Brassica tournefortii: Phenology, interactions and management of an invasive mustard

Brassica tournefortii (Gouan), or Sahara mustard, is a nonnative, invasive annual forb currently invading the deserts of North America. Despite its increasing distribution and dominance in desert plant communities, little is known about the biology or impacts of B. tournefortii , and few options exist for management. This dissertation sought to answer three basic questions. First, this dissertation questioned "Why is B. tournefortii able to invade desert ecosystems, whereas closely-related invasive mustards are not?" Four biotypes of invasive Brassicaceae, including desert and more mesic populations of B. tournefortii, Brassica nigra, and Hirschfeldia incana were grown with climate and watering treatments over three years. Results show that all biotypes are capable of equal fitness under desert and drought conditions. Although no differences were found between mesic and desert populations of B. tournefortii, the species had a more rapid phenology than its congeners, suggesting that B. tournefortii succeeds because it can reproduce quickly. Second, this dissertation asks, "What are the interactions of B. tournefortii with native annual forbs?" Because native annuals fill a similar ecological niche, I hypothesized that B. tournefortii would have negative impacts on natives with increasing density and cover. Success of the plant community and individual native species was correlated to B. tournefortii dominance. Results show that B. tournefortii has mostly negative interactions with natives with high precipitation, but positive relationships with low precipitation, suggesting that interactions of B. tournefortii with natives change from negative to positive based on resource availability. Finally, this dissertation asks, "Can B tournefortii be selectively managed in desert ecosystems?" This research compared hand-weeding, a common control technique, to an emergence-stage application and rosette-stage application of glyphosate, vs. no treatment. Emergence-stage application was hypothesized to selectively control B. tournefortii and other invasives by taking advantage of their non-specific germination requirements and rapid emergence. This hypothesis was supported by results showing that native cover can be maintained by applying herbicide at emergence while reducing exotic cover. Hand-weeding selectively removed B. tournefortii, but promoted annual cover and richness only underneath shrubs at a site with few other invasives present. Late herbicide produced high mortality in all species. Results suggest that herbicide can be used as a selective technique to remove most desert invasives, not just B. tournefortii. In conclusion, B. tournefortii is a unique case study for biological invasions in extreme ecosystems, and presents challenges for ecologists and land managers alike.