Sphagnum cuspidatum, S. magellanicum and S. rubellum are three co-occurring peat mosses, which naturally have a different distribution along the microtopographical gradient of the surface of peatlands. We set out an experiment to assess the interactive effects of water table (low: À10 cm and high: À1 cm) and precipitation (present or absent) on the CO 2 assimilation and evaporation of these species over a 23-day period. Additionally, we measured which sections of the moss layer were responsible for light absorption and bulk carbon uptake. Thereafter, we investigated how water content affected carbon uptake by the mosses. Our results show that at high water table, CO 2 assimilation of all species gradually increased over time, irrespective of the precipitation. At low water table, net CO 2 assimilation of all species declined over time, with the earliest onset and highest rate of decline for S. cuspidatum. Precipitation compensated for reduced water tables and positively affected the carbon uptake of all species. Almost all light absorption occurred in the first centimeter of the Sphagnum vegetation and so did net CO 2 assimilation. CO 2 assimilation rate showed species-specific relationships with capitulum water content, with narrow but contrasting optima for S. cuspidatum and S. rubellum. Assimilation by S. magellanicum was constant at a relatively low rate over a broad range of capitulum water contents. Our study indicates that prolonged drought may alter the competitive balance between species, favoring hummock species over hollow species. Moreover, this study shows that precipitation is at least equally important as water table drawdown and should be taken into account in predictions about the fate of peatlands with respect to climate change.

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To evaluate the effects of changes in water level and temperatures on performance of four Sphagnum mosses, S. magellanicum, S. rubellum, S. imbricatum and S. fuscum were grown at two water levels, -5 cm and -15 cm, and at two temperatures, 15°C and 20°C. These species differ in their position along the microtopographical gradient and in their geographical distribution. Height increment, subcapitulum bulk density, biomass production, capitulum water content and cumulative evaporation were measured. Height increment and biomass production of S. magellanicum was lower at low water table than at high water table, whereas height increment and biomass production of S. rubellum, S. imbricatum and S. fuscum were unaffected. Height increment of S. magellanicum, S. rubellum and S. imbricatum was higher at high temperature than at low temperature. Biomass production of only S. magellanicum and S. rubellum was higher at high temperature than at low temperature, corresponding with their more southern distribution. Cumulative evaporation of S. magellanicum and S. rubellum was lower at low water table and could be explained by hampered water transport towards the capitula. We conclude that changes in water table and temperature may alter the Sphagnum composition on raised bogs, which may result in changes to important ecosystem processes. Therefore, it is important that species composition and changes therein are taken into account when evaluating global change effects on raised bog ecosystems.