Intraspecies differences in phenotypic plasticity: Invasive versus non-invasive populations of Ceratophyllum demersum

High phenotypic plasticity has been hypothesized to affect the invasiveness of plants, as high plasticity may enlarge the breath of environments in which the plants can survive and reproduce. Here we compare the phenotypic plasticity of invasive and non-invasive populations of the same species in response to growth temperature. Populations of the submerged macrophyte Ceratophyllum demersum from New Zealand, where the species is introduced and invasive, and from Denmark, where the species is native and non-invasive, were grown in a common garden setup at temperatures of 12, 18, 25 and 35 °C. We hypothesized that the phenotypic plasticity in fitness-related traits like growth and photosynthesis were higher in the invasive than in the non-invasive population. The invasive population acclimated to elevated temperatures through increased rates of photosynthesis (range: P_amb: 8-452 μmol O₂ g⁻¹ DM h⁻¹) and relative growth rates (range: 0.01-0.05 d⁻¹) and associated regulations in the photosynthetic machinery. The non-invasive population had a lower acclimation potential (range: P_amb: 43-173 μmol O₂ g⁻¹ DM h⁻¹; RGR: 0.01-0.03 d⁻¹), but was better at acclimating to cooler conditions by regulation of the light-harvesting complex. Hence, the invasive population of C. demersum from New Zealand had higher phenotypic plasticity in response to temperature than the non-invasive Danish population. This might be the result of genetic evolution since its introduction to New Zealand five decades ago, but further studies are needed to test this hypothesis. The study also indicate, that the global increase in temperature may exacerbate the problems experienced with the invasive C. demersum in New Zealand, as the performance and fitness of this population appear to be favoured at elevated temperatures.