Summary 1We studied the effects of elevated atmospheric CO 2 and increased N deposition on the plant species composition of a Sphagnum -dominated bog ecosystem in the Netherlands. Large peat monoliths (surface area 1 m 2 , depth 0.6 m) with intact bog vegetation were kept outdoors in large containers and were exposed to elevated CO 2 or increased N deposition for three growing seasons. Elevated CO 2 conditions (target concentration 560 µ mol CO 2 mol -1 ) were created using MiniFACE technology. In a separate experiment, N deposition was increased by 5 g N m -2 year -1 by adding dissolved NH 4 NO 3 at 3 week intervals during the growing season. 2 Elevated atmospheric CO 2 increased height growth of Sphagnum magellanicum , the dominant Sphagnum species, in the second and third growing seasons. Vascular plant biomass was not significantly affected by elevated CO 2 , but growth of species growing close to the moss surface was influenced negatively by the increased Sphagnum height growth. Elevated CO 2 did not change allocation to below-ground plant parts. 3 Adding N increased above-ground vascular plant biomass. The shallow-rooted species Vaccinium oxycoccus responded most to the increased N deposition. Sphagnum growth was significantly reduced in the third growing season. This reduction was likely the result of the increased vascular plant cover, given the observed negative relation between vascular plant cover and Sphagnum growth. 4 The observed shifts in species composition as a result of species-specific responses to treatments, and interactions between peat mosses and vascular plants will have important consequences for the sequestration of carbon in the bog ecosystem.

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[1] Mosses are an important component of the boreal forest, but little is known about their contribution to ecosystem carbon, water, and energy exchange. We studied the role of mosses in boreal forest evapotranspiration by conducting two experiments in a black spruce forest in Fairbanks, Alaska. Moss evaporation was measured using lysimeters filled with Hylocomium splendens or Sphagnum capillifolium. Microclimate and moisture content were varied by placing the lysimeters in different habitats (dense forest, open forest, bog), and by manipulating the water supply (no water, natural rainfall, water added). Moss evaporation rates between 1 June and 8 September averaged 0.3, 0.9, and 1.5 mm day À1 in the dense forest (Hylocomium), open forest (Hylocomium and Sphagnum), and bog (Sphagnum) respectively. Assuming a total forest evapotranspiration rate of 2 mm day À1 , this study shows that moss evaporation contributes considerably to boreal black spruce forest evapotranspiration. Moss evaporation rates depended strongly on the openness of the forest and to a lesser degree on the density of the vascular plant canopy and on moss species. The strong influence of habitat suggests that microclimate is the primary factor determining moss evaporation rates. Hylocomium evaporation reacted strongly to experimental water additions, indicating that precipitation frequency is an important factor in addition to microclimate for this species. The large moss evaporation rates in this study suggest a potential cooling effect of mosses, of Sphagnum in particular.

We determined evapotranspiration in three experiments designed to study the effects of elevated CO2 and increased N deposition on ombrotrophic bog vegetation. Two experiments used peat monoliths with intact bog vegetation in containers, with one experiment outdoors and the other in a greenhouse. A third experiment involved monocultures and mixtures of Sphagnum magellanicum and Eriophorum angustifolium in containers in the same greenhouse. To determine water use of the bog vegetation in July–August for each experiment and each year we measured water inputs and outputs from the containers. We studied the effects of elevated CO2 and N supply on evapotranspiration in relation to vascular plant biomass and exposure of the moss surface (measured as height of the moss surface relative to the container edge). Elevated CO2 reduced water use of the bog vegetation in all three experiments, but the CO2 effect on evapotranspiration interacted with vascular plant biomass and exposure of the moss surface. Evapotranspiration in the outdoor experiment was largely determined by evaporation from the Sphagnum moss surface (as affected by exposure to wind) and less so by vascular plant transpiration. Nevertheless, elevated CO2 significantly reduced evapotranspiration by 9–10% in the outdoor experiment. Vascular plants reduced evapotranspiration in the outdoor experiment, but increased water use in the greenhouse experiments. The relation between vascular plant abundance and evapotranspiration appears to depend on wind conditions; suggesting that vascular plants reduce water losses mainly by reducing wind speed at the moss surface. Sphagnum growth is very sensitive to changes in water level; low water availability can have deleterious effects. As a consequence, reduced evapotranspiration in summer, whether caused by elevated CO2 or by small increases in vascular plant cover, is expected to favour Sphagnum growth in ombrotrophic bog vegetation.