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Feature garden - Saving threatened species at the Australian Botanic Garden Mount Annan – one ecological community at a time
Dr Peter Cuneo, Honorary Research Associate, Australian PlantBank, Australian Botanic Garden Mount Annan and David Khoury, Supervisor Natural Areas & Open Spaces, Australian Botanic Garden Mount Annan
In the face of ongoing biodiversity loss, the conservation role of botanic gardens continues to be a critical part of ex situ actions to safeguard against species extinctions. Since inception, the Australian Botanic Garden Mount Annan (ABGMA), has progressively built a range of conservation programs including high profile species such as the Wollemi Pine, and more recently the establishment of the internationally recognised Australian PlantBank. In parallel with these significant ex situ and scientific programs has been several decades of in situ conservation and regeneration work focused on the management of remnant Cumberland Plain Woodland (CPW) that occurs naturally at the ABGMA site.
The gently undulating topography of the Cumberland Plain region in western Sydney and its fertile clay soils were targeted for agricultural land clearing and subsequent urban development, which continues to this day. Sometimes seen as the ‘poor cousin’ when compared to the adjacent Sydney sandstone vegetation, the CPW contains amazing diversity in its rich grassy understorey layer. This grassy woodland ecosystem, which once formed a mighty forest across western Sydney, is now reduced to 6% of its original extent. As a result of extensive clearing and weed invasion impacts, CPW was listed under New South Wales legislation as a Critically Endangered ecological community in 1997.
At ABGMA, the CPW conservation areas have an open forest structure with a tree canopy dominated by three distinctive eucalypts: Grey Box Eucalyptus moluccana, Forest Red Gum Eucalyptus tereticornis and Narrow Leaved Ironbark Eucalyptus crebra. The midstorey is largely dominated by the shrub Blackthorn Bursaria spinosa, which flowers prolifically in midsummer, attracting a huge range of invertebrates. The grassy understorey layer has a wide range of grasses, native herbs and lilies, with some of the key species being Kangaroo Grass Themeda triandra, Weeping Meadow Grass Microlaena stipoides, Blue Trumpet Brunoniella australis, Kidney Weed Dichondra repens, Pale Grass Lily Caesia parviflora var. parviflora and Common Woodruff Asperula conferta. Western Sydney is a rain shadow area, and CPW has many similarities with the drier woodlands west of the Great Dividing Range, and includes plants such as several Einadia species from the saltbush family Chenopodiaceae. CPW plants are well adapted to drought and heavy clay soils, and many are able to resprout from underground lignotubers when rainfall occurs. Set in the heart of the Cumberland Plain region, the ABGMA’s early planning during the mid 1980s recognised the importance of remnant patches of CPW, which had not been disturbed by cultivation. At this time, renowned Royal Botanic Garden Sydney ecologist Doug Benson identified areas of high species diversity, which were set aside as conservation areas. From these early days of ABGMA, Doug and his colleague Jocelyn Howell generated great interest in western Sydney vegetation through many publications and recognised that native vegetation was poorly conserved. Doug and Jocelyn also commenced a long term monitoring program in the ABGMA woodland, where they have documented changes in understorey diversity and abundance in CPW vegetation. Today, the CPW conservation woodlands have expanded since the opening of ABGMA in 1988, covering approximately 35 hectares, and are an integral part of the diverse landscape mosaic. Importantly, the woodlands are now some of the best regional examples of CPW and include threatened species Pimelea spicata and the Cumberland Land Snail Meridolum corneovirens.
The Spiked Rice Flower Pimelea spicata is a small spreading understorey shrub with white, pink tinged flowers. It is largely restricted to the clay soils of the Cumberland Plain region, growing in CPW understorey that is in good condition. Flowering occurs in spring; however, it has also been observed after favourable rainfall, where the plant is able to resprout from a small lignotuber. Due to its restricted distribution, loss of habitat and the impacts of weed invasion, the Spiked Rice Flower is listed at the state and federal levels as Endangered. The occurrence of the Spiked Rice Flower in the ABGMA woodland areas has long been the focus of horticultural and scientific work at ABGMA. Projects include the propagation of this species to include in horticultural displays, monitoring of local populations and a recent project at the Western Sydney Airport site requiring propagation, seed collection/seedbanking and a regional genetic study. The primary goal is to maintain this species in the ABGMA conservation woodlands through maintaining the health and diversity of the understorey layer.
