Forests in urban and suburban landscapes are often threatened by increased pressures from urbanization and plant invasion. Forests provide important ecosystem services for cities, but urban stressors and non-native plant invasions may adversely affect biodiversity and nutrient cycling in these environments. To assess the combined effects of urbanization and invasion on tree recruitment, forest regeneration, and native understory composition, we established a network of forest sites along an urbanization and an invasion gradient. We hypothesized that native plant understory richness and density would be negatively affected as invasion and urbanization increase across the gradient, with a larger negative effect on the sapling layer. Conversely, non-native species are expected to respond positively to urbanization. Using a subset of 9 sites previously established as part of a long-term study in the FRAME (Forest Fragments in Managed Ecosystems), we sampled the abundance of plant species at each site in 20-m2 plots (n = 5-10 plots site-1; 70 plots total). We quantified both herbaceous/seedling (height ≤ 1 m) and sapling layers (height > 1 m; DBH < 2.54 cm), and measured canopy cover at 0.5 m and 1.37 m above ground as a proxy for available sunlight to these layers.
As expected, preliminary results indicate that the mean density of non-native species, both in the seedling and sapling layers increased across the invasion gradient (seedlings: r2 = 0.426, p = 0.06; saplings: r2 = 0.746, p < 0.01). Non-native saplings mean species richness showed a positive trend (r2 = 0.412, p = 0.09) with increasing invasive pressure. Our hypothesis that native species would respond negatively to invasion was not supported. Instead, native sapling mean density showed a positive trend as invasive pressure increased (r2 = 0.355, p = 0.06) suggesting that invasion may enhance recruitment of native tree and shrub seedlings to the sapling layer, but mechanisms warrant exploration. In response to urbanization, seedlings and saplings were unaffected, although non-native sapling density showed a negative trend, decreasing in response to urbanization (r2 = 0.336, p = 0.10). This was unexpected, since previous studies have reported that urbanization is often associated with higher non-native species richness and abundance. Understory soil sample analysis and land use history data at the sites will allow a more mechanistic understanding of how invasion impacts forest ecosystems, as these unexpected results may be linked to changes in soil nutrient cycling and land use history.