Edge and the city: Evaluating the role of edge effects on urban forest structure and tree species composition

Abstract

Urban forests are highly valued for the important services they provide (e.g., carbon sequestration, supporting biodiversity), many of which are closely linked to ecosystem structure and tree species composition. High pressure from non-native species and a prevalence of edge effects often characterizes urban forests, but how the confluence of these factors alters tree species composition and structure remains uncertain. We hypothesized that edge effects would increase urban forest basal area and stem density but also shift tree communities towards higher abundances of non-native species. We tested these hypotheses in the forests of New York City, New York using two approaches that employ: (1) high spatial resolution data from 8 transect-style plots and (2) a New York City-wide ecological assessment that includes 1124 plots. We find that urbanization and fragmentation each play a distinct role in shaping the structure and composition of the urban forest. Forest basal area and stem density were significantly higher near forest edges compared to the interior. By contrast, edge effects had only a small impact on the occupancy rates of non-natives in the canopy. Instead, we found evidence of compositional shifts between the canopy and regenerating forest strata exemplified by native oaks (Quercus spp.), which currently dominate the canopy, being replaced within the regenerating stratum by other native and non-native species. Taken together, these findings suggest that urban forests likely store more carbon than initially assumed, but also that the urban forests of tomorrow will likely be compositionally different, with important implications for ecosystem function.