Soil environmental nutrient constraint changes the negative association between alien species proliferation and native functional niche dimensionality in urban wastelands

Urban wastelands are hotspots of alien plant invasions in the context of urbanization and habitat fragmentation. Understanding the mechanisms behind plant dynamics in these areas is crucial. We established 100 plots in urban wastelands in Guiyang, China, measuring 11 functional traits for both native and alien plants, and assessing trait integration (with and without intraspecific variation). We derived an environmental nutrition constraint index based on soil carbon, nitrogen, and phosphorus content, and quantified the alien species proliferation index. We analyzed the individual and interactive effects of environmental nutrition constraint and trait integration on alien species proliferation. We found that alien species proliferation was positively correlated with trait integration in native plants and with the native–alien trait integration difference, but negatively correlated with trait integration in alien plants. However, increased environmental nutrition constraint negatively moderated the positive relationship between alien species proliferation and the trait integration difference. Specifically, the lowest proliferation occurred when both the trait integration difference and soil environmental constraint were low, whereas high constraint levels resulted in high proliferation even when the trait integration difference was small. This study demonstrates that high trait integration in native plants promotes alien species proliferation in urban wastelands, while alien success is primarily driven by its high intraspecific variation and functional flexibility (low trait integration). Furthermore, elevated environmental nutrition constraint undermines native communities’ functional advantage, facilitating alien species proliferation. We recommend context-dependent management: under low nutrient constraint, focus on enhancing native resistance through functional composition and habitat improvement; under high constraint, prioritize alien removal before restoring native resilience.