COS 294-3 - Non-native plant abundance and composition varies across urban forest soil seed banks

Abstract: Soil seed banks are living reservoirs that structure plant communities across both space and time in urban forest ecosystems. While a large body of literature exists on the dynamics and composition of soil seed banks in temperate deciduous forests, relatively little is known about soil seed banks in urban forested natural areas within these same regions. Urban forests are generally considered to have a greater amount of non-native and invasive species, especially in their understories, than their rural counterparts. To what extent this aboveground invasion can be explained by recruitment from the seed bank is not well understood. Previous studies have restricted analyses to urban forest seed banks of single cities or single spatial scales. A better understanding of how non-native composition in the seed bank varies across cities and urban forest gradients will allow us to more successfully prioritize interventions for restoration.

To survey seed bank composition, we collected soil samples from 25 urban forests across five US east coast cities between North Carolina and Massachusetts. We took five soil cores from each urban forest and conducted seedling emergence trials from these samples in greenhouses at the University of Delaware. Throughout the 2022 growing season, all emergents from soil samples were counted and identified to species.

We observed averages greater than 50% non-native emergents in six of the 25 forests included in our study, while an additional five forests had averages above 25%. The most abundant non-native emergents were princess tree (Paulownia tomentosa), Japanese stiltgrass (Microstegium vimineum), and Japanese angelica tree (Aralia elata). Paulownia tomentosa and Microstegium vimineum were found in four out of five cities, but Aralia elata was restricted to forests in Philadelphia. While the most invaded seed banks occurred in Philadelphia and Baltimore, Bayesian models suggests that non-native plant abundance in the seed bank varies most at the forest-level, rather than at city or within-forest levels. Our results suggest that non-native seeds are a major component of many urban forest seed banks but vary widely across the spectrum of urban forest gradients observed in our study. While this may mean that the seed bank may be a prominent pathway for invasives to become established in urban forests, the success of these species will likely be a product of both their competitive ability and forest disturbance events.