Community assembly explains invasion differences between two contrasting forest types

Plant communities within a metacommunity can vary widely in their degree
of invasion by introduced species. Disturbance, propagule pressure, and
biotic resistance are common explanations for this variation, but
empirical evidence for these hypotheses is mixed. Alternatively, the
community assembly framework predicts that local assembly filters
determine both native and exotic composition, but lower trait variation in
the introduced species pool may exclude them from certain sites. We
examined evidence for this framework using observational data from forests
and woodlands of Long Island, NY, USA. These forests vary in vegetation
composition and invasion along a soil gradient. They are also highly
disturbed and fragmented, yet some stands have almost no introduced
plants. Using data collected in 1998 and 2021-22, we quantified
relationships between community composition, soil characteristics, and
functional traits for native and exotic assemblages, as indicators of
environmental filtering. We found similar trait-environment relationships
in native and introduced species, suggesting that both groups follow the
same local assembly rules. Introduced species were predominantly found in
sites with more nutrient-rich soils and were absent from sites with
nutrient-poor soils. At the regional scale, the exotic species pool was
biased toward trait values favored in more nutrient-rich environments,
particularly high growth rates and low leaf C:N ratios, which explains
their absence from nutrient-poor environments. These patterns were
consistent over time, and stands that were uninvaded in 1998 remained so
in 2021-22, supporting the robustness and reliability of short-term
studies. Synthesis: This study shows that invasion patterns in plant
communities can be explained by the assembly rules that govern native
species. By linking local environmental filtering with regional species
pool characteristics, this work advances our understanding of how some
communities remain uninvaded despite high disturbance and propagule
pressure. Overall, these results highlight the utility of the community
assembly framework, and emphasize the importance of regional processes in
constraining the local distribution of introduced
species.