Invasion of Centaurea nigrescens, Tyrol knapweed, in North America
Grella, Rebecca Ann
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Centaurea nigrescens is a native plant in Europe that was introduced to North America in the 1800s. This dissertation focuses on the introduction and spread of C. nigrescens across North America using herbaria records and genetic differences between native and non-native populations to construct likely invasion routes. Herbarium records and the results of a Minimum Cost Arborescence model coupled with chloroplast DNA analysis support the hypothesis that there was an initial introduction of C. nigrescens near Worcester, Massachusetts in 1830. Significant haplotype differences between the populations from the northeastern and the northwestern US support the hypothesis that there was a second, independent introduction of C. nigrescens in the Pacific northwest, USA. To compare the performance of plants from the native and non-native range of C. nigrescens, seeds from 8 European (native) and 8 North American (non-native) populations were collected. Germination and early developmental stages were assessed by studying germination rates, time to germination, time to first leaf, and time to second and third leaf formation at 18??C and 28??C. Seeds from non-native populations had a greater germination rate than those from native populations at both temperatures. However, there was no difference in time to germination or developmental rates between plants from native and non- native populations at 18??C or 28??C. Three greenhouse experiments were used to assess traits that could affect the competitive ability of C. nigrescens when exposed to low and high light, low and high water, and low and high nutrients. Surprisingly there were relatively few traits that differed between plants from native and non-native populations in any of the three experiments. Plants from the non- native populations were capable of adapting to the different treatments by differences in resource allocation. For the non-native plants in the light and water experiments, leaf structure changed significantly without affecting specific leaf area, a trait associated with competitive ability, but, there was no difference in specific leaf area between native and non- native populations in any of the treatments. By incorporating the history of invasion, early life history traits, and responses to stress, this dissertation research provides important knowledge about the introduction of an invader, and indicates the importance of studies that include many different factors that affect the introduction and success of new invaders.