Anthropogenic activities greatly increase the spread of invasive species into fragmented urban ecosystems. Our current knowledge of nonnative invasive plant effects on ecosystems has focused mainly on comparing invaded and uninvaded sites, whereas our understanding of how invasive plant abundance impacts ecosystem structure and function is less well known. The goal of our investigation was to assess the vegetation composition and structure of forests across an abundance gradient of nonnative plant invasion. We hypothesized that decreasing species richness and density would be linked to increasing abundance of nonnative plant species. In order to address our hypothesis, we studied vegetation characteristics across 38 forest fragments along an invasion gradient in northern DE and southeastern PA. In each forest fragment, we identified and measured all trees and shrubs > 2.54 cm dbh in ten 401 m2 subplots (380 total subplots), and we measured stem density of all species ≤ 1 m height (trees, shrubs, vines, herbaceous plants) in ten 6.25 m2 subplots. Non-metric Multidimensional Scaling (NMS) was used to assess patterns in vegetation composition and structure across forests. Finally, we analyzed landscape metrics with the objective of identifying specific factors responsible for the vegetation structure patterns observed in urban forest fragments.
Preliminary results from the NMS analyses support our hypothesis of greater tree and shrub (> 2.54 cm dbh) richness and density in forest fragments with fewer nonnative species present (richness: r2 = 0.107, p = 0.07; density: r2 = 0.292, p < 0.01). In contrast, we found understory species richness and density increased in forest fragments with more nonnative species present (richness: r2 = 0.319, p < 0.01; density: r2 = 0.672, p < 0.001) suggesting that there is an increasing contribution of nonnative species to the stem density and richness lower in the canopy. Furthermore, we found a strong direct relationship between nonnative stem density and total stem density across forests (r2 = 0.889, p < 0.001) providing further evidence that nonnative invasive plants are potentially altering the vertical forest structure. Our analyses also revealed that more nonnative species were present in younger forest sites suggesting older forest fragments may be resistant to invasion by nonnative invasive plants. Future analysis of direct relationships between the abundance of nonnative plant invasion and plant productivity will provide greater insight in to specific invasive plant mechanisms that alter ecosystem structure and function across urban forest fragments.