A comparison of life history traits of an invasive Mediterranean forb in its native range versus two invasive ranges

Background/Question/Methods

Introductions of invasive plants expose non-native populations to founder effects, genetic isolation, and novel abiotic conditions and biotic interactions. These potentially contribute to genetic differentiation from native populations, and sometimes result in increased competitive ability in invasive populations. To determine whether genetic differentiation has contributed to invasiveness for a European annual forb, we compared life history traits and phenotypic plasticity in those traits in a common greenhouse for Centaurea melitensis collected from three regions of similar climate type: the native range (southern Spain) and two invaded ranges (California and central Chile). In each region, seeds were collected from six sites, germinated under common conditions, and planted in a common greenhouse under four nutrient concentrations. The life history traits examined were seed size, germination time, early season growth rate, late season growth rate, days to first flower, final number of inflorescences, final height, shoot mass, inflorescence mass, above-ground biomass, fine root mass, tap root mass, below-ground biomass, root/shoot ratio, and specific leaf area. Multilevel mixed-effects linear regressions were used to explain variation by region, nutrient treatment and their interaction, controlling for seed mass and elevation, and accounting for the non-independence in sites and mothers, using random effects. 

Results/Conclusions

Californian and Chilean seeds were larger than Spanish seeds. Seeds from Chile germinated slower than those from Spain and California. Plants from Spain flowered 22 days later than Chilean plants, and 13 days later than Californian ones. There was no difference in final height or final number of inflorescences among the three regions, however reaction norms for Spain showed a greater response for height and above-ground biomass to increased nutrients at low nutrient levels than those for invasive populations. Spanish plants had smaller inflorescences, lower below-ground biomass and shallower reaction norms for both variables than Californian and Chilean plants. Spanish plants also had lower root/shoot ratios. Early season growth rate was higher for leaf length in Spanish plants under lower nutrient conditions and also for number of leaves under higher nutrients, but late season growth rate was lower. Plants from the invasive ranges were more similar to each other than to those from the native range. Contrary to expectations, plants from invasive ranges did not outperform those from the native range overall and generally showed greater phenotypic plasticity. Inflorescences were heavier on invasive plants than native ones, suggesting greater seed production, which is an important trait for invasiveness.