UC Santa Cruz

Invasive plants significantly threaten ecosystems, especially when they expand away from anthropogenic environments into natural habitats. This dissertation investigates the ecological and evolutionary drivers of invasion in Dittrichia graveolens (stinkwort), a recent California invader primarily associated with roadsides but showing signs of spread into adjacent vegetated areas. Chapter 1 examines whether adaptive differentiation facilitates this shift. We found no evidence that D. graveolens populations have evolved traits favoring performance in grassland habitats, suggesting that roadside populations are not yet adapted to compete in established vegetation. Chapter 2 reveals that the roadside association of D. graveolens is likely due to its poor competitive ability. Resident grassland species severely limited D. graveolens growth in both greenhouse and field experiments. This highlights the interacting roles of competition and disturbance in driving invasion patterns in this species. Chapter 3 explores the dormancy and germination traits of D. graveolens, seeking evidence for the evolution of reduced dormancy in edge populations. Contrary to prediction, seed bank behavior was similar between core and edge populations. Most seeds germinated in the first year immediately after the first rain, but burial strongly affected germination. Together, these chapters deepen our understanding of the constraints and drivers of D. graveolens invasion. Effective management practices should prioritize limiting soil disturbance, promoting competition to restrict D. graveolens spread, and strategically manipulating seed burial in conjunction with control efforts.