Competitive superiority of aliens over natives under abiotic and biotic stresses in legume and nonlegume woody species

Resource utilization is considered a crucial determinant of alien plant species in terrestrial ecosystems under abiotic and biotic conditions of global change. Alien plants are often favored over natives in stress-free or resource-rich ecosystems. However, certain resource-poor ecosystems have also been heavily invaded, particularly by legume woody species. How alien and native woody species compete in various abiotic and biotic stress environments and whether the functional traits associated with resource utilization promote their performance remain unknown. To test this, we grew six naturalized alien and six native woody species, grouped into three pairs of legumes and three pairs of nonlegumes, individually or in competition, under benign and two abiotic stress (drought, limited nutrients) and two biotic stress (aboveground enemies, belowground enemies) conditions. Overall, the four stress conditions had more negative effects on native plants than on alien ones, especially for nonlegumes under abiotic stresses. Moreover, when grown in competition, the presence of stress increased the growth asymmetry between alien and native plants in favor of the alien plants, but this was less pronounced in the legume group than in the non-legume group. Our study suggests that alien woody plants may have a competitive advantage over native ones under diverse abiotic and biotic stress conditions, but that this depends on their nitrogen-fixing ability. This is likely to affect the coexistence of alien and native woody species and may facilitate the spread of alien plants into stressful habitats.