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African Olive control program
Peter Cuneo, Manager Natural Heritage, the Australian Botanic Garden, Mount Annan
Exotic plant invasions are a major threat to biodiversity, and are often linked to changes in ecosystem function and species diversity. In Australia, it is estimated that 28,000 exotic plant species have been introduced since European settlement, and over 1700 are now considered invasive environmental weeds. A large proportion of these environmental weeds originated as garden plants, which have ‘jumped the garden fence’ and are now established in natural ecosystems. For ‘megadiverse’ countries such as Australia, the ecological impact of persistent woody weeds is now a major environmental issue, and research is now required to better understand weed ecology, improve control measures and develop restoration techniques for ecosystems degraded by weed invasion.
An exotic invasive tree of increasing spread and conservation concern in the Cumberland Plain region (western Sydney, NSW, Australia) is African olive, Olea europaea ssp. cuspidata. African olive is a dense-crowned small to medium tree (up to 15 m) which is part of the tropical wild olive group, and well adapted to temperate and sub tropical regions. The major area of natural distribution for African olive is eastern Africa. Unlike the related European olive from the Mediterranean region, African olive has small black fruits with no commercial value; however, these fruits (~7 mm) are readily consumed by both native and introduced birds and spread through the landscape.
African olive was introduced into cultivation in NSW as a hedging plant and rootstock during the mid-1800s, and is closely linked to agricultural pioneers John and William Macarthur and the development of the famous Camden Park estate, near Camden NSW. Since the 1970s African olive started to spread rapidly throughout the Camden district and nearby locations such as the Australian Botanic Garden, Mount Annan quickly developed dense infestations on steep hillsides, particularly as grazing ceased in the mid 1980s. By 2000, over 60 hectares of dense African olive forest had established throughout the Australian Botanic Garden, however control work was primarily focussed on keeping the high conservation value Cumberland Plain Woodland areas free of olive invasion. Despite the rapid spread of African olive throughout western Sydney and its apparent impact on the fragmented native vegetation, very little literature existed on the ecology and origins of this invader, which became the stimulus for my PhD research project. Through this seven-year research project, in conjunction with the Plant Invasion and Restoration Ecology group at Macquarie University, the key aspects of African olive ecology and management have been documented, and are now being used to guide control measures for dense infestations and subsequent ecological restoration at the Australian Botanic Garden and at other locations.
The key ecological impact of African olive is its ability to invade grassy woodland communities, and form a dense mid-canopy which eventually eliminates the highly diverse understorey layer through shading. Surveys in Cumberland Plain Woodland identified that there were 78 per cent fewer native species beneath African olive canopy in the field compared to uninvaded woodland sites. An experiment that simulated the dense shade levels of the African olive canopy demonstrated a high mortality for native species, compared to an 88 per cent survival rate for African olive seedlings. Also adding to these impressive credentials as an invader, is the fact that African olive can produce 25,000+ fruits on a tree at early maturity - but how long can they last in the soil?
A seed burial experiment was used to determine the lifespan of seed in the soil seed bank, and provided some good news. There was a slow decrease in viability down to 70.3 per cent during the first year, followed by a rapid decline down to 14.7 per cent in the second year. Survival analysis indicated that under field conditions, maximum seed persistence in the soil was ~29 months (2.4 years). This short persistence of seed in the soil (compared to hard seeded natives such as wattles) provides an opportunity for managers to achieve control of African olive once mature plants are removed.
Having established some of the fundamental aspects of African olive biology, we then faced the question of how to achieve control of dense African olive forests, and then restore the previous native woodland vegetation. To achieve control of these olive forest ‘monocultures’ requires a radical approach to bush regeneration, and we use a range of contractors with machines that woodchip the entire olive tree to create a thick ground mulch layer. Olive stumps are also left in the ground to maintain soil stability and are treated with herbicide. To work out the best approach to restoring these mechanically cleared ‘ground zero’ sites, a three-year field experiment was established to determine the effectiveness of direct seeding native species and fire for restoring native woodland. Amazingly, fire was able to stimulate the germination of native species such as wattles from the soil seed bank even after 15 years of olive invasion, a great indication of the resilience of these grassy woodland communities! Direct seeding using kangaroo grass (Themeda) and meadow grass (Microlaena) was able to re-establish a vigorous native grass cover which was resistant to subsequent weed invasion. This establishment of native grasses on these cleared sites follows natural succession processes, and is the basis of a ‘bottom up’ approach to restoring the grassy woodland understorey. African olive seedlings that emerge in the grasslands are controlled using (broad leaf) selective herbicides or fire. Trees and shrubs are progressively added either using direct seeding or planting of tubestock.
Looking to the future, over 40 hectares of dense African olive have now been cleared at the Australian Botanic Garden through the Endeavour Energy funded African olive control program. Now in its third year, this project is transforming the landscape at the Australian Botanic Garden and beautiful hills, gullies, gentle landscape contours and remnant eucalypts previously buried under the olive forest are reappearing. Landscape planning for these areas is well advanced, and some of these ‘rediscovered’ landscapes will become future horticultural exhibits, whereas some will become conservation zones restored with local native species. Areas with existing native vegetation will be targeted for conservation zones, however this restoration will be achieved over many years. Restoration ecology is now at the forefront of conservation science, and will be a key program for the Australian PlantBank, demonstrating how seed and seed technology can be used to restore sustainable landscapes and habitat.
>> Download African Olive fact sheet