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Peatlands store globally significant quantities of soil carbon, and Sphagnum moss is the main peat forming vegetation type in bogs. Sphagnum moss productivity is driven by the moisture content of its apical cluster of branches, the capitulum. Capitulum moisture content is dependent on the arrangement of leaves, branches and stems for a given species and also on hydrological conditions of the underlying peat. Despite this link, the response of CO 2 exchange in Sphagnum-dominated peatlands to extreme drought is still unclear, particularly under field conditions.We used drainage to expose Sphagnum rubellum to extreme drought and monitored water table, volumetric water content (VWC), gross ecosystem photosynthesis (GEP), ecosystem respiration (ER) and net ecosystem exchange of CO 2 (NEE) at plots in a 25 m transect perpendicular to a deep drainage ditch and compared results to an undrained site. VWC in the upper 10 cm of peat was strongly related to water table at depths shallower than 55 cm. Below this depth, near surface VWC remained relatively constant between 25 and 28% and Sphagnum GEP was effectively shut down. This also resulted in decreased ER at these locations. The combined effect was a linear relationship between VWC and NEE with moist sites acting as net CO 2 sinks (up to 5 g CO 2 m 2 day 1 ) whereas sites closest to the ditch were consistently small carbon sources. We suggest that understanding how climate change will alter peatland hydrology relative to the moisture thresholds of Sphagnum mosses is critical to determining the fate of their carbon sink function.

Northern peatlands store approximately one-third of the World's soil carbon through the long-term accumulation of carbon as peat. However, when peatlands are exploited for Sphagnum moss and horticultural peat, they become degraded and large, persistent sources of atmospheric carbon dioxide. Recent advances in peatland restoration techniques have succeeded in the re-vegetation of Sphagnum moss on previously cutover surfaces. The long-term success of peatland restoration depends on the development of a sufficiently thick new peat layer that has ecohydrological and hydrophysical properties similar to natural peatlands. We determined these properties for an upper (0-4 cm) and lower (8-12 cm) peat layer in a recently restored peatland, a naturally re-vegetated cutover peatland, and a natural peatland in eastern Québec. The properties of the new peat layer differed significantly between the sites, especially for the lower layer samples. Lower samples for the natural and naturally re-vegetated sites had a bulk density of 43 š 5 and 41 š 11 kg m 3 , respectively, almost twice as high as the value for lower samples from the restored site (24 š 4 kg m 3 ). Sphagnum rubellum capitula density ( C ) was significantly higher (p < 0Ð05) for the restored peatland (28 726 # m 2 ) compared to the natural site (26 050 # m 2 ). Residual moisture content at a soil water pressure of 200 mb (Â r ) was significantly lower (p < 0Ð05) for the restored site in comparison to the natural and naturally re-vegetated sites for the lower samples. This suggests that S. rubellum in a natural peatland is able to hold onto more moisture under increasing soil tension than the same species growing in a restored site likely due to its higher bulk density and relatively more decomposed state. We suggest that it is possible that S. rubellum growing in the harsher cutover peatland environment has possibly allocated more carbohydrates to vertical growth and as a consequence has a lower bulk density and poorer water retention.

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