The eu-oceanic woodland herb Ceratocapnos claviculata has been expanding north- and eastwards into north temperate and subcontinental regions during the past decades. The rapid range expansion of this therophyte may be an example of a species that strongly benefits from global change. The aims of this dissertation were thus to study filters and processes which are related to the expansion success and the persistence of colonizing plant populations in native and neophytic range sites. To this end, the project focused on the following objectives: to study the biology, habitat requirements and the effects of ecological filters for colonization of C. claviculata across the entire range, to test for phenotypic plasticity and local adaptation in native and neophytic populations, to analyze population genetic patterns (diversity, founder effects) in the native and neophytic range and to characterize possible dispersal pathways.In the first study I analyzed the community composition of vegetation plot data containing C. claviculata using ordination techniques (CCA, DCA) on three different scales: across the entire range, between the native and invaded range and during the past decades within the native range. C. claviculata is characterized by a broad macroclimatic amplitude. Relevés in the neophytic range showed more indicators of anthropogenic disturbance than native sites and abundance of C. claviculata was positively linked to silvicultural intensity. Newer relevés from after 1990 showed, e.g., higher Ellenberg nutrient indicator values, lower species diversity and higher proportions of neophytic and hemerobic species than old relevés from before 1970. In the second study I compiled all available data and reviewed taxonomy, morphology, distribution, habitat requirements, life cycle and biology of the species. In the third study I carried out a reciprocal seeding experiment in the native and in the neophytic range, and a common garden experiment in order to test for the presence and effects of local adaptation and phenotypic plasticity. Additionally, we tested whether the species is able to establish beyond the current range. C. claviculata showed phenotypic plasticity towards the factors nitrogen availability and sun exposure. However, differences between plants of different seed origins and homesite advantage in some fitness-related plant traits indicated also an ongoing local adapation to environmental conditions. Probably due to climatic reasons, native seeding regions featured higher establishment than neophytic regions. Future expansion into more continental region seems improbable as the species did not establish successfully beyond the current range. In the last study I investigated patterns of genetic diversity across the native range to assess the current population structure and phylogeographic patterns. I tested, whether genetic diversity is reduced in the neophytic range and tried to identify source regions of the expansion. C. claviculata shows generally a very low genetic diversity (mean He=0.004) and strong population differentiation (FST=0.812). Genetic variation decreased within the native range towards the north-east, possibly due to the postglacial recolonization from putative Pleistocene refugia in south-western Europe. Variation decreased also from the native to the neophytic range. Bayesian cluster analysis suggested that putative source regions of the neophytic range are situated in north-western Germany and adjacent regions.Beyond potential positive effects of soil eutrophication and mild winters, direct anthropogenic impact seems to be the most important filter which determines invasibility of plant communities with C. claviculata. Anthropogenic impact may directly facilitate seed dispersal and provides sites and resource conditions suitable for germination and establishment of the short-lived species. Thus, despite very low genetic variation C. claviculata expanded successfully its range and occurs as opportunistic neophyte. This may be also enhanced through general purpose genotypes and phenotypic plasticity. However, in my field experiments a higher establishment success was found in the native range, which may indicate more favorable environmental conditions such as nutrient availability, soil moisture, climate and floristic composition.
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