Competition between Sphagnum mosses in European raised bogs: the effects of a changing climate

Sphagnum mosses are key-stone species in European raised bogs, and as such play an important role in the functioning of the ecosystem and its response to environmental change. Within Sphagnum, species may vary widely in their characteristics, making the identity of the dominant Sphagnum species an important factor explaining ecosystem response. We conducted several glasshouse and field studies, aimed at investigating how the main Sphagnum species in European raised bog respond to changes in climatic variables such as temperature, water table and precipitation with special focus on their effect on interspecific competition. Our results showed that Sphagnum species differ in their response to water table, although the effect of water table was not as straightforward as hitherto expected. Despite differences in the potential to transport water from the water table towards their apical parts, co-occurring Sphagnum species possessed similar water contents. Nevertheless, we found species which naturally occur closest to the water table (hollow) to be ousted by species that naturally occur further from the water table (hummock), indicating that competition is related to interspecific (physiological) differences in the response to capitulum water content. We show that carbon assimilation of hollow species has a narrow optimum at relatively high capitulum water contents, whereas carbon assimilation of hummock species had its optimum under dryer conditions. Additionally, our results show that precipitation plays an important role in determining the water content of Sphagnum mosses. Especially at low water tables, precipitation was found to be an important driver for carbon assimilation. Particularly, the competitive ability of hollow species may largely be determined by precipitation. A considerable part of the competitive performance of Sphagnum mosses may be explained by the size of anisotropic peat moss vegetation. Our results show that the persistence of Sphagnum species which are transplanted generally increases with increasing patch size. Our results indicate that with increasing patch size the ability of Sphagnum mosses to create their own optimal micro-hydrology increases. Especially, under drier conditions, this ability may increase the competitive strength of species. Summarizing, competition between Sphagnum species is influenced by the photosynthetic capacity at distinct capitulum water content. In turn, the capitulum water content of Sphagnum species depends on its capillary system, its water holding capacity (i.e. size of hyaline cells) and on the surrounding vegetation. The results presented in this thesis show that if the availability of water decreases, due to decreasing water tables or prolonged dry periods, hummock species will benefit at the expense of hollow species. Additionally, vascular plants like ericoids will increase. Since Sphagnum species differ in their production rates, changes in the species composition in raised bogs may result in changes in the production of the vegetation, and ultimately change the functioning of these ecosystems. It is important that peatland scientist continue to monitor the effects of environmental changes on changes in the species composition of peatlands. Especially long term measurement may provide a lot of information, which will help us to understand the mechanisms of competition between Sphagnum species to a higher degree.