Ursachen des Invasionserfolges von Rhododendron ponticum L. auf den Britischen Inseln: Einfluss von Habitat und Genotyp

Abstract

In recent years, biological invasions have become a focus of ecological research. The success of invading plant species can be driven, both, by specific traits of the invader (invasiveness) as well as by the susceptibility of a new habitat for such invasions (invasibility). Furthermore, in many invasion processes, specific interactions between invasive populations and their environment are involved. The present study analyses the contribution of genotypes and habitat conditions to the invasion success of Rhododendron ponticum L. in the British Isles.Rhododendron ponticum is an evergreen Ericaceae with natural occurrences in the Black Sea area, in Lebanon and with further natural stands on the Iberian Peninsula in Southern Spain and Portugal. In 1763, the species was first introduced to Great Britain for ornamental reasons. Since its introduction, R. ponticum has spread successfully in Atlantic Western Europe into many communities, mainly in Great Britain and Ireland. Until now, single aspects of the species invasion and population biology have been the topic of several studies, conducted either in the invaded or in the native range. The present study is the first extensive comparison of native and invasive populations combined with experimental analyses. Six native populations in Georgia (Caucasus) and Spain, and six invasive populations in Ireland were studied with a standardised hierarchical sampling design.Patterns of invasion and site conditions in the three countries were analysed in a comparative approach and correlated to assess the meaning of abiotic factors for explaining the invasion success. Experimental approaches in the field and in the greenhouse were performed to detect a genetic basis for the invasiveness of R. ponticum and putative adaptations to the new habitat.The systematic comparison of biometrical, morphological and demographical data on the basis of three investigation periods in six native Rhododendron populations in Georgia and Spain and six invasive populations in Ireland could demonstrate that the invasive populations differed significantly from native occurrences in higher annual shoot growth rates and a more successful establishment of seedlings in the field. Georgian individuals were clearly separated from both, Spanish and Irish ones by morphological leaf traits. These results confirm the high probability of an Iberian descent of the invasive populations.In a second study, differences in microclimate, soil conditions and structural parameters between the Georgian, Spanish and Irish Rhododendron stands were assessed and showed that invasive populations had a higher nutrient supply than the populations in the native area. High growth rates and a high number of seedlings in the field were positively correlated with the contents of exchangeable magnesium and potassium and of organic carbon. In contrast, a high temperature amplitude in the course of the year was negatively correlated with the growth rates in the field.Rhododendron cuttings of the three origins were reciprocally (re-)transplanted in a range-wide transplant experiment. With highest survival rates in Ireland, the invasion area was most appropriate for all origins. In this experiment, no indication of a general superiority of the invasive genotypes could be detected. Among all three countries tested, the invasive Irish origins responded positively only to the invasion area by means of a high increase in shoot length. The study could demonstrate that among the factors habitat, genotype and adaptation, the favourable edaphic and climatic conditions explained the invasion success to a major degree.In green house and climate chamber experiments, we tested for the evidence of a genetic background for the successful spread of Rhododendron ponticum. Germination experiments at four different temperature levels with seeds from all 18 native and invasive populations showed no significant differences in the absolute germination rates, but the seeds from the invasive Irish populations responded much faster independent of the temperature tested. Growth experiments with Rhododendron cuttings from native and invasive populations resulted in higher growth rates of the invasive origins, and demonstrated, thus, that the observed pattern of a different annual increase in shoot length in the field is also due to genetically fixed traits.These results reflect a specific adaptation of the populations to the site conditions in their respective home country: limited resource availability and harsh abiotic site conditions act less as selection pressure in the new habitat than in the native range. In contrast, the ability of the invasive genotypes to transfer available resources into rapid growth and fast reproduction has to be considered as an adaptive response to the conditions in the invasion area. Summarising, one might conclude that the favourable conditions of the habitat and resulting adapted life strategies of the invasive populations explain the invasion success of Rhododendron ponticum to a high degree. To which extent biotic factors are involved in explaining the favourability of the new environment needs to be analysed in additional studies.