A plant species in a community often grows with some other plant species. While many studies have assessed interspecific interactions between two target plant species, few have considered the impacts of the other plant species (e.g., the third, fourth, and fifth plant species) on these interactions. To assess the impacts, we grew one seedling of each of the five herbaceous plant species that are common in China (Cynodon dactylon, Plantago asiatica, Taraxacum mongolicum, Nepeta cataria, and Leonurus japonicus) alone (no competition) or with one seedling of one, two, three, or four of the other species. The presence of a neighbor plant generally reduced the growth of the target species, suggesting that the interspecific relationships were mostly competitive. The presence of other neighbor species (the third, fourth, and fifth species) could alter the interspecific interactions between two target species, but such effects varied depending on both the identity of the target species and the identity of the other species. Additionally, the effects of the third species depended little on the presence of the fourth and fifth species. We conclude that interspecific interactions between two plant species are commonly regulated by the presence of other species, facilitating species coexistence. However, our findings do not support the idea that the impacts of the fourth and fifth species on interactions among three plant species are common. This study highlights the complex interactions among multiple plant species within a community and also the importance of including these high-order interactions when modelling community dynamics and species coexistence.

Aims Considerable evidence suggests that more diverse communities of native plants more strongly resist invasion by introduced plants. Here we tested whether biotic plant-soil feedbacks can explain this relationship independently of other factors, via either soil richness, as based on the number of different plant species interacting with soil; or soil heterogeneity, the degree to which plant-soil feedbacks involving different plant species are spatially separated. No previous study appears to have tested both soil richness and heterogeneity as components of biotic plant-soil feedbacks that might explain why more diverse native plant communities are less invasible. Methods We conditioned soils with monocultures of six native plant species and grew five introduced plant species individually in sterilized soil inoculated with one, two, or four of the conditioned soils, keeping the conditioned soils separate or mixing them. Results Soil richness had little effect on the final dry mass of any introduced species. Higher soil heterogeneity did not decrease final mass in any introduced species and instead increased it in one. Conclusion Results suggest that biotic plant-soil feedbacks are not in themselves an important mechanism by which diversity limits invasibility but do not rule out the possibility that such feedbacks play a role in combination with other mechanisms such as abiotic feedbacks or plant competition.