Ecological diversity depends on mechanisms that promote species coexistence, yet some non-native plants form monospecific stands, suggesting an ability to escape self-limiting processes that constrain native flora. Whereas invasion success is often attributed to enhanced interspecific competition, it may also result from reduced intraspecific competition within introduced populations. Using a controlled greenhouse experiment, I compared competitive outcomes among single native and introduced populations of five plant species (Gleditsia triacanthos, Pinus sylvestris, Paulownia tomentosa, Solidago canadensis, Viola sororia). Individuals were grown in within-range (native × native, introduced × introduced) and across-range (native × introduced) pairings to test whether introduced populations exhibit weaker intraspecific competition. Introduced populations generally showed reduced self-limitation compared with native conspecifics, consistent with relaxed intraspecific density dependence. Because only one population per range was tested, these findings reflect population-level contrasts rather than species- or range-wide generalizations. Even so, the results indicate that modest reductions in self-limitation at the population level can substantially enhance the potential for invasive dominance, highlighting relaxed intraspecific competition as an underrecognized pathway to invasion success.

Experimental set up

Greenhouse competition experiments were conducted from April to August in 2019 (P. sylvestris), 2020 (G. triacanthos, S. canadensis), and 2021 (P. sylvestris, P. tomentosa, V. sororia). Groups of 4–6 introduced- and home-range seeds of the same species were sown 2 cm apart in 10 × 10 cm pots filled with commercial potting soil (Fox Farm ocean forest, Arcata, CA USA). After germination, seedlings were thinned to ensure a 1:1 competition pairing of similar-sized individuals. Each experiment included three competition pairings: (1) Introduced × Introduced (n = 132 individual plants), (2) Home × Home (n = 148) and (3) Introduced × Home (n = 183). This resulted in a total of n = 463 plant replicates, with variation in species-level replication due to differences in recruitment success: G. triacanthos (n = 160), P. sylvestris (n = 115), P. tomentosa (n = 108), S. canadensis (n = 50) and V. sororia (n = 30). The plants were grown under ambient spring/summer light conditions (~1200–1500 max. PAR), watered three times per week, and lightly fertilized (FoxFarm Happy Frog all purpose fertilizer, 6-5-4 NPK, Arcata, CA USA). Greenhouse temperatures ranged from 13–30°C. After five months, plants were harvested, oven-dried at 70°C for three days, and weighed for aboveground biomass. Aboveground biomass was selected as the primary response variable due to the difficulty of partitioning belowground tissues and because it sufficiently represents plant competition under controlled conditions (Zou et al. 2008, Kiær et al. 2013, Zhang et al. 2019).

Data analysis

Plant biomass (recruitment growth) was analyzed as a function of origin (home-range, introduced-range) and competition type (within-range, across-range). Given that the study focused on competition type by origin rather than species-level effects, I employed a linear mixed-effects model with species identity as a random effect, using R statistical software (R Core Team, 2023). The random effect accounted for species-level autocorrelation while allowing treatment effects to be assessed across species. In the random-effects model, species were assumed to be drawn from a common distribution, facilitating partial pooling of estimates, to account for the uneven number of replicates between species. This approach mitigates the influence of groups with fewer observations by shrinking their estimates toward the overall mean, thereby preventing inflated or overgeneralized effects from smaller sample sizes. Because the experiments were conducted in different years (2019, 2020, 2021), year was included as a blocking factor to account for temporal variation. Additionally, since the study included both woody and herbaceous species, which differ in growth form and developmental trajectories, growth habit (woody, herbaceous) was included as a covariate to capture potential effects during seedling recruitment. Finally, as the hypothesis predicted that the effect of competition type would vary by origin, I included a type × origin interaction term to assess this relationship.