Ecological restoration in a biodiversity hotspot

A key resource centre (silica and building products) operated by Rocla is located 30km northeast of Perth (Gnangara) on the Swan Coastal Plain on the suburban outskirts of Perth. The development of the rapidly urbanised areas of metropolitan Perth is highly dependent on sand resources that are restricted to isolated pockets in specific geological units within the geological feature known as the Bassendean dune system.  The resource area sits within the southwest Australian biodiversity hotspot reflecting the high level of threat to the plants and animals mostly as a result of extensive clearing for agriculture and urban development.

As result, Rocla?s quarrying activities impact on the biodiverse Banksia woodland plant communities in the path of the mine, which grow on the deep siliceous Bassendean dune sands (Figure 1). The process of sand extraction requires clearing of the Banksia woodland vegetation from undulating hill sites, stripping the topsoil and removing the underlying white and yellow quartz sand horizons constituting 18m to 40m of the soil profile, thereby reducing the soil profile depth by at least 20m. A major priority of the company is to restore the post-sand extracted sites with a plant community closely resembling the pre-sand extracted Banksia woodland plant community.

The Restoration Science team at Kings Park and Botanic Garden (Botanic Gardens and Parks Authority) commenced a research partnership with Rocla in 1995, with a highly ambitious plan to restore as many as possible of the 280 plant species found in Banksia woodlands following sand extraction. Kings Park had been developing intervention restoration capabilities, but the complex species composition, radical alteration of the soil profile and limited knowledge of restoration technologies applicable to these woodlands required development of an integrated research programme linking seed science, topsoil technology and ecological restoration techniques.

The vision of the project was to develop international leading practice in the key areas of topsoil management, seed broadcasting, seed germination enhancement technology, mulch management and smoke technology. To facilitate the incorporation of research findings, an adaptive management approach was applied (management applied on the basis of lessons learnt).  Such an approach had not been attempted previously for a species rich ecosystem in a biodiversity hotspot.

Recognising that post-sand extracted sites would provide an ideal and important opportunity for describing restoration and management needs of the neglected Banksia woodland ecosystem, Kings Park developed and implemented a research and development programme with Rocla – resulting in a collaboration comprising 14 years of research and involving graduate and post-graduate research student programmes.

Prior to the commencement of the collaborative research programme, the diversity and sustainability of post-mined Banksia woodland was limited both in species diversity and plant abundance values (Figure 2). Initially, it was hypothesised that reasons for the failure to establish biodiverse plant communities was related to poor topsoil handling procedures, inadequate soil storage methods, changed soil environment and lack of knowledge of seed dormancy.

Rocla?s Gnangara site (>70ha) became the first focus site for the restoration research and development of Banksia woodlands. The research programme rigorously applied the adaptive management approach resulting in a cycle of continuous improvement through research analysis and inputs into the operational aspects of the restoration programme.

The collaborative research programme focused on two key scientific areas: (1) seedling recruitment and plant survival, and (2) plant growth and development responses to a reconstructed soil environment. Kings Park scientists believed that these aspects would yield essential information for the development of appropriate ecological restoration practices.  As a result, the research programme has demonstrated innovation and environmental research excellence through:
? Compiling the largest database on soil and plant development data for reinstated Banksia woodland species benchmarked with natural Banksia communities.
? Optimisation of the regenerative potential of the topsoil seed bank.
? Optimisation of topsoil handling and storage.
? Development of innovative seed germination enhancement pre-treatments.
? Rigorous testing of greenstock-enabling treatments (eg tree-guards, anti-transpirants).
? Rigorous testing site treatments (eg mulching, irrigation and soil ripping practices and application of soil stabilisers).
? Researching the ecological impact and selective control of dominant weeds species impacting upon native plant survival in restored sites.
? Investigating ecophysiological parameters (nutrient and soil water relations) limiting plant survival and growth.

As a result, the number of species returning to restoration sites has increased ten-fold (from six native species to over 80 native species in restored sites).  A key operational approach involved a more strategic and precision-based approach to stripping fresh and dry topsoil to 10cm and re-spreading to only 5cm.  Respreading of topsoil was combined with deep ripping the soil profile to decrease soil compaction and improve root architecture and depth-seeking (for water harvesting).  Finally, rigorous research showed that the common practice of mulching and replacing the overburden sand profile to depth were not only costly exercises but resulted in higher plant mortality.

Through this unique, long-term research partnership, Rocla has demonstrated restoration performance that is among the highest achieved in a biodiverse plant community.  The research partnership has resulted in the restoration performance of over 100 restored native plants per five square metres, double the value for the industry leader, Alcoa World Alumina (50 plants per five square metres) with a lift of ten-fold in species richness in restored sites over the 14 year programme.

Restoration ecologist Dr Deanna Rokich and Director of Science Professor Kingsley Dixon are based at Kings Park and Botanic Gardens, Perth.

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