Exotic plant species invade many native communities, yet some communities are less susceptible than others. Community properties that determine resistance to or influence on invasive species performance are less studied. The target-neighbor experimental design, originally used for studies of root and shoot competition (Chapter 1), is now commonly used to determine effects of communities of species on a target plant of interest. However, intensity of competition varies between species both within and across seasons, therefore interactions between species are not static. Thus I compared the competitive response of two widespread but relatively little studied invasive species, early flowering Hesperis matronalis and later flowering Nepeta cataria. Both invasives were introduced into native species neighbor communities composed of either early, late, or a mixture of early and late growth and flowering phenologies and measured over the course of two years (Chapter 2). Additionally, invasive species introduction time into native communities was manipulated by either coestablishing the invasive with the neighbors, or introducing the invasive into established neighbor communities (Chapter 3).

I predicted that target invasives would experience the most intense competition (1) when sharing the same growth and flowering phenology as their surrounding native community, due to temporal niche overlap (2) when introduced into an established native community versus when coestablished with native neighbors, and also that (3) the overall effect of neighbor presence and neighbor identity would vary with the age of the competitors. Finally, I utilized field soil probes and greenhouse plants labeled with 15N isotopes to determine the relationship between phenology and nitrogen uptake to provide a mechanistic basis for temporal niche overlap (Chapter 4).

The competitive response of both invasive species' performance was dependent on plant trait measured, community age, and native neighbor phenology treatment. I found evidence for a negative effect of temporal niche overlap, but resource pre-emption by the early phenology neighbors was more prevalent than stronger competition due to a shared phenology. Despite this, field nitrogen levels were still correlated with species phenology. This body of work supports the need for more research on the factors associated with native communities' ability to resist invasion.