Department Chair

Dr. Martha Skerrett

Date of Award


Access Control

Open Access

Degree Name

Biology, M.A.


Biology Department


Dr. Robert Warren II

Department Home page

First Reader

Dr. Robert Warren II

Second Reader

Dr. Gary Pettibone

Third Reader

Dr. Daniel Potts


Invasive plants often dominate novel habitats where they did not co-evolve with local species. Several hypotheses suggest mechanisms that explain increased exotic plant success, including 'novel weapons' and 'degraded mutualisms'. Japanese knotweed (Fallopia japonica) and European buckthorn (Rhamnus cathartica) are widespread plant invaders in North America that can dominate ecosystems. The goal of this study is to test whether these impacts are more consistent with novel weapons or degraded mutualism hypotheses. I examine tree seedling recruitment, (germination and initial survival) growth, (biomass) and mycorrhizal invasion (AMF content) as a function of F. japonica and R. cathartica root exudates. Given that species co-evolved with these invasive species may have compensatory mechanisms for the allelochemicals, I use arbuscular (AMF) and ectomycorrhizal (ECM) tree congeners that both co-occur and do not co-occur with the invasive species. My results suggest that novel weapons both attack the seedlings directly and indirectly degrade their mutualisms. Novel weapons imposed the greatest impact on Ulmus tree seedling germination as the root exudates significantly reduced germination in the Ulmus species that evolved in the absence of the invasive plants. However, the Ulmus species during later life stages (seedling survivorship and growth), appeared more dependent on mycorrhizal fungi, and R. cathartica degraded the AMF of Ulmus congeners. These results suggest that both novel weapons and degraded mutualisms help explain the success of these widespread invaders, and that the impacts depend on life stage. Hence, successful species invasion may bring a suite of weapons rather than a single magic bullet.