Many studies have attempted to test whether certain leaf traits are associated with invasive plants, resulting in discrepant conclusions that may be due to species-specificity. However, no effort has been made to test for effects of species identity on invasive-native comparisons. Here, we compared 20 leaf traits between 97 pairs of invasive and native plant species in seven disturbed sites along a southwest-to-northeast transect in China using phylogenetically controlled within-study meta-analyses. The invasive relative to the native species on average had significantly higher leaf nutrients concentrations, photosynthetic rates, photosynthetic nutrients- and energy-use efficiencies, leaf litter decomposition rates, and lower payback time and carbon-to-nitrogen ratios. However, these differences disappeared when comparing weakly invasive species with co-occurring natives and when comparing invasives with co-occurring widespread dominant natives. Furthermore, the magnitudes of the differences in some traits decreased or even reversed when a random subset of strongly to moderately invasive species was excluded from the species pool. Removing rare to common natives produced the same effect, while exclusion of weakly to moderately invasives and dominant to common natives enhanced the differences. Our study indicates that the results of invasive-native comparisons are species-specific, providing a possible explanation for discrepant results in previous studies, such that we may be unable to detect general patterns regarding traits promoting exotic plant invasions through multi-species comparisons.

Global changes such as atmospheric CO₂ enrichment often facilitate exotic plant invasions and alter soil arbuscular mycorrhizal fungi (AMF) community. However, it is still unclear whether the effects of CO₂ enrichment on exotic plant invasions are associated with its effects on root-AMF symbiosis of invasive and native plants. To address this issue, the annual invasive plant Xanthium strumarium and two phylogenetically related annual natives were compared under ambient and elevated CO₂ concentrations for three consecutive years. Atmospheric CO₂ enrichment increased AMF colonization rates for the species only in few cases, and the invader did not benefit more from CO₂ enrichment in terms of AMF colonization. Under ambient CO₂ concentration, however, the invader had a higher AMF colonization rate than the natives in the first year of the study, which disappeared in the second and third year of the study due to the increase of AMF colonization rates in the natives but not in the invader. The influences of species, CO₂ concentrations and planting year on AMF colonization were associated with their effects on both soil nutrient and AMF community, and the former may be more important as it also influenced the latter. Our results indicate that the invader could more quickly form symbiosis with soil AMF, contributing to adaptation and occupation of new habitats, and that it is necessary to consider the roles of AMF and the effects of time when determining the effects of global changes such as atmospheric CO₂ enrichment on exotic plant invasions.

High nitrogen (N) uptake is one of the main reasons for invasive alien plant invasions. However, little effort has been made to compare the effects of different N forms on N uptake between invasive and native plants, especially those on N form acquisition strategies (preference and plasticity), which influence N uptake, and thus exotic plant invasions. Related studies are particularly few in barren habitats, where the effects of N deposition on invasiveness are considered to be much weaker than in fertile habitats. In this study, we grew Solanum rostratum, a noxious invader in barren habitats, and the native plants Leymus chinensis and Agropyron cristatum in both mono- and mixed cultures under nitrate and ammonium addition treatments, and analyzed the effects of the soil N availability and forms on the growth, N uptake, and N form acquisition strategies for these plants. The invader outperformed the natives in N uptake (in most cases) and growth (always) in both mono- and mixed cultures under all N treatments. N addition increased the N uptake and growth of the invader. The advantages of the invader over the natives were higher under ammonium relative to nitrate addition. The growth advantage of the invader was associated with its higher N uptake and higher N-use efficiency. Higher plasticity in N form uptake may contribute to the higher N uptake for the invader when grown in mixed cultures. Our findings indicate that N deposition, particularly in the form of ammonium, may accelerate exotic plant invasions in barren habitats.

Global changes, such as atmospheric nitrogen deposition, can facilitate alien plant invasions, which are often attributed to the increase in soil nitrogen availability. However, few studies have considered the effects of global change-driven alterations in soil nitrogen forms, especially under conditions with interspecific competition. In this study, we first determined the differences in growth, biomass allocation, and photosynthesis under different nitrogen forms and addition levels between three noxious invasive species (Xanthium strumarium, Ambrosia trifida, and Bidens frondosa) and their respective related natives grown with and without interspecific competition and then assessed the interspecific difference in nitrogen form preference using the ¹⁵N labeling technique. Interspecific competition significantly decreased the positive responses of growth to nitrogen addition for all three natives, while increasing the responses for all three invaders, particularly under nitrate addition. When grown in competition, all invaders showed significant growth advantages over their related natives in most cases, and responded more positively to the addition of nitrate relative to ammonium, while the natives responded more positively to ammonium addition. These findings indicate that the invaders prefer nitrate, while the natives prefer ammonium. Consistently, the growth advantages are more pronounced for the invaders under nitrate relative to ammonium addition, indicating that nitrate-rich habitats may be more vulnerable to the invaders. When grown in monoculture, however, the growth advantage of the invaders became smaller or even disappeared. Nitrogen form preference also disappeared in Siegesbeckia glabrescens (native) and Bidens frondosa (invasive). Interestingly, the native plant Xanthium sibiricum showed significantly higher total biomass than its invasive congener under ammonium addition in both mixed and monoculture conditions. Our ¹⁵N labeling experiment showed that all six species preferred nitrate over ammonium, although this was not significant for two natives (S. glabrescens and X. sibiricum), which is not completely consistent with the results from our nitrogen addition experiment. Our results indicate that global change-driven alterations in soil nitrogen forms, particularly the shift from ammonium to nitrate, may facilitate alien plant invasions. Planting patterns significantly affect the responses of invasive and native species to nitrogen forms and addition levels, with mixed-culture experiments providing better insights into the invasiveness of alien species.