UC study shows early action on small acacia populations is key to halting their spread
Australian acacias have gradually become one of the major environmental threats in the Mediterranean region, according to new findings.
A study led by Raquel Juan Ovejero, researcher at the Centre for Functional Ecology (CFE) of the Faculty of Sciences and Technology of the University of Coimbra (FCTUC) and the University of Vigo, has found that tackling small acacia populations early on is crucial to stop their spread. The findings are published in the scientific journal Neobiota.
The invasion of acacias poses serious risks to the stability of forests along the Atlantic fringe of the Iberian Peninsula. Even in small numbers, their impact is clear, disrupting both vegetation and soil. The study was conducted in the Lousã Mountains, a patchwork of pine and other introduced conifers, native oak and chestnut groves, and Mediterranean scrub. Scientists examined the effects of Acacia dealbata (silver wattle) and Acacia melanoxylon (blackwood) on vegetation, soil quality, and leaf composition, as well as on springtails – tiny soil invertebrates vital for recycling nutrients. They also explored the knock-on effects these changes can have on the overall health of the ecosystem.
“As acacia cover increases, herbaceous plants and species richness decline sharply, leading to a clear loss of biodiversity,” explains lead researcher Raquel Ovejero. “We observed a drop in the carbon-to-nitrogen ratio in the leaves and an increase in soil organic carbon, both of which alter nutrient availability and decomposition processes. There were also impacts on soil fauna, with different springtail groups reacting in distinct ways to the changes, revealing subtle but important shifts in ecosystem dynamics.”
The study warns that Australian acacias – with their ability to fix nitrogen, form dense stands and outcompete native plants – have become a major environmental problem in the Mediterranean.
“In Portugal, the situation is particularly severe. It is the Mediterranean country with the highest number of invasive acacia species, a trend driven by rural abandonment and fragmented forests,” Ovejero stresses. Galicia faces a similar scenario, with these species also spreading rapidly. “These factors increase the vulnerability of forests and scrubland, where acacias advance quickly, leading to biodiversity loss, soil changes and greater challenges in forest management.”
The researchers conclude that “early intervention is the most effective and least costly strategy, and it reduces the risk of severe ecological impacts. However, management requires continuous monitoring, as both species have persistent seed banks and strong resprouting ability after disturbance.” They also emphasise that restoring native habitats is crucial to strengthening ecosystem stability and preventing reinvasion.
In Portugal and Galicia, control measures include manual or mechanical removal of seedlings and small patches, bark-stripping, or injecting herbicide into individual trees. Larger stands are cut at the base, but these often require follow-up treatments such as repeated cutting before regrowth exceeds one metre, applying herbicide to new shoots, or treating stumps directly.
“In all scenarios, continuous follow-up is essential, as Acacia dealbata and Acacia melanoxylon both have persistent seed banks and high resprouting capacity after cutting or fire,” Ovejero concludes, reinforcing that restoring affected native habitats supports ecosystem recovery and reduces the risk of reinvasion.
This research was conducted within the MyForest pilot project, part of the regional initiative Forest For Future (F4F), a regional initiative funded by CCDRC to promote sustainable forestry in central Portugal.