Date of Award


Document Type


Degree Name

Master of Science (MS)


Plant Biology

First Advisor

Stephen R. Keller


Introduced plants face many ecological and evolutionary challenges when establishing in a new range, such as strong abiotic stressors and potentially novel selective environments. One such abiotic stress is water availability, which is a strong selective force shaping physiological and phenological traits that enable plants to tolerate or avoid drought stress. Despite the challenges of establishing in a new range, thousands of species have become invasive in recent centuries. Two hypotheses that may explain how a species is able to withstand stress in its introduced range are preadaptation, which posits that species are adapted to similar environments in their native ranges before being introduced to a new range, and post-introduction adaptation, which posits that species are able to rapidly adapt after being introduced.The Centaurea jacea hybrid complex is a complex of the species Brown Knapweed (C. jacea), Black Knapweed (C. nigra), and their fertile hybrid Meadow Knapweed (C. × moncktonii), which readily backcrosses with its parental species. This complex is native to western Europe and invasive in North America. In this study, I investigated differences in drought response between native European populations and invasive North American populations to test the hypotheses of preadaptation versus post-introduction adaptation. I grew individuals from 11 populations from the European range and 11 from the Pacific Northwest region of the invaded range in a greenhouse experiment, which included a control group and a drought treatment group. Over the 8-week drought treatment, I measured stress response physiology, phenology, size, and reproductive effort in order to discern if invasive populations of this species complex show evidence of (1) pre-adaptation to the environmental niche, (2) preadaptation as an ecological generalist, (3) post-introduction evolution of invasiveness, or (4) post-introduction evolution of clinal adaptation. I found evidence of post-adaptation evolution towards larger biomass and a higher number of capitula and in physiological traits related to stomatal conductance in invasive Pacific Northwest populations. There was also evidence of preadaptation in traits such as first year flowering, chlorophyll absorbance, and total seed production. These results suggest that both preadaptation and post-introduction evolution have contributed to the invasion of the Centaurea jacea hybrid complex in the Pacific Northwest of the United States.



Number of Pages

64 p.