Competitive Effects and Spatial Dynamics of Bromus inermis Invasion of a Rough Fescue Grassland, Canada

Rough fescue grasslands are increasingly threatened by invasion of Bromus inermis Leyss. an introduced, cool season perennial that has transformed grasslands of the Aspen Parkland. However, its ecological impacts and mechanisms of dominance remain incompletely understood. Bromus inermis invasion was assessed across spatial scales by characterizing macroscale species distributions and quantifying mesoscale canopy structure, stem density, species composition and litter at the invasion edge. Cluster and indicator species analyses identified three plant assemblages along an invasion gradient, from native species dominated by Festuca campestris Hack. to non-native species dominated by B. inermis. Invasion significantly and consistently reduced species richness and diversity with reduced abundance of subordinate species and potential competitive exclusion. Dominance diversity patterns revealed a shift toward highly uneven assemblages; these patterns were consistent with greater niche overlap and expansion of B. inermis dominance within the disturbed plant community. Structural evidence suggests that B. inermis invasion is associated with reduced species richness, diversity, and evenness; increased dominance; and altered vegetation structure, while increased canopy height and litter accumulation suggest potential constraints on native species establishment. The results of this study indicate that B. inermis altered both plant assemblage composition and ecosystem structure and highlight the need to prioritize management strategies to address disturbance driven invasion pathways.