Introduced annual grasses, not native species, benefit from successful removal of yellow starthistle (Centaurea solstitialis) from a California grassland

Background/Question/Methods

For managers of natural areas contemplating an invasive species control program it is important to understand how the removal of an invasive species will impact management targets such as native plants. Even where presence of an invasive coincides with a reduction in native diversity, the impacts of removal on native diversity may be untested. Additionally, the trajectory of the invasive if left untreated is often unknown. Post-removal monitoring can provide insight into drivers of community structure and inform future control programs.

In a Sonoma County California grassland, 60-1m² plots were placed randomly within four homogeneous areas -- 20 plots each within two areas invaded by Centaurea solstitialis (Asteraceae; yellow starthistle), and 10 plots placed within each of two similar, nearby, uninvaded areas. Half of the plots within each invaded area were randomly selected for C. solstitialis removal, by glyophosate applied to each plot plus a 1 meter buffer with a backpack sprayer in early June 2011 and 2012. What little C. solstitialis remained in these removal plots was hand pulled in 2013. All plots were visited in early May of 2011, 2012 and 2013 and percent cover of all vegetation was recorded.

Results/Conclusions

Untreated, C. soltitialis cover increased in already infested areas. Initially, cover of introduced annual grasses and richness of native species were both about twice as high in the uninvaded plots than in either the treated or untreated invaded plots. After two years of treatment, cover of introduced annual grasses in the removal plots had increased to levels no different from the uninvaded plots. In contrast, native species richness remained about twice as high in uninvaded plots than in either removal or untreated plots.

While higher levels of C. solstitialis may be correlated with depressed native diversity, simply removing C. solstitialis does not, within two years, result in an increase in native diversity. In contrast, it appears to be introduced grasses which respond to the resources made available from C. solstitialis removal. Where management goals include native species increases, controlling the invader may not be sufficient to achieve this goal.

While present, C. solstitialis’s effective competition for water may be an important driver of community structure. However existing sources of seed along with reproductive plant traits such as seed production and dispersal mechanisms may best explain community composition post-removal.