing the compositional and structural recognisability of ecological mine rehabilitation as target vegetation types, including the use of the PCT Assignment Tool; a range of criteria under New South Wales and Commonwealth listed TEC descriptions; cover of specific growth forms; and tree abundance. Potential self-sustainability (function) performance measures identified by Oliver and Dorrough (2019) were further assessed as part of this study and the majority of those identified were assessed as suitable for use in completion criteria.
Proposed rehabilitation objectives, completion criteria and performance indicators for ecological mine rehabilitation were developed using the results of this study, to assist with the development of more prescriptive project-specific completion criteria, based on the target vegetation types to be established in rehabilitation. The project recommends three recognisability and six function completion criteria. We consider that the requirement to meet all the completion criteria relating to structure and function may be unnecessarily prohibitive for these reconstructed ecosystems, as supported by the results of this study, and we therefore put forward an approach where a select proportion of completion criteria should be met for the corresponding rehabilitation objective to be satisfied.
The project found that mine rehabilitation can support ecological communities which are recognisable as three different PCTs in the Hunter Valley and the results are promising in relation to the likelihood of establishing recognisable ecological rehabilitation elsewhere in temperate woodlands in Australia.