Open Cut » Environment
Long-term research has provided valuable data on the best species selection and conditions for native understorey species regeneration in NSW coal mines. NSW mining lease conditions require rehabilitation to at least the pre-mining Rural Land Capability and, for the Hunter Valley region, this generally means a return to grazing land. Rhodes grass, a high maintenance, low productivity pasture, has been the predominent species used for rehabilitation. While many rehabilitation programs have aimed to return the land to improved pasture for grazing, there is also interest in establishing self-sustaining and relatively maintenance-free native vegetation communities. For some mines, this is to provide wildlife habitat and an aesthetically pleasing variation in the landscape, while other operations want to return the post-mined land to native vegetation, consistent with neighbouring plant communities. Due to the increased use and diversity of native flora in rehabilitation, operators need to understand the germination characteristics and establishment requirements to maximise the success of new native species communities.
In 1995, researchers began to investigate seed germination and viability of a wide range of native understorey species at nine New South Wales coal mines (C4009) - Hunter Valley No 1, Mount Owen, Ravensworth, Warkworth, Westside, Baal Bone, Clarence, Tahmoor and West Cliff. This project also considered methods of establishing an appropriate range of understorey species on specific soils and spoils. In March 1999, researchers began an extension project (C7010) driven by the same overall objective as C4009, but with the following specific objectives:
- Continuation of seed viability and germination optimisation studies for those species with a useful role in the composition and structure of the reconstructed plant community.
- Further monitoring of established field trials to provide the end-users with the most reliable information and a guide to the long-term stability of the newly-created ecosystems.
- Initiation and assessment of additional field trials that rationalise species establishment differences between non-water limiting glasshouse conditions and the field environment. These studies concentrate on issues such as seedbed preparation and other cultural practices that have a major impact on field success.
- Integration of complementary studies such as the native grass project (C6004).
- Establishment of other field trials using selected species under canopy protection with a microclimate less exposed than those areas previously trialled.
Research for the extension project was conducted at Westside, Hunter Valley No 1 and Ravensworth mines.
Species with high germination (greater than 80 percent) and recommended for rehabilitation are:
No pretreatment: Allocasuarina torulosa and Banksia marginata.
Pretreatment: Acacia filicifolia, A mearnsii, Bossiaea heterophylla, Daviesia leptophylla and Pultenaea retusa.
Species with reasonable germination (between 40 percent and 80 percent) and recommended for rehabilitation are:
No pre-treatment: Acacia melanoxylon, Allocasuarina distyla, A littoralis, A nana, A verticillata, Angophora floribunda, Dodonaea viscosa, Kunzea parvifolia and Melaleuca nodosa.
Pre-treatment: Acacia brownii, A buxifolia, A suaveolens, Daviesia genistifolia, D latifolia, D retorta and Jacksonia scoparia.
The 51 species sown in the field trials represent 25 genera and 10 families. Fabacease, Mimosaceae, Myrtaceae and Proteaceae families contributed to the largest number of species with eight, 12, eight and 10 representatives, respectively. Of the 10 families, the Fabaceae family had the most genera used through this trial.
The main conclusions from this experiment are that seedling emergence and species richness is improved with irrigation and that mulched topsoil is a more appropriate medium for native understorey species re-establishment than spoil, regardless of irrigation or any media amelioration. Although total species emergence for understorey species was lower in the bare topsoil than some spoil treatments, species richness and growth and persistence of seedlings on the bare topsoil was far greater than any treatments of the spoil. Despite the fact that results were variable, some general recommendations regarding species selection, seedbed preparation and watering regimes can be made.
Species selection: For the topsoil material used in the trial, all species sown except for Geijera parviflora are recommended for inclusion in seed mixes, particularly Acacia falcata, Allocasuarina verticillata, Jacksonia scoparia, Melia azedarach and Swainsona galegifolia. For the spoil, those species which emerged were Acacia falcata, Allocasuarina verticillata, Jacksonia scoparia, Melia azedarach, Olearia elliptica and Swainsona galegifolia. Of these, Acacia falcata, Allocasuarina verticillata and Swainsona galegifolia had more than one percent emergence in the spoil and are, therefore, recommended.
The understorey species and the native grasses could be sown simultaneously as, apart from a couple of treatment combinations that showed minor advantages to omitting the grasses, negligible differences were noted between the vegetation treatments within media.
Seedbed preparation: In the mulched topsoil plots, trends indicated that species success was greater in the absence of biosolids, particularly where irrigation was not supplied. The regular differences between the mulched and bare topsoils indicated that the use of mulch in the topsoil was more favourable for seedling emergence and species richness. Given the numbers of treatment variable used in the spoil area, trends were more difficult to determine.
Watering regime: Irrigation was aimed at allowing the accumulation of sufficient data to make comparisons between media and vegetation treatments. Overall, for the topsoils, trend data indicated that more seedlings emerged under irrigation than in the non-irrigated areas, and that the longer water remained available, the more emergence occurred.
In general, the numbers of sown species that emerged were low, with Acacia species being the most common. Of the 16 species sown across the three sites, only the Acacia species, Banksia spinulosa, Melaleuca armillaris, Oxylobium ilicifolium and Swainsonia galegifolia recorded emergence. Numbers of emergence were low at all sites, although Banksia spinulosa had more than one percent emergence at Woodside mine. The surface scarification treatment made very little difference to emergence results. However, the very low numbers of emerged seedlings at each site meant the two treatments could not be fully analysed.
Low emergence numbers were due to inappropriate seedbed conditions, competition from high, dense overstorey species, unfavourable rainfalls over the trial period, low seeding rate, dormancy or low viability of sown species, and possible grazing of seed and seedlings.
Westside mine had the best results in terms of species richness and percentage of emergence -assisted by the lower litter cover, bare areas and only patchy grass and weed cover. Hunter Valley No 1 and Ravensworth mines had very little herbaceous and shrub ground cover and high litter loads.
Sowing native understorey species into an existing canopy presents the germinating seeds with potential problems, including the lack of adequate soil:seed contact for root establishment, nutrients, light and space.
Potential revegetation areas need to be adequately prepared before sowing. Understorey species should be included in the initial seed mix along with canopy species.