Plant species surplus in recent peri-urban forests: the role of forest connectivity, species’ habitat requirements and dispersal types

In peri-urban regions, the establishment of new forest patches can be affected by specific community assembly rules driving species extinction debts and colonization credits. This might result in species surpluses or deficits and shifts in species composition along forest establishment, whose association with human-driven landscape transformation remains largely unknown. We have compared plant assemblages in peri-urban Mediterranean forests of contrasting history (recent vs. preexisting; post- vs. pre-1956) according to species’ habitat requirements (forest, scrub-grassland, synanthropic or other) and dispersal mode (vertebrate- vs. non-vertebrate), while considering site and landscape characteristics. Species richness and cover were mostly associated to site characteristics, but also to forest connectivity and history. Connectivity increased total species cover, forest species richness and both richness and cover of vertebrate-dispersed species, while reducing non-forest species richness and synanthropic species cover. Regarding forest history, recent stands showed higher total and non-forest species richness, higher scrub-grassland species cover and lower cover of vertebrate-dispersed species than preexisting forests. Thus, recent stands exhibit a species surplus due to the extinction debt of scrub-grassland species and the transient abundance of synanthropic species, despite the observed colonization credit of vertebrate-dispersed species. These results emphasize the importance of forest connectivity when managing plant species assemblage in peri-urban forest patches but also highlight the need to consider recent history of habitat patches to manage time lags in biodiversity change in human-transformed landscapes.