Seasonality of sprouting in the exotic tree Robinia pseudoacacia L. in Hokkaido, northern Japan

Robinia pseudoacacia L., one of the major invasive tree species in Japan, has been used for coppice wood production worldwide. However, we have little knowledge of the species’ sprouting characteristics. Here, we tested seasonality in R. pseudoacacia sprout development. We removed aboveground parts of R. pseudoacacia at different times throughout the year (spring, summer, autumn, and winter) and censused sprout demographics for three consecutive growing seasons thereafter. The most vigorous sprouting was observed among trees cut in winter, while sprouting was lowest among those cut in summer. Sprout abundance in the spring- and autumn-cut treatments was intermediate between that of winter- and summer-cut treatments. Both trunk diameter and season affected stump sprout biomass after three growing seasons, following the same seasonal pattern as above. We found a negative relationship between the herbaceous vegetation cover height and the biomass of root suckers. Both trunk removal in summer and consecutive sprout-clipping treatments contributed to the decline of living stumps. Our results show that R. pseudoacacia trees should be cut during the dormant season to facilitate coppicing and in the summer to facilitate control or removal. Conservation of herbaceous vegetation cover, together with consecutive sprout clipping, will aid the complete removal of this species.