Rebuilding a War Zone The

REBUILDING A WAR ZONE

In autumn 2021 almost 7,500 trees were planted on the slopes of Mount Annan

WITH THE BATTLE AGAINST A WILE WEED ALMOST WON, THE AUSTRALIAN BOTANIC GARDEN MOUNT ANNAN IS FOCUSED ON REGENERATING A DEGRADED LANDSCAPE. JOHN SIEMON REPORTS.

As Foundation & Friends members are well aware, the Australian Botanic Garden Mount Annan has long been at war with a frighteningly formidable foe: the African Olive (Olea europaea ssp. cuspidata). At one stage this aggressive woody weed covered almost 20% of the Garden’s 416 ha landscape.

With hindsight, I don’t think any of our predecessors ever anticipated the African Olive would mount such a successful assault. The weed’s growth, while noticeable, seemed of secondary importance to establishing the Garden’s core credentials and, before anyone really appreciated the scale of infestation, it had spread to such an extent that staff were simply unable to tackle the problem by themselves.

Peter Cuneo, Manager Seedbank & Restoration Research, completed his PhD in 2012 on ‘The ecology and management of invasive African Olive in southwest Sydney’. In a joint paper, he and Professor Michelle Leishman painted a rather dire picture of the enemy: “individual African Olive trees are capable of producing more than 25,000 fruits per annum, and able to establish up to 950 seedlings per square metre as a ‘seedling bank’ beneath the crown of mature plants”. Images of Mount Annan, captured by Cuneo across successive decades, highlight just how hostile and transformative this weed is in decimating biodiversity and exterminating all floristic life from the landscape.

Through a concerted effort over the last decade, the Garden has now reduced the total coverage of African Olive by a staggering 85%, with less

‘In autumn 2021 almost 7,500 trees were planted around the slopes of Mount Annan’

than 12 ha remaining. This has been predominately through a mechanical mulching process, where the trees are pulverised into woodchips, but also through bush regeneration contractors, volunteers and a Boer goat trial in 2016.

In September 2019, we undertook our most assertive effort to date, removing an enormous 15 ha of olive from the slopes of Mount Annan. The results were confronting and, post-clearing, it was hard not to feel a sense of loss at the mass destruction of trees. To the untrained eye, the naked and fragile landscape was raw, almost war-zone like in its appearance. The land, on such a grand scale, devoid of vegetation, seemed counterproductive to the core mission of an organisation intent on protecting and conserving biodiversity. Some may even argue that a more sensitive approach was required. In reality, we were left with little choice because taking a ‘softly softly’ approach will never outpace the fecundity of the African Olive.

On the flip side, the ridge line of the Garden, part of western Sydney’s Scenic Hills, has been cleared of a serious scourge. For the first time in more than 20 years, phenomenal vistas have been restored, a trait more commonly considered as superior at our other two well-known Gardens.

Not only has the Garden landscape been riddled with African Olive, but so too our neighbours’ lands. It has been impossible for Garden management to have meaningful dialogue with these neighbours while we failed to deal with our own problem. Now, however, with a potential light at the end of the tunnel, we have commenced dialogue with adjacent landholders to ensure the seed from their olive stands, which is easily spread by birds, will not continue to reinfect the Garden long into the future. We intend to share our knowledge

African Olive infestation captured on the slopes of Mount Annan in 1984

The same slopes of Mount Annan in 2004, almost completely covered in African Olive

Boer goats were trialled as part of the African Olive eradication solution in 2016

‘The Garden has now reduced the total coverage of African Olive by a staggering 85%’

Over the last decade, mechanical mulching has helped reduce African Olive coverage by 85% Tree planting in autumn 2021 and expertise to help them tackle the problem and substantially reduce the biomass of African Olive in the region.

The restoration of Dharawal Country and the Garden landscape has also begun with a number of trials implemented to revegetate the degraded landscape with native grass seed, which was harvested or ‘orcharded’ from the Garden itself and returned to the landscape in the millions. This native pasture is certainly helping to stabilise the soils of the degraded Garden landscape, but significant tree planting – to return canopy, enhance biodiversity and contribute to reducing the heat-island effect of western Sydney – is also needed.

Fortunately, thanks to the Greening our City program (which is aiming to plant five million trees by 2030) and the NSW Government’s Climate Change Fund, more than $1.4 million was allocated to accelerate African Olive removal and commence the restorative work of planting trees.

In autumn 2021, almost 7,500 trees (including Acacia falcata, A. decurrens, Melaleuca styphelioides and Eucalyptus tereticornis), grown in the Garden’s new production nursery from seed stored in the Australian PlantBank, were planted around the slopes of Mount Annan. It was initially hard not to feel as though our plantings had become a war cemetery with cardboard tree guards, designed to protect the trees (but apparently very tasty for resident kangaroos). Thanks largely to wonderful La Niña rains, these trees are thriving.

By early winter 2022 we will see the installation of a further 15,000 trees, which will be studied as part of our Australian Institute of Botanical Science programs.

Over the years, Foundation & Friends has been a key ally in the fight against African Olive, donating over $662,000 to help eradicate the weed. As always, thank you for your support!