Interspecific competition between Sphagnum mosses at different water tables

1. Effects of climate change may affect the Sphagnum species composition in bogs, and ultimately the functioning of the whole ecosystem. We investigated the effect of different water tables on the competition between six Sphagnum species in the glasshouse. The amount of precipitation (234 mm year(-1)) and precipitation frequency (every 2 weeks) were kept low to encourage water-table effects. Relevant species combinations and monocultures were grown at different water tables for a 16-month period. We studied changes in cover, height increment and capitulum water content (WCcap) in order to understand competitive responses. 2. Species naturally occurring further above the water table generally showed higher competitive strength than species naturally occurring closer to the water table. Surprisingly, this effect was irrespective of the water table, indicating a minor role for capillary water transport. Cover change seemed to be related to differences in length growth, but not to water table or WCcap. 3. The WCcap of species within a mixture did not differ, but was lower than the WCcap of the individual species growing in monoculture, indicating differences in ability to supply water to the capitula between mono- and mixed cultures. Subcapitulum bulk densities between mono- and mixed cultures did not differ, or were even lower in monocultures, but did differ between species within mixed cultures. 4. Our results indicate that structural heterogeneity of the peat in mixed cultures has a negative effect on WCcap of both species. Furthermore, we show that sustained periods of drought cause species that naturally occur further above the water table to oust species that naturally occur closer to the water table, even if the water table remains high. Ultimately, the Sphagnum vegetation in raised bogs may shift from hollow to hummock species, evening out the natural microtopography of raised bogs.