Distribution and characteristics of the soil seed bank of the black locust (Robinia pseudoacacia) in a headwater basin in northern Japan

Invasive expansion of Robinia pseudoacacia is a worldwide problem. A method to control existing R. pseudoacacia populations is urgently needed. In addition to plants above the ground, seed banks should be targets for the management of R. pseudoacacia. We targeted an old plantation of R. pseudoacacia and its surroundings in a headwater basin to determine the spatial characteristics of the distribution. We established 49 square plots in which the number of buried seeds and seeds dispersed by wind, and several other related environmental factors were surveyed. The relationship between the spatial extent of seeds dispersed by wind and the distribution of the seed bank was analyzed. Environmental factors that potentially influenced the density of buried seeds were also analyzed. We found that barochory and anemochory were the main factors that contributed to R. pseudoacacia forming seed banks. The greatest factor controlling the density of buried seeds was slope angle; thus, transfer of seeds by rolling is important for the formation of seed banks in headwater basins. From these results, some guidelines for the management of seed banks in headwater basin are presented. First, unnecessary soil disturbance within about a 100-m buffer area around seeding trees should be avoided. Second, plantation soil from gentle slopes should not be applied for revegetation of other locations. In situations where R. pseudoacacia populations are planted on steep slopes near streams, there is a chance that seeds will be carried away and form seed banks at lower or downstream sites.