ABSTRACT UNDERSTORY PLANT COMMUNITY STRUCTURE IN FORESTS INVADED BY GARLIC MUSTARD (ALLIARIA PETIOLATA) MAY 2016 JASON ALLEN AYLWARD, B.S., PAUL SMITH’S COLLEGE M.S. UNIVERSITY OF MASSACHUSETTS AMHERST Directed by: Professor Kristina Stinson Plant invasions represent a significant threat to the structure and function of natural ecosystems. Garlic mustard (Alliaria petiolata) has been identified as a threat to native communities mostly through small-scale studies and focused experiments. In this in situ observational study I examined the effects of garlic mustard invasion on species composition across multiple sites by comparing plant diversity and composition in invaded and adjacent non-invaded communities. Mean Shannon diversity was higher in invaded compared to non-invaded plots and invasion was associated with greater densities of invasive species such as burning bush (Euonymus alatus), and greater celandine (Chelidonium majus). In ordination space, the sites grouped more closely by geographic region than by invasion status, suggesting that regional environmental variation is important for community structure. My findings indicate that garlic mustard invasion is associated with other invasive plants, and that the understory plant community varies considerably across the region regardless of invasion status. Overall we show that community structure in garlic mustard invaded forest understories is not consistent across the landscape.