Species composition, competition, and environmental stress influence early growth dynamics in bottomland forest communities

Abiotic and biotic stressors can impede bottomland forest restoration. Native bottomland species’ performance when growing alongside invasive species under varying shade and flooding regimes may identify suitable candidates for restoration. Two greenhouse experiments tested how species composition, interaction, and environmental stress (shade and flooding) influence seedling growth and biomass. Both experiments included one invasive species, Chinese tallow , and four native species: baldcypress, green ash, sugarberry , and water tupelo. The first experiment tested competition using all pairwise species combinations and their monocultures. The second experiment grouped species into invasive, native, and mixed communities and subjected them to two treatments: light (high vs. low irradiance) and flooding (non-flooded vs. flooded). In both experiments, seedling height and root collar diameter were monitored throughout a growing season, with total biomass measured at the end to compare the actual and expected biomass difference in the mixture (in terms of biodiversity effects). Tallow and baldcypress suppressed the growth of ash, sugarberry, and tupelo, while the latter three species grew well with each other. Composition and light availability, not flooding conditions, explained more variation in growth. Growth of both invasive and native communities was highest under high irradiance. Biodiversity effects were generally governed by selection effects, or dominant species leading to higher productivity. High irradiance and flooded conditions yielded positive selection and complementary effects, while low irradiance and non-flooded conditions yielded low values. Removing invasive species alone may not be effective for restoring bottomland forests. Native species interactions under stress should be a key consideration.