Region-wide assessment of fine-scale associations between invasive plants and forest regeneration

Invasive plants are widely spread throughout the forests of the southern United States (US) and are expected to rapidly increase their distributional ranges over the next few decades. Multiple studies have shown that invasive plants pose great challenges to forest regeneration at local spatial scales; however, little is known about how those local-scale impacts of invasive plants may collectively influence forest regeneration at a regional scale. In this study, we hypothesized that invasive plants influence forest regeneration across southern US forests by altering tree seedling and sapling abundance and diversity. We used a total of 52,690 southern US forestland plots surveyed between 2015 and 2019 by the Forest Inventory and Analysis (FIA) program of the US Department of Agriculture Forest Service. We compiled presence/absence data from 16 major invasive plant taxa and calculated the number, richness, and diversity of seedlings and saplings for all major tree species present in the dataset. We used Generalized Linear Models to examine the relationship of the selected invasive plants to tree seedlings and saplings. We then used principal component analysis to evaluate whether negative relationships appeared to be related to niche differences. We found that the presence of invasive plants generally was negatively associated with tree seedling and sapling abundance, and that these negative interactions appeared to be independent of niche associations. Moreover, fast-growing hardwood tree species had the fewest negative species-wise associations with invasive plants, which suggests that the influence of plant invasions on forest regeneration may be partly mediated by the functional traits of regenerating tree species. We also found that the invasive plants evaluated in this study differentially influenced seedling versus sapling abundance and diversity, which could have complex implications for decision-making related to forest management strategies. Ultimately, the potential impact of these invasive plants on tree regeneration is likely to hamper the sustainable management of southern US forests (natural and managed) by the reduction of extant biodiversity. Thus, invasive plants should be incorporated into management strategies to help ensure the persistence of native forest communities and the ecological services they provide.