Plant invasion depresses native species richness, but control of invasive species does little to restore it

ABSTRACT Background Invasive plants are associated with the decline of native plant richness, but the impact of removal of invasives on native plant richness is often unknown. Aims We investigated whether the presence of the introduced plant Centaurea solstitialis (Asteraceae) was correlated with reduced native plant richness; whether rain in late spring, when C. solstitialis is virtually the only plant actively growing, increased its cover; and whether native species richness increased following the control of C. solstitialis. Methods From 2011 to 2017 in a grassland in Sonoma County, California, USA, we treated 20 1-m2 plots in C. solstitialis-invaded patches with chemical and mechanical removal. We monitored cover of all plants in those plots, plus 20 invaded untreated and 20 uninvaded plots, for a total of 60 plots in two blocks. Results Native plant richness was lower in invaded than in non-invaded plots. More late spring rainfall resulted in greater C. solstitialis cover in the following year. Native species richness in the six years after initial removal was slightly higher in removal plots than in untreated control plots. Conclusions Centaurea solstitialis removal alone results in modest benefits for native plant species richness. Managing this invasive requires more resources in years with more late spring rainfall.