Ecological interactions of the cadmium- and zinc-hyperaccumulating plant, Thlaspi caerulescens, and their implications for phytoremediation

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The success of invasive species can be attributed to a combination of abiotic factors, such as abundant resources and favorable climate, and biotic factors, such as low levels of competition and predation or herbivory, at the introduced location. While studies have demonstrated the effects of these factors on known invasive species, the degree to which these factors affect a non-native species can be used to predict its likelihood of becoming invasive. The metal-hyperaccumulating plant Thlaspi caerulescens (Brassicaceae) is potentially useful for remediating soils that are moderately contaminated with Cd and Zn, and has been experimentally introduced to contaminated sites outside of its native range for phytoremediation. To assess the ecological risks involved in introducing metal-hyperaccumulating plants for phytoremediation, including their potential invasiveness, I have performed three studies to examine the abiotic and biotic factors that could influence the establishment of T. caerulescens at three contaminated sites near the Rocky Mountain Biological Laboratory in Gothic, Colorado. In the first two studies, I test the effects of soil metal concentrations and interspecific competition on plant performance, and in the third study I examine the strength of herbivore pressure on this plant. Results from these studies show that the growth rate of T. caerulescens in field conditions is generally low, but higher where there are high concentrations of soil Zn and low concentrations of soil Cu. Interspecific competition between T. caerulescens and a native congener is weak overall, and herbivory pressure from a native Lepidopteran herbivore is also low. Therefore, abiotic conditions are more limiting to T. caerulescens than biotic interactions, and would likely prevent T. caerulescens from becoming invasive or spreading outside of contaminated soils at these sites. In the fourth chapter, I use a long-term dataset to describe the demography of Frasera speciosa (Gentianaceae), a long-lived monocarpic plant. Results show that the population is stable, and despite the low elasticity values for the reproductive stages, masting events must be observed to describe accurately the population dynamics of this species.