Abstract: Phosphorus is a key resource influencing alien plant invasion. To investigate the effects of phosphorus addition on the invasion of Chromolaena odorata in karst rocky desertification area, we conducted a pot experiment to examine the growth and competitive responses of the invasive plant C. odorata and the native plants Eupatorium chinense and Vitex negundo under three phosphorus addition levels (0, 50, and 100 mg·kg^-1). The results showed that phosphorus addition significantly increased plant height and biomass in both C. odorata and E. chinense, but did not affect V. negundo. When grown in competition with E. chinense, C. odorata consistently demonstrated greater biomass across all treatments. Under high-phosphorus conditions, C. odorata exhibited significantly higher root mass ratio (RMR) and root-to-shoot ratio (R/S) than E. chinense. Moreover, its competitive indices-the relative interaction intensity (RII) and relative dominance index (RDI) showed significant increases. When grown in competition with V. negundo, plant biomass showed no significant differences between C. odorata and V. negundo across all phosphorus levels. Phosphorus addition did not affect growth parameters in either species. Both species exhibited negative RII values under all treatments. Furthermore, there was no difference in either RII (except low phosphorus treatment) or RDI between the two species. Phosphorus addition enhanced the competitive advantage of C. odorata over E. chinense, but did not alter the competition between C. odorata and V. negundo. These findings demonstrate that phosphorus-mediated effects on invasion success depend critically on the identity of native competitor. This research provides novel insights into the mechanisms driving C. odorata invasion in rocky desertification areas and provides a scientific basis for developing management strategies.

Key words: phosphorus, competitive relation, Chromolaena odoratum, native plant, alien invasive plant