Effects of tree size and park maintenance on soil seed bank of Gleditsia triacanthos, an exotic tree in urban green areas.

The exotic honey locust (Gleditsia triacanthos) is often planted as ornamental tree in urban parks. In Hungary, it occasionally escapes cultivation, in other countries it has already become invasive, and thus, further spread cannot be ruled out. The production of copious long-lived seeds may contribute to its invasiveness. We investigated the soil seed bank of honey locust growing in urban parks of Budapest (Hungary). Soil samples of 1200 cm3 were taken under the canopy of 48 solitary female trees, seeds were extracted by washing, and their viability was assessed by germination test following scarification. For each tree, trunk circumference and level of park maintenance were recorded. Seed bank density varied between 0 and 500 seeds/m2 and in 17 out of 48 cases, it exceeded 100 seeds/m2. Larger trees tended to form better-developed soil seed bank than smaller ones, yet the level of park maintenance has a much stronger effect: trees in neglected parks produced five-times higher density on average than those in perfectly managed parks (126.4 and 24.5 seeds/m2, respectively). For a better understanding of the species' invasion risk, detailed seed ecological studies are needed and to prevent its further spreading, a more careful treatment of its litter is recommended.