COS 249-5 - CANCELLED - Short-term dynamics of understory plant communities in small forests

Abstract: As human populations continue moving to urban areas, there is a growing need to understand urban forests and their capacity for regeneration. Studies have shown urban forest canopies consist mainly of native species, while understory composition contains many non-native invasive species. However, little is known about how invasion and urbanization pressures influence the seedling (impacts successional trajectories) and herbaceous (often contains greatest diversity) layers in these forests over time. Additionally, land-use history often plays a large role in the assembly of plant communities and establishment of invasive plant species populations, and is important for explaining patterns of species distributions across small forests. The objectives of this study were to assess the responses of herbaceous and seedling communities to non-native plant species across 10 small forest sites experiencing a range of non-native plant invasion, and investigate the potential for native tree and forest regeneration. We further established whether or not non-native species were increasing in abundance and/or dominance in these forest sites. We found native richness was highest and non-native richness lowest in the herbaceous plant layer, and native herb dominance differed by site but was stable. Native woody species were becoming less dominant within sites, and species that were no longer present in the mature tree/shrub canopy layer were not replaced in the seedling layer. Furthermore, we found that canopy cover, proximity to water, trails, roads, and forest edge were useful for explaining patterns of species distributions across sites. These results indicate that while the herbaceous layer seems to resist invasion more than other layers, small forests are becoming more invaded, and non-native species are increasing in abundance at faster rates than displaced native species. Invasion and urbanization influence forest plant communities and the potential for native regeneration, but the responses differ by forest strata, and these results suggest an altered future trajectory of the understory plant community.