Regeneration of urban forests as influenced by fragmentation, seed dispersal mode and the legacy effect of reforestation interventions

Urban forests are highly fragmented in mega-cites, acting as islands in terms of preserving species diversity. To maintain the ecological services of urban forests, management measures such as reforestation have been implemented, which might have a long-term effect on biodiversity. To understand how fragmentation and reforestation affect the natural regeneration of urban forests, we investigated the relationship between forest area, isolation and seed dispersal mode and the diversity and composition of woody species at the tree, seedling, and sapling stages in 28 secondary forests of the mega-city of Wuhan, China. We found that the alpha diversity of woody species was positively correlated with forest area, while their beta diversity was negatively correlated with forest area. The beta diversity of nonanimal-dispersed species significantly correlated with isolation. Animal-dispersed plants had consistently a higher alpha diversity from trees to seedlings and saplings, while their beta diversity was lower than nonanimal-dispersed plants at the seedling and sapling stage. The community composition of woody plants in urban forests was largely congruent among the three life stages. However, only the communities of animal-dispersed plants were consistent across life stages in small or highly-isolated forest patches. The results show that the woody plant diversity of urban forests is largely similar to that expected by island theory. Animal-dispersed trees are more likely to regenerate successfully due to a more diverse set of species used in reforestation and their higher tolerance to urban forest fragmentation. More management measures for nonanimal-dispersed species, such as enriching and repeating reforestation, will be required to maintain their high biodiversity in urban forests.