The woodland areas also support a wide range of fauna, including one of the more cryptic species, the Endangered Cumberland Land Snail Meridolum corneovirens, which is also restricted to the Cumberland Plain region. This land snail has a much flatter and thinner shell than the common garden snail. Like the Spiked Rice Flower, the Cumberland Land Snail depends on a healthy diverse CPW understorey, where it lives under logs and leaf litter and eats fungi.
The management of the woodlands, particularly the understorey, comes with many challenges. The 416‑hectare AGMA site is now largely surrounded by urban development, and managing CPW woodlands at this urban interface is complex due to ‘edge effects’ and weed invasion. One of the key threats to the woodland, and the threatened species habitat it provides, is African Olive Olea europaea subsp. cuspidata. African Olive is a highly invasive tree that was introduced as a hedge/rootstock plant in the mid 1800s and ‘jumped the garden fence’ from the nearby Camden Park estate. As is the case with many invasive species, the prolific fruit of the African Olive is dispersed by birds. This has resulted in the rapid spread of this long lived tree throughout the Cumberland Plain since the 1980s. Bird dispersal of seed makes African Olive highly mobile in the landscape, as it establishes in ‘halos’ around large eucalypt perch site trees. It spreads from these establishment sites, and if not contained will form a dense mid storey canopy shading out the diverse grassy understorey layer and preventing recruitment of eucalypt canopy trees.
African Olive is a highly conspicuous invader of woodland areas, and a dedicated program established in 2000, which included volunteers, has successfully controlled its significant invasion of the key woodland areas. Less conspicuous but significant threats to the woodland are invasive grasses such as Rhodes Grass Chloris gayana and more recently, Chilean Needle Grass Nassella neesiana. Invasive grasses easily blend in with the native grassy understorey, and species such as Chilean Needle Grass can create monocultures displacing all native plants, producing thousands of seeds that remain viable for up to 15 years. Chilean Needle Grass control is achieved by physical removal and the use of selective herbicides by staff and a network of specialist contractors.
Key to maintaining diversity in grassy woodlands are appropriate fire regimes.
At ABGMA we have used spring or autumn ecological mosaic burning focusing on the understorey, which can also provide a reduced hazard risk. The natural fire regime for CPW would have included occasional higher intensity summer fires, however, this is not possible at the urban interface. Future directions include a new program of cultural burning of the woodland, which is an opportunity to engage with locally based indigenous knowledge and ecological management.
The conservation woodlands are an important component of the ABGMA landscape mosaic, which has become a regionally significant fauna habitat. With the garden transformed from a grazing property with open paddocks in the 1980s to a mosaic of woodlands, grasslands, lakes, arboreta and horticultural display gardens, it now supports over 190 bird species and is regarded by ornithologists as an outstanding site in the Sydney region. Fauna also includes a range of mammals such as wallaroos, swamp wallabies and eastern grey kangaroos.
Maintaining and developing this wider landscape mosaic is a huge and expensive task, including the control of 80 hectares of dense African Olive forest since 2009.
As the hilly areas along the eastern ridge of ABGMA are progressively cleared of African Olive, there are opportunities to conserve and expand other threatened ecological communities, such as Western Sydney Dry Rainforest, as well as CPW. Dense olive infestation sites once cleared have limited remaining native plant diversity, and their restoration often requires seed sourced from the existing woodland areas. Importantly, the woodland areas that are in good condition also provide a template or ‘reference ecosystem’ to guide restoration efforts.
In a region of intense urban development, the size of ABGMA is significant, and it is now considered a ‘green nucleus’ with much valued native flora and fauna. The biodiversity of the woodlands, combined with the established horticultural areas (focusing exclusively on native plants) provide a unique landscape, and add an important dimension to the visitor experience. Future planning will look beyond the ‘garden walls’ to ensure that native vegetation and fauna corridor connections are maintained with the Nepean River, and other local reserves as part of a regional conservation network.
The biodiversity of the woodlands, combined with the established horticultural areas provide a unique landscape
