Aminopyralid is the most commonly used herbicide for the control of yellow starthistle and other invasive thistles in annual grasslands of California. Although the effects of aminopyralid on native plant communities over a 2-yr period have been evaluated in prairies dominated by perennial species in the northern central states, similar evaluations have not been conducted in grassland communities of California, which are generally composed of a high diversity of native and nonnative annual species. In this study we monitored the effects of 53 and 123 g ae aminopyralid ha⁻¹ on individual species cover and species richness over three growing seasons in two locations on California annual grassland. Treated plots were compared to untreated plots in randomized complete-block designs. Results were largely consistent between the two trials. In the first season after treatment, both rates of aminopyralid reduced dicot cover significantly, particularly members of the Asteraceae and Fabaceae. Treated plots also showed reduced species richness. However, these differences were less pronounced in the second season after treatment, particularly at the low rate. By the third season after treatment in both sites, there were no longer any significant effects on cover or species richness at the low herbicide rate. On California annual grasslands, winter applications of low rates of aminopyralid have been shown to give excellent control of yellow starthistle, providing long-term benefits to grassland ecosystems. Results of the current study suggest that negative impacts of aminopyralid on the desirable native forb community are transitory.

Nomenclature: Aminopyralid; yellow starthistle; Centaurea solstitialis L.

Management Implications: Although land managers should, and generally do, consider the impact of invasive plant control methods on the desired vegetation and the ability to achieve their management goals, they often do not have the necessary information on the long-term effects of these control options. The primary control method for yellow starthistle and other invasive species, particularly thistles, is the herbicide aminopyralid. It is well recognized that members of the Asteraceae, Fabaceae, Apiaceae, and a few other families are particularly sensitive to aminopyralid. In this repeated study, we monitored the effect of high and low registered rates of aminopyralid on species cover and richness on California grasslands dominated by annual grasses and forbs. Our results demonstrated a significant impact on forb cover in the first season after treatment with both herbicide rates, but the response was transitory and by the third season after treatment there were no longer any negative impacts on the native plant community, including nearly all sensitive species. Although we do not know the long-term effect of aminopyralid when it is applied in multiple years, we previously showed that following a summer prescribed burn with a winter aminopyralid application provided nearly complete control of yellow starthistle without requiring multiple years of aminopyralid treatment. Under this integrated approach, it may be possible to achieve the desired control of invasive thistles with only a transitory effect on nontarget native plants.