Ecology and control of goatgrass (Aegilops triuncialis) and medusahead (Taeniatherum caput-medusae) in California annual grasslands

Studies on invasive species have explored the impacts of invasions as well as methods of control. Barbed goatgrass (Aegilops triuncialis L.) and medusahead (Taeniatherum caput-medusae ssp. asperum [Simk.] Melderis) are annual grasses from the Mediterranean Basin that have invaded rangelands of the western United States. Both species are highly unpalatable to livestock and can significantly decrease grazing capacity of rangelands. At a site where both species occur in the Sierra Nevada foothills of California, I compared the relative impacts of goatgrass and medusahead and studied the effectiveness of using prescribed fire to control both species. In addition, through a mulch manipulation experiment, I examined the impact grazing intensity can have on the establishment of goatgrass, which currently has a smaller range than medusahead. Both species had some effects on the annual grassland ecosystem, such as effects on soil water content, total soil nitrogen, net N mineralization and nitrification rates, and inorganic N pools; some effects were more evident following fire. However, neither species appeared to have a larger impact than the other, and many of the observed effects were variable within and between growing seasons. Both species were controlled effectively using prescribed fire; three consecutive summer burns reduced goatgrass and medusahead to undetectable levels. Single burns reduced goatgrass and medusahead cover, but post-burn levels were high enough to allow the populations to reestablish. Post-burn fire communities were also more desirable from a livestock production standpoint. Grazing appears to negatively affect goatgrass establishment; goatgrass was more likely to germinate and reach maturity in ungrazed areas. Mature plants in these areas were also more robust and produced more seeds. The grazing effect likely operates through differences in seed predation and microclimate in areas with high and low residual mulch. Overall, medusahead's success as an invader relative to goatgrass is likely due to historical (dates/locations of introductions) and demographic factors (seed production). Medusahead may tolerate a larger range of climatic conditions, but goatgrass may not have been introduced to these areas yet. If goatgrass has similar climatic tolerances as medusahead, the range and impact of goatgrass will likely increase.