Alien Trees Use Logging Roads to Invade Borneo Forests

One tropical tree from Central and South America has made itself at home in Borneo where it is crowding  out local vegetation and interfering with forest recovery. Although this tree has been slow to spread through Borneo since its introduction in 1952, new logging roads appear to be driving the species farther afield. Read more below to find out more about this invasive species and how scientists studied this threat.

Read more at Mongabay.

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A recently cleared logging road without P. aduncum (top), and an older logging road with heavy growth (bottom). Photo by: Padmanaba and Sheil.

A recently cleared logging road without P. aduncum (top), and an older logging road with heavy growth (bottom). Photo by: Padmanaba and Sheil.

The spiked pepper tree (Piper aduncum) is native to the American tropics, but has made itself at home in a variety of other locales where it can crowd out local vegetation and interfere with forest recovery. Although it’s been slow to spread through Borneo since its introduction to Indonesia in 1952, new logging roads appear to be driving the species farther afield. A study in mongabay.com’s open access journal Tropical Conservation Science raises concerns that these roads may bring unintended plant colonists to new areas – putting the biodiversity of forests at risk.

Invasive species such P. aduncum are organisms that leave their natural habitat and spread to new environments, often displacing native species and upsetting the balance of ecosystems in the process. Logging roads are already known to create environmental problems by dividing wildlife habitats. Now, using the spiked pepper tree as a test species, scientists examined the potential for these roads to increase the spread of invasive plants. Combining observations with road-building history, the study shows these trees advanced along logging routes at a minimum rate of five kilometers (three miles) each year. At that pace, the pepper trees are set to steadily march towards the Malinau region of Kalimantan, threatening one of Borneo’s richest areas of flora. 

“These threats and the underlying processes that lead to them are poorly studied in much of the species-rich tropics and require more attention from researchers and conservation managers,” the scientists write in their paper. 

This logging road in Sabah, Borneo may also be a travel route for invasive plant species. Photo by: Rhett A. Butler.

This logging road in Sabah, Borneo may also be a travel route for invasive plant species. Photo by: Rhett A. Butler.

The researchers created a distribution map of pepper tree density along 200 kilometers (124 miles) of roads that have been gradually extended in a logging concession in the East Kalimantan area of Borneo over a 27-year time span. Their sampling route started in the West Kutai District where pepper trees were already well established at the southern end of a road that was built in 1980, running northward along more recently built roads in the Malinau District where the trees were not yet found. They recorded pepper tree sightings within five meters (16 feet) of the road and counted tree stems in plots every five kilometers (three miles). By assessing their tree counts against a timeline of road construction based on logging company records, they estimated the rate at which the trees advanced along roadways.

The researchers note that their study didn’t look at the role of riversides or agriculture in creating open land areas that may also have helped the shade-averse pepper trees to spread. The Boh River runs through both districts and there were five villages that used small-scale farming in the study area. 

“We consider this assessment to be a case-study ‘snapshot’ of one alien species at a particular time and place,” the scientists write. They conclude that immediate and coordinated control over the entire road network would be needed to prevent further spread of the spiked pepper tree. Their work further suggests that conservation management practices should reflect the role roads play in the spread of invasive species.