Urban Forests Form Isolated Archipelagos

Urbanization decreases native species diversity by converting natural landscapes to anthropogenic cityscapes, and it indirectly degrades native species diversity in remaining natural patches by fragmenting and isolating them. Remnant forest patches embedded in an urban matrix may act as ‘islands’ separated from large populations of trees in rural ‘mainland’ forests. As such, native species richness should increase with forest patch size within the urban landscape and increased connectivity to other urban forest patches and rural forests. Our objective was to link urban land cover with forest patch richness and recruitment. We used U.S. National Park Service (NPS) data from parks in the Eastern USA to examine forest patchiness and connectivity in relatively large, intact landscapes. We applied a similar approach to field data collected along an urban–rural gradient in Western New York (WNY). We predicted that urban forest patches are isolated habitats with tree seedling and canopy richness driven by patch size and connectivity with surrounding forest patches. The WNY forest patches were surrounded by much less forest cover compared to NPS forests, and native tree species richness and abundance were negatively impacted by urban land cover. In all parks, we found increased tree seedling richness with park area, and decreasing species richness with the incursion of non-native species. Urban land-limited forest patch connectivity, particularly with rural forests, and the patches were increasingly limited to bird- and wind-dispersed trees with distance from rural areas. Our data support a conceptual model of urban forest patches behaving as island archipelagos in an anthropogenic ocean of less suitable habitat.