We maintain long term observations and experiments investigating the ecology and spread of the invasive plant, Alliaria petiolata (garlic mustard). We have demonstrated that this species interferes with tree seedling growth by competition as well as by suppressing symbioses with beneficial root-colonizing fungi. The original observational plots are now in their 21st year. Our work thus far has shown that net dispersal of garlic mustard seeds from forest edges contributes to invasion of forested sites and that some populations may be adapting to local environmental conditions (e.g., light, water, nutrients and temperature in sites with varying degrees of forest cover) in tandem with demographic spread. These studies have provided more than a decade of continuous observation and over 20 years of observations of garlic mustard’s spread into forest habitats at Harvard Forest and across New England. The present study continues bi-annual surveys of garlic mustard abundances and reproductive success, with the additional context of environmental changes that are affecting all forest understory species. We are now investigating how garlic mustard itself responds to changes in temperature, soil moisture, and nutrient conditions as well as the impacts it may have on forest regeneration. We are also conducting field surveys and experiments to specifically test how red maple, sugar maple, white ash, and red oak tree seedlings respond to simultaneous ecological stressors that include garlic mustard invasion, warming temperatures, and other environmental changes.