Competition between the invasive alien species Solidago altissima and the native Pueraria lobata in Japan

ABSTRACT Understanding the functions, invasiveness, and control methods of invasive alien species is an important societal concern because these species have a significant impact on ecosystem functions and services, biodiversity and agricultural production. Solidago altissima, a perennial herb native to North America, has invaded and seen explosive growth in many parts of the world. However, the expansion has currently ceased in Japan, one of the severely invaded regions. We test the hypothesis that S. altissima is being outcompeted by Pueraria lobata, an expansive native species which has been spreading recently. By removing one of the two species growing together in a vegetation, their interspecific relationship was verified. After four months of regular clipping of P. lobata, the aboveground biomass of S. altissima increased significantly (2.8-fold) while that of other species remained unchanged. On the contrary, the removal of S. altissima did not change the biomass of P. lobata. This indicates that the growth of S. altissima was severely suppressed by P. lobata. We further found that the restrained growth of S. altissima was due to the reduction in the net assimilation rate, which was caused by P. lobata overshading the vegetation and preventing S. altissima from intercepting light. In the past, P. lobata was frequently utilized by humans as a source of food and medicine. As it is being harvested less these days, the constraints on its growth has reduced. The release of P. lobata from human pressure thus appears to be involved in the recent stagnation of S. altissima.