Invasibility or invasiveness? Effects of habitat, genotype, and their interaction on invasive Rhododendron ponticum populations

The extent and nature of biological invasions are mainly influenced by either the genotype of the invading species, the suitability of the new habitat or by genotype-habitat interactions expressed in adaptations to the new environment. The relevance of these factors was assessed for the invasive evergreen shrub Rhododendron ponticum. Habitat characteristics of soil, climate and community properties were analysed in six native populations in both Georgia (Caucasus) and Spain and in six invasive ones in Ireland. Growth variables of rhododendron individuals and seedling occurrences in the field served as response variables. We performed a reciprocal transplant experiment with rhododendron cuttings and determined survival of transplants in all countries. Due to low survival rates in Georgia and Spain, vegetative increase was only analysed for Ireland. The Irish sites benefited from significantly higher nutrient supply than the Spanish and Georgian sites. We found both strong positive correlations of nutrient supply and negative correlations of seasonal temperature amplitude with growth variables of shoots and seedling density. Origin, target site and interaction effects were significant in the survival of transplanted rhododendron individuals. The Irish site was more favorable for all genotypes, but the invasive genotypes did not perform better than the native ones. The total increase in shoot length of transplants in Ireland was highest in the Irish genotypes, which might suggest adaptation of the Irish populations to their new area. In conclusion, we found evidence for invasiveness of Irish Rhododendron ponticum populations, but only in the invaded habitat. Nonetheless, habitats in the new range also seem to be well suited to native Spanish populations, supporting the idea that invasibility of these new sites also contributes to rhododendron invasion success.