An Investigation of the Factors that Facilitate and Inhibit the Range Expansion of an Invasive Plant

All species on Earth occupy limited geographic space. More than a century of observational, experimental, and theoretical work investigating the factors that drive species distributions have demonstrated the importance of the interactions between abiotic, biotic, and demographic factors in determining why species are found where they are. However, it is still unclear when and where these factors interact to set species range limits. Filling the existing knowledge gaps is imperative for the accurate predictions of how species will respond to global change, and particularly for invasive species, many of which are expected to benefit from global change. Here, I sought to investigate the mechanisms that enable, as well as limit, the range expansion of the globally invasive plant Sorghum halepense (L.) Pers. (Johnsongrass). I performed a series of field and laboratory experiments to study population and range dynamics throughout Johnsongrass's North American distribution, and test for the effects of climate, local habitat, and competition on multiple functional traits. I found Johnsongrass consistently demonstrated impressive performance across varying environments, often growing more than 3 m tall, producing hundreds of flowering culms within a single growing season, and maintaining positive population growth rates, even under intense competition with resident weeds. I also found evidence that seed germination has adapted to varying climates encountered during Johnsongrass's range expansion resulting in a shift in the germination temperature niche from warmer to cooler as Johnsongrass spread from warmer climates in the south to more temperate climates in higher latitudes. This shift in the germination temperature niche may have been an important contributing factor in the range expansion of Johnsongrass by enabling the optimization of seed germination in varying climates. On the other hand, results from a field study suggested a possible trade-off between flowering time and growth in populations originating from the range periphery (i.e., range boundary) which may be limiting, or slowing, continued range expansion of Johnsongrass. Together, the outcomes of this work contribute to our understanding of the factors involved in the distribution of species, which is a fundamental goal of Ecology, and essential to accurately predict how invasive species will respond to global change.