Initially prized for its timber, Acacia trees now bring unexpected consequences to Brunei Darussalam’s biodiversity as it has become an alien invasive species to rare local Kerangas (heath) forests in the country, as discovered by researchers at the Institute for Biodiversity and Environmental Research (IBER) in Universiti Brunei Darussalam (UBD).

Due to its ability to survive in harsh conditions and regenerate quickly, Acacia was first introduced to Brunei to restore and rehabilitate degraded habitats. However, preliminary findings by IBER show that Acacia invasion has resulted in significant loss in native plant diversity in coastal Kerangas forests that make up less than one per cent of Brunei’s forests.

The FINDING, discovered by Dr Rahayu Sukmaria Sukri, Associate Professor Dr Kushan Tennakoon, Associate Professor Hajah Zohrah Haji Sulaiman, and their team of student researchers from the Bachelors and Masters programmes, calls for remedial action to restore native forest types and manage the population of Acacias in Brunei. 

Despite the absence of recent fires, the research team recorded a high density of newly germinated Acacia seedlings in invaded areas. This is a cause for concern as it is an indication that Acacias - which are fire-dependent - can grow in areas without fire triggers.

The impact of the rapid regeneration of Acacias is likely to have negative effects on Brunei’s biodiversity, according to principal investigator and tropical forest ecology expert of the project, Dr Rahayu. “One of the most detrimental effects of Acacia is that it increases the flammability of an invaded area, thus increasing fire occurrences that can potentially destroy nearby intact forests,” she said. A high number of Acacias can be found in the coastal areas of Brunei, which seem to be getting increasingly fire-prone due to the considerable number of the highly flammable plant.

The team predicts that Acacias will also negatively lower fauna diversity and affect other ecosystem attributes such as soil properties, biogeochemical cycling and water supply. Amongst many key traits which allow Acacia to dominate in an invaded area, a crucial one is their ability to convert nitrogen in the atmosphere into fixed nitrogen compounds which can then be taken up via its roots, enabling them to survive in degraded soil where few other plant species would thrive. The leaf litter of Acacias also show negative allelopathic properties, secreting biochemicals that can inhibit the growth and establishment of native plants.

The invasion of Acacia into agricultural areas can affect the economy as native crops are at a disadvantage in competing for space with these Acacias. In Brunei, for instance, the invasion of Acacias has led to the abandonment of coffee and fruit plantations in Tutong, resulting in the plantations being declared economically unviable.

According to the International Union for Conservation of Nature (IUCN), 22 invasive species of insects, birds, fish and plants have been recorded in Brunei, six of which are plant species. The list, however, has not included Acacia, which the research team hopes to add with the evidence from their study.

According to Dr Kushan, co-investigator and plant physiology expert of the project, Acacia has all the characteristics of an invasive species: fast growth, rapid reproduction, high dispersal ability, ability to alter growth form to suit current conditions, tolerance of a wide range of environmental conditions, association with humans (i.e. thought by humans to be native species of the invaded area), and prior successful invasions in other countries.

“It is high time that we incorporate Acacia into that list now that we have solid information [and so] can take the necessary steps. At the same time, Brunei is [also] moving in the right direction towards bio-security,” he said.

General concerns include pests and diseases in the destruction of plantations, but invasive plants such as Acacia also play a vital role. Dr Kushan added that the evidence-based research can serve as a stepping point for awareness of alien plant species threats.

The degeneration of the unique Kerangas forests, which once covered over six million hectares in Borneo, is a wake-up call for remedial action to be taken to protect the unique flora and fauna of these forests, including the endangered tree Agathis borneensis, locally known as Tulong and prized for its timber. 

Effective fire management and enrichment planting are two of the recommendations put forth by the team to control the invasion of Acacias. Since Kerangas forests are vulnerable to fire, effective fire management in their ecosystems is vital to prevent disturbance and subsequent invasion of Acacias.

In addition, the team is working with the Brunei Forestry Department in enrichment planting and rehabilitation programmes of already degraded forests with native species, especially those with commercial value. Dr Rahayu added that these particular native species can help restore ecosystems productivity in these areas and subsequently help boost Brunei’s economy.

Funded by a research grant from UBD and possibly the first systematic investigation undertaken in Brunei Darussalam on invasive plants, the project has drawn international interest.