* Biotic resistance, the ability of species in a community to limit invasion, is central to our understanding of how communities at risk of invasion assemble after disturbances, but it has yet to translate into guiding principles for the restoration of invasion-resistant plant communities. We combined experimental, functional, and modelling approaches to investigate processes of community assembly contributing to biotic resistance to an introduced lineage of Phragmites australis, a model invasive species in North America. We hypothesized that (i) functional group identity would be a good predictor of biotic resistance to P. australis, while species identity effect would be redundant within functional group (ii) mixtures of species would be more invasion resistant than monocultures. * We classified 36 resident wetland plants into four functional groups based on eight functional traits. We conducted two competition experiments based on the additive competition design with P. australis and monocultures or mixtures of wetland plants. As an indicator of biotic resistance, we calculated a relative competition index (RCIavg) based on the average performance of P. australis in competition treatment compared with control. To explain diversity effect further, we partitioned it into selection effect and complementarity effect and tested several diversity–interaction models. * In monoculture treatments, RCIavg of wetland plants was significantly different among functional groups, but not within each functional group. We found the highest RCIavg for fast-growing annuals, suggesting priority effect. * RCIavg of wetland plants was significantly greater in mixture than in monoculture mainly due to complementarity–diversity effect among functional groups. In diversity–interaction models, species interaction patterns in mixtures were described best by interactions between functional groups when fitted to RCIavg or biomass, implying niche partitioning. * Synthesis. Functional group identity and diversity of resident plant communities are good indicators of biotic resistance to invasion by introduced Phragmites australis, suggesting niche pre-emption (priority effect) and niche partitioning (diversity effect) as underlying mechanisms. Guiding principles to understand and/or manage biological invasion could emerge from advances in community theory and the use of a functional framework. Targeting widely distributed invasive plants in different contexts and scaling up to field situations will facilitate generalization.