Plant responses
Plant species richness decreased with increasing invasion across all gradients (Figure 1a, Appendix S1: Table S1). Plant community (Figure 2a, Appendix S1: Table S2) and functional (Figure 3a, Appendix S1: Table S3) composition differed significantly across invasion levels. In Montana, B. arvensis invasion was negatively correlated with C3 perennial grass, C4 perennial grass, forb, and sub-shrub/shrub cover (Appendix S1: Figure S3a, Table S4). In Wyoming, B. arvensis and B. tectorum cover were strongly negatively correlated with C4 perennial grass cover and weakly negatively correlated with C3 annual grass;B. tectorum cover was also negatively correlated with C3 perennial grass cover (Appendix S1: Figure S3a, Table S4).