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The response of plant growth to rising CO2 levels appears todepend on nutrient availability, but it is not known whether the growth of bogplants reacts similarly. We therefore studied the effects of elevatedCO2 in combination with N supply on the growth of Sphagnum mosses and vascular plants in ombrotrophic bogvegetation. Because the growth of Sphagnum is lessnutrient-limited than that of vascular plants, we hypothesized that Sphagnum would benefit from elevated CO2. In ourgreenhouse experiment, peat monoliths (34 cm diameter, 40cm deep) with intact bog vegetation were exposed to ambient (350ppmv) or elevated (560 ppmv) atmosphericCO2 combined with low (no N addition) or high (5 g Nm−2 yr−1 added) N deposition for twogrowing seasons. Elevated atmospheric CO2 had unexpected deleterious effectson the growth of Sphagnum magellanicum, the dominant Sphagnumspecies. Growth was greatly reduced, particularly in the second growing seasonwhen, regardless of N supply, the mosses looked unhealthy. The negativeCO2 effect was strongest in the warmest months, suggesting a combinedeffect of elevated CO2 and the raised temperatures in the greenhouse.High N deposition favored Rhynchospora alba, which became the dominant vascular plant speciesduring the experiment. Biomass increased more when N supply was high. There wereno significant effects of elevated CO2 on vascular plants, althoughelevated CO2 combined with high N supply tended to increase theaboveground vascular plant biomass. As Sphagnum is the maincarbon-sequestrating species in bogs and rising atmospheric CO2levels are likely to be followed by increases in temperature, there is an urgentneed for further research on the combined effects of elevated CO2 andincreased temperature on Sphagnum growth in bog ecosystems. [ABSTRACT FROM AUTHOR]