Abstract
Interactions with natural enemies are important to the success of invasive plants in their invaded range. However, little is known of the relative impacts of above- and belowground interactions, or how these interactions may change across the invaded range or between growing seasons. My research investigated these questions using Asteraceae occurring in Ontario, Canada. First, I quantified aboveground herbivory for exotic and native Asteraceae across a latitudinal gradient with the expectation that marginal or more northern populations would experience reduced enemy pressure. Herbivory did not always decline with increasing latitude, but often depended upon the causal herbivorous organism(s) or whether the species was native or exotic. I then further investigated a single non-native species exhibiting strong latitudinal trends in herbivory, Canada thistle (Cirsium arvense). I manipulated distance between plots of C. arvense to see if local spatial and/or temporal isolation also allowed escape from aboveground enemy pressure. Herbivory patterns changed significantly over 4 years, with initial patterns of reduced damage with isolation disappearing by the final year. However, variation in aboveground damage did not explain performance. I then investigated the effects of belowground interactions using a soil feedback experiment with soil collected from across the invaded range, and found C. arvense performed best in range edge soils with shorter invasion histories. This was reflected in compositional shifts in soil mesofauna, and most strikingly, the soil microbiota. Finally, to determine the relative importance of these interactions, I created replicate common gardens in which C. arvense was treated with above- and/or belowground exclosures. Regardless of garden location within the invaded range, protection from both aboveground and belowground enemies improved performance, but belowground interactions tended to have a greater negative impact. Together these results emphasize the importance of spatial and temporal scale in studies of invasions, as well as the need to consider both aboveground and belowground interactions when describing invader performance.