How is Robinia pseudoacacia riparian forest formed at Chikumagawa River in central Japan?

Background/Question/Methods

Tree is focused for its capability of fixing carbon as a countermeasure against global warming. Robinia pseudoacacia (Black Locust), which is a tree species native to North America, has been planted worldwide because of its remarkable growth. R. pseudoacacia, however, is regarded as problematic tree species, as its trait of biological invasion is recognized. In Japan, R. pseudoacacia has been planted for greening and preventing soil outflow. The distribution of R. pseudoacacia has been expanding through natural regeneration, especially into riparian areas and bringing on some problems such as narrowing of river width and reduction of biodiversity. In order to control useful alien species properly, it is important to understand the process of its expanding and establishing outsides of its plantations. We investigated forming processes of riparian R. pseudoacacia forests through dendroecological and microsatellite polymorphism analyses. Trees were sampled from three sites (sites A, B, and C); including two plots (plots A1 and A2). For genetic analysis, a fragment of cambial tissue was excised from every R. pseudoacacia tree with a DBH >5 cm at two plots in site A and 25 R. pseudoacacia trees randomly chosen from 600 m length in sites B and C.

Results/Conclusions

The dendroecological analysis was helpful to presume that R. pseudoacacia trees reproduced asexually or sexually. The growth increments at first year of possible individuals with asexual reproduction were significantly larger than those with sexual reproduction. AMOVA with seven microsatellite markers showed low genetic differentiation among site-by-site populations (among and within population variation contributed 3% and 97% to the total variation, respectively). Parentage analysis demonstrated most individuals had parent-child relationships within their own study sites or plots. Individual identification clarified that ramet trees belonging to the same genet clustered within the plot and the existence of two places where R. pseudoacacia trees of asexual reproduction were dominating and those of sexual reproduction were dominating. Formation processes of R. pseudoacacia in our study sites were speculated as bellow. At first, some R. pseudoacacia seeds derived from mother trees in a wide area were dispersed at random. Once R. pseudoacacia trees established, they started growing and bearing seeds there, with some trees expanding their distribution asexually through adventitious buds on roots. After that new-born seeds were dispersed near to their mother trees and started germinating under comfortable conditions.