Sexuelle Reproduktion und klonales Wachstum in kleinen Populationen von Antennaria dioica (L.) Gaertner

Abstract

Ongoing habitat fragmentation and land use changes pose major threats leading to reduced plant fitness in small and isolated populations. For plant species that can propagate either sexually or asexually, this is assumed to cause a shift towards the vegetative reproduction. Plant populations that exhibit prolonged clonal growth have been referred to as remnant populations, since the loss of genets and consequently the reduction of effective population size induce mechanisms which accelerate further declining of populations. Hence, for clonally reproducing species, it was frequently shown that declining populations exhibit so-called sexual extinction which may be particularly pronounced in dioecious species due to skewed sex ratios.Antennaria dioica is a dioecious species that reproduces mainly clonally. In Central European grasslands, A. dioica is a rapidly decreasing species which is associated to semi-natural plant communities. In our study, we examined the effect of decreasing population size on the clonal structure and the sex ratio which both inherently determine the effective population size. In consequence, we asked whether sexual reproduction is derogated in small populations with skewed sex ratios.Based on field surveys, we estimated population size and sex ratio of 32 populations. Furthermore, we acquired data on plant fitness and habitat conditions (soil profundity, height of surrounding vegetation, nitrogen content in the soil). In order to evaluate the clonal structure of the populations, we collected leaf samples for subsequent AFLP analysis. In addition, we estimated the seed set and the germination of 17 populations.The majority of the examined populations were rather small consisting of less than ten patches. Our results demonstrated that increasing height of the surrounding vegetation leads to considerable loss of plant fitness. In the AFLP analysis, we found striking evidence that the small populations consists of only a few genets. Our analyses revealed that stochasticity in sex ratio increased with decreasing population size while fruit set increased with the size of the sampled stands.We reasoned that the loss of clonal diversity and reduced sexual reproduction contribute to the recently observed declines of population sizes. Hence, we argue that prolonged and almost exclusive clonal growth may ultimately result in sexual extinction which implies fundamental consequences for population viability. Furthermore, we hypothesise that successful sexual reproduction is crucial to balance stochastic losses of genets. Without appropriate conservation activities, the vast majority of our examined A. dioica-populations are subjected to face an extremely high risk of extinction in the near future.Key words: clonal diversity, oligoclonality, dioecy, sex ratio, sexual extinction