Spread of the invasive alien species Piper aduncum via logging roads in Borneo

We examine how spiked pepper (Piper aduncum L., Piperaceae), a shade intolerant, animal-dispersed Neotropical tree, is spreading in the interior of Borneo. Concerned that logging roads might be facilitating this spread, we made a series of observations, relating tree distribution, location and road history, in a concession in East Kalimantan. These roads will connect West Kutai and Malinau Districts and may allow alien plants to disperse from one to the other. We observed that P. aduncum was already well established on the oldest, southern portions of the logging road network, but was absent on the newest roads to the north. A few scattered individuals occur on the roadside as much as 150 km beyond the main areas dominated by P. aduncum, suggesting an occasional ability to achieve long-hop dispersal. Rivers of 30 m width are not a barrier to P. aduncum’s spread. Based on road age, we estimate a minimum rate of spread between five and seven km per year. We infer that logging roads are assisting P. aduncum to spread and the tree will become widely established in Malinau District. Prevention of this spread would require urgent, intensive and coordinated control over the length of the road network and, more generally, major restrictions on how such roads are located and managed.