The endangered Dracocephalum ruyschiana: a study of factors affecting reproduction and recruitment in Norwegian populations

Dracocephalum ruyschiana is a rare species in Europe and its conservation is considered a priority by the Norwegian Environment Agency. Specialist species in fragmented and rare habitats such as dry, calcareous grasslands may be especially vulnerable to changes in the environment. Eutrophication and overgrowth of habitats is thought to favour more competitive species, potentially leaving specialists like D. ruyschiana at a disadvantage. Efficient conservation of D. ruyschiana calls for more knowledge on the survival and reproduction of the species. This thesis provides a study of factors that may influence survival, seed set and germination in D. ruyschiana, including an investigation of the presence of D. ruyschiana soil seed banks in Norway. The aim is to illuminate factors that may explain the observed decline of D. ruyschiana populations in Europe and to contribute knowledge that may be helpful for conserving and restoring populations of the species. The research questions address (1) the effect of neighbour competition on flower size, seed set and germination, (2) the relationship between population size and genetic diversity, seed set and germination, (3) whether light and scarification promote seed germination, and (4) the representation of D. ruyschiana in natural soil seed banks. The effects of increased competition through habitat overgrowth on D. ruyschiana reproduction were investigated by a vegetation removal experiment. Seed set, seed germination percentage, and flower size measurements for 77 individual D. ruyschiana were compared to measurements of a control group of 82 individuals. D. ruyschiana that had surrounding vegetation trimmed produced significantly more seeds per flower. A small positive effect of vegetation removal was also found on seed germination percentage. The effects of population size on genetic diversity, seed set, and germination percentage, and the effects of seed set on germination was investigated for 11 Norwegian populations of D. ruyschiana. Seed set was a significant predictor of germination percentage. Population size also appeared to have a positive effect on genetic diversity, although this effect was not statistically significant. Population size could not predict variation in seed set or germination. Seed scarification was found to increase D. ruyschiana seed germination percentage. Although not statistically significant, exposure to light seemed to have a small, positive effect on germination. Similarly, the population from which the seeds came had a near-significant influence on seed germination percentage. No interaction between light and scarification could be found. The presence of D. ruyschiana seeds was found in soil from two out of three populations, using two different methods of seed detection, suggesting that D. ruyschiana has a soil seed bank.