The black locust Robinia pseudoacacia is a common invasive tree species inhabiting Japanese riverbanks. Native to the southern Appalachian Mountains in North America, it was introduced around the world for forestation and apiculture purposes. Its first introduction to Japan was reported in 1873 but it has since spread rapidly across the country’s riparian ecosystems. In the 2014 National Census on River Environments, R. pseudoacacia was found to be present in 84 % of river basins in Japan. Invasion of the black locust results in a loss of native plant diversity and reduces the floodflow capacity within river channels. When flood events occur, R. pseudoacacia tends to be washed out, and its trunks prevent downstream water flow. However, R. psudoacacia populations also tend to recover quickly after clearing. Understanding how the species spreads could allow us to develop more effective methods to clear it from riparian areas.
In their new study published in AoBP, Yaegashi et al. investigated the expansion strategy of the invasive tree based on gene flow across three sites on the Tama River, Tokyo, Japan using microsatellites. A restoration project had been conducted at one site in 2002, whilst one of the other sites was situated several kilometres upstream of this restoration site and the other several kilometres downstream. Subpopulations from the three sites showed small genetic distance (i.e. no barrier or slightly limited) and a genetically mixed population structure. Migrants seemed to settle on the riverside immediately after the clearing project through sympatric dispersal. River water then carried new migrants for settlement downstream. The authors conclude that for effective management of this highly invasive species, migrants should be removed regularly following the initial removal of invaders during site restoration.