Restoration challenges in a semi-arid grassland following removal of the invasive plant, spotted knapweed (Centaurea stoebe)

Invasive plants like Centaurea stoebe are pervasive and pose ecological and economic challenges, particularly in vulnerable ecosystems such as semi-arid grasslands. Measures, such as herbicide applications and other amendments (like ash), are frequently employed to control the spread of C. stoebe . While these interventions are effective at removing invasive plants, post-application impacts on plant community dynamics are rarely studied. The removal of C. stoebe often leaves a potential niche unfilled, which could result in secondary invasion, though this remains understudied. This research examines post-intervention plant community dynamics following herbicide and wood ash treatments in an invaded semi-arid grassland in British Columbia, Canada. We used taxonomic and phylogenetic alpha and beta diversity, along with species-level dominance patterns, to assess whether treatment effects were limited to the suppression of target invaders or reflected broader changes in vegetation structure. Results revealed that herbicide application was the primary driver of taxonomic and phylogenetic restructuring of plant communities post-treatment. Community partitioning analyses showed that herbicide application had contrasting temporal effects on species loss (the nestedness component) and species replacement (the turnover component). However, after herbicide application, the relative cover of C. stoebe declined while that of B. tectorum increased, shifting invasive plant dominance without a clear decline in their combined relative cover. Wood ash effects were limited, with a richness difference detected only in unsprayed plots. Together, these results show why suppressing the target invader alone does not demonstrate recovery of the grassland community.