Trait-mediated responses of native plant diversity to combined urban and alien pressures in wastelands

Urban wastelands, noval anthropogenic ecosystems, remain understudied using integrative frameworks linking community assembly to ecosystem management. Current research of these systems largely treats urbanization and alien spread as isolated, overlooking their combined effects. We examined how the combined impacts of urbanization and alien plants drive native plant community restructuring in urban wastelands, quantifying native plant diversity loss and assessing soil stoichiometric mediation in these interactions. We measured trait-niche relationships along urbanization-alien gradients to reveal adaptive trait shifts. The results revealed that high native plant diversity was sustained under conditions of low urbanization combined with low alien plant diversity. Soil nitrogen emerged as a keystone mediator driving the loss of native plants. Native niche centroids shifted along urbanization and alien plant gradients, with leaf length decreasing and leaf economic traits shifting acquisitively, characterized by increased nitrogen and decreased dry matter and carbon-to-nitrogen ratio. The combined effects of urbanization and alien diversity reduced native diversity in urban wastelands, mediated in part by nitrogen-induced soil imbalances. These shifts push natives toward acquisitive strategies, compromising stress tolerance and accelerating functional homogenization. Effective management must integrate alien plant control, nitrogen mitigation and trait-based restoration to counter the urban-alien combined effects on native diversity and composition.