Moisture controls on CO2 exchange in a Sphagnum‐dominated peatland: results from an extreme drought field experiment

Abstract 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. Copyright © 2009 John Wiley & Sons, Ltd.