Goulden, M.L. and Crill P.M.  (1997)  Tree Physiology, 17:537-542.

We used an automated, multiplexing gas-exchange system to measure the net exchