Nonnative vegetation dynamics in the understory of a fragmented temperate forest

Although the conditions that render forests likely to support nonnative plants are well documented, there are surprisingly few data on the long-term dynamics of nonnative invasive species in eastern temperate forests. We examined 11 years of compositional and structural change in a small (60 ha) forest preserve in Connecticut with abundant invasive plants, a diverse land use history, and varied edaphic characteristics. We quantified the extent to which vegetation composition changed at the species, life form, and community scales and then assessed the possible factors driving these changes. Fifty-four plots were sampled in 2004, 2009, and 2015; these plots spanned two major forest types, (a) Acer rubrum L.-Fraxinus americana L and (b) Quercus montana Willd.-Quercus coccinea Muenchh. Significant changes in composition occurred over time with a greater magnitude of change in the Acer-Fraxinus forest than in the Quercus forest at all scales of analysis. Large increases in the abundance of the invasive Microstegium vimineum (Trin.) A. Camus (+1760%), Berberis thunbergii DC. (+180%), and Rosa multiflora Thunb. ex Murr (+1790%), along with a surprisingly large decline in the dominant Alliaria petiolata (Bieb.) Cavara & Grande (-86%) occurred in the Acer-Fraxinus forest. Native shrub and herb life forms growing alongside these invasive species remained unchanged or increased in abundance. Soil moisture predicted change in abundance of R. multiflora and B. thunbergii. Our results suggest that A. petiolata may be less persistent over time than previously believed and underscore the difficulties of generalizing about invasive species dynamics and relationships with native plants.