Moisture controls on Sphagnum growth and CO 2 exchange on a cutover bog

Summary 1.  Abandoned cutover peatlands are persistent sources of atmospheric CO 2 . Net ecosystem CO 2 exchange and Sphagnum net primary production of an abandoned block‐cut bog were measured in the field and in the laboratory using gas exchange techniques to determine the processes controlling CO 2 exchange in these ecosystems. 2.   Sphagnum net primary production was offset by peat respiration, resulting in the peatland becoming a net source of CO 2 during the summer months. 3.   Sphagnum photosynthesis was greatest at wet sites. In addition, sites with vascular plant cover photosynthesized at approximately twice the rate of sites where vascular plants were removed. 4.  Laboratory results indicate that drying and wetting cycles negatively affect Sphagnum net primary production and net ecosystem CO 2 exchange. Sphagnum and peat respiration increased 4–14‐fold upon rewetting, whereas Sphagnum photosynthesis did not recover until 20 days of saturation. 5.   Synthesis and applications. This research emphasizes the importance of stable moisture availability for the growth of Sphagnum and the eventual development of a new acrotelm on the cutover bog surface. Restoration techniques must therefore include companion species and a constant moisture supply above the minimum threshold for Sphagnum mosses.