Abstract
Biological invasions occur when species are transported into novel regions outside of their native range, and have been a frequent occurrence since the start of global travel and trade. Although invasions by these non-native species are common in temperate regions, non-native plant species rarely invade higher latitudes; there has been relatively little study on the barriers that are setting the range limits of these poleward invasions. In Churchill, Manitoba, Canada, a subarctic treeline environment, the usage of the grain and deep-sea shipping port for trade has brought over a hundred non-native plant species into the area. Dozens have persisted in town and human-disturbed areas, however almost none have spread from this reservoir into the natural tundra and boreal forest ecosystems. Reasons for the failure of these non-native species to spread are unclear. I experimentally simulated a suite of anthropogenic impacts to test whether abiotic factors might limit six focal non-native species (Capsella bursa-pastoris, Linaria vulgaris, Plantago major, Silene alba, Taraxacum officinale, Thlaspi arvense), and to investigate how climate and global change may benefit these non-native species. I first investigated the effects of anthropogenic disturbances that perturb the top layer of the soil and remove native vegetation, and found that such disturbances increased non-native seed germination and growth. I also found that even though negative soil feedbacks developed in previously invaded human-modified soils, transplants still benefitted from growing in such soils, which are likely warmer, contain elevated nutrients, and can create a suitable microsite for non-native species in an otherwise inhospitable environment. Additional inputs of nitrogen and phosphorous, as may occur in human-occupied sites, also improved non-native growth, but I found no positive effect on survival and germination. Finally, I found increased survival and seed germination with warmer summer temperatures, which are expected with future climate change. Across all field experiments, mortality was high, reflecting the failure of these species to invade despite experimental ameliorations. Altogether, this thesis provides evidence of multiple abiotic constraints of poleward plant invasions, contributes to the scarce data on non-native species at high latitudes, and predicts how future climate warming and increasing levels of anthropogenic impacts may change this current trend of invasion failure.