A comparison of native and invasive populations of three clonal plant species in Germany and New Zealand

Our aim was to test for changes in growth patterns of three clonally growing plant species (Achillea millefolium, Hieracium pilosella and Hypericum perforatum) between native and invaded regions. We addressed the hypotheses that with differing important life-history traits, invasive populations perform better than native populations, and that this expected better performance is linked to weakened trade-offs between individual growth and sexual and clonal reproduction. Germany and New Zealand. We conducted field surveys for the three above-mentioned species in both native German and invasive New Zealand populations, and collected data at both population and individual levels. At the population level, the proportion of flowering plants, population size and population density were all higher in invasive populations. Similarly, at the individual level, the number of stolons per plant, stolon-biomass ratio and population crowdedness (local plant density in a specified area around a target plant) were significantly higher in New Zealand. Plant height did not differ between countries, and plant biomass was lower in New Zealand than in Germany for Achillea millefolium and Hypericum perforatum. These two species showed significant trade-offs between individual growth and sexual and clonal reproduction. Achillea millefolium exhibited a weakened trade-off in its invaded range, where the same proportion of flowering plants was sustained at much higher levels of population crowdedness than in its native range. The apparent invasion success of the three study species is generally due to better overall performance in their respective invaded ranges. In respect of both Achillea millefolium and Hypericum perforatum, this is driven primarily by increased vegetative reproduction. In contrast, Hieracium pilosella seems to benefit more from increased sexual reproduction in its invaded range. Shifts in trade-offs as a general trend seem to be of minor importance.