In Search of Similarities in Invasive Plant Species: Comparing Native and Invasive Populations of Six Clonal Plant Species in Germany and New Zealand

Humans have introduced non-native species into almost every corner of the world. These, often invasive, alien species have caused severely negative ecological and economical consequences and are regarded to be one of the most problematic aspects of human caused global environmental change. This thesis investigates the invasion of six clonal growing plant species in New Zealand: Achillea millefolium L., Hieracium pilosella L., Lotus pedunculatus Cav., Leucanthemum vulgare Lam., Prunella vulgaris L. and Hypericum perforatum L. This thesis combines four comparisons of native (German) and alien (New Zealand) populations. The studies include a biogeographical multi-species comparison of field populations, a multi-species common garden experiment, a germination experiment including three study species and a UV-B experiment focusing on one species. When grown from seeds, alien plants were less tolerant to simulated herbivory on clonal organs. The germination experiment uncovered higher germination rates in alien populations of three species in medium temperature conditions. In another growth chamber experiment, Hieracium pilosella responded to increased UV-B intensities with high phenotypic plasticity and increased the length of foliar hairs. The outcomes of the common garden experiment indicate that local adaptation and contemporary evolution might be responsible for the observed lower tolerance of alien plants to simulated herbivory on clonal organs. In case of the germination and UV-B experiments, alternative explanations such as phenotypic plasticity or maternal effects are more likely to be the source of different responses. Through the different responses found for different types of clonal growth, this thesis also emphasises the need to more explicitly address clonal growth in future studies in invasion science