Understanding the functional differences between urban and rural habitats is a critical focus for urban ecology and the sustainable management of urban natural areas. Early recruitment processes are critical to determining future forest structure and composition. Some studies have observed suppressed recruitment of native plant species in urban forests, but the ecological barriers to regeneration are not well understood. Here we present the results from a three-year experimental field study that examines seed production, seed dispersal, seed predation, and early establishment in urban and rural oak-hickory forests in and around New York City. Using a series of nested experiments in six urban and rural forest sites, this research asks: (1) What is the difference in the composition and abundance of woody plant species in the seed rain, seed bank, and vegetation, including seedling, sapling and canopy layers? (2) What is the difference in seed removal rates for native and non-native species? (3) What is the difference in the contribution of seed production, seed dispersal, post-dispersal, and early-establishment recruitment limitation in urban and rural forests? Answers to these questions were framed using a combination of seed traps, caged and uncaged seed addition plots, natural regeneration plots, and a cafeteria experiment.
Results/Conclusions
This experiment identifies multiple life-history stages that contribute to increased recruitment limitation for native plants in urban forests. In both urban and rural sites, significant differences were observed in the composition of the seed rain, seed banks, and vegetation strata. However, greater dissimilarity and abundance of non-native species were observed in urban forests. Seed predation rates fluctuated among years in rural sites, while in urban forests seed removal was sustained, and relatively high, across all three years. Additionally, native species experienced greater seed removal than non-native species in both forest types. These findings suggest that urban forests deviate from temporal patterns of seed predation of rural forests dominated by masting tree species, a significant urban-driven shift in a basic community ecology dynamic. Finally, the seed addition experiment determined that urban forests were significantly more recruitment limited than rural forests. Urban forests are characterized by greater seed and site limitation. Collectively, these findings provide evidence of urban-driven changes in forest ecological processes. By determining processes that may limit regeneration in cities, we may begin to develop restoration and management strategies better able to sustain our urban forests, so important to urban well-being.