Invasive plants may dramatically impact forest ecosystems by establishing dense populations and suppressing the recruitment of native tree species. One invasive shrub currently spreading throughout eastern deciduous forests of North America, Japanese barberry (Berberis thunbergii DC), may be limiting tree recruitment in stands where it invades. Once established, B. thunbergii becomes densely populated within forest understories and suppresses native plants by competing for limited resources, altering soil conditions, and changing the understory microclimate. To quantify native seedling inhibition caused by B. thunbergii invasion, we conducted an observational study on seedling abundance within forest plots that were either invaded or not invaded by B. thunbergii and used survey data to generate Bayesian models of native seedling densities along gradients of increasing B. thunbergii stem counts and aboveground plant dry mass. Model outputs predicted that B. thunbergii–invaded plots had 82% lower seedling densities compared with uninvaded plots. Native tree seedling densities were very low even in areas with moderate B. thunbergii density, suggesting that reduced tree seedling densities are observed even at low densities of invasion. Our findings indicate that forests invaded with B. thunbergii harbor substantially lower densities of native tree seedlings, with potentially significant long-term consequences for forest ecological integrity, biodiversity, and ecosystem services.

For over a century the foundation of biological conservation has been the development of open space networks either through outright public land acquisition or appropriate management of private lands. Because both approaches come with significant trade-offs, it is critical to understand which species are found across various land ownership types so that policy tools and management actions can efficiently be targeted to do the most good. In this paper, presence-only biological data were used to create species distribution maps for 18 imperilled forest bird species that breed within the deciduous forests of New Jersey (USA). These maps, combined with publicly available, spatially explicit information on land ownership, document who owns the habitat relied on by each of these 18 species. There were significant variations in both species- and guild-specific reliance on public versus private lands, with the latter preferentially supporting nearly twice as many species as the former. Subcategories of land ownership provided support for the role of both state-owned forests and privately-owned agricultural lands in forest bird conservation; however, each landownership type supports a distinct set of species. While explicitly recognizing the need to employ diverse conservation strategies, the approach provides a solid framework for structuring forest conservation planning and policy at regional scales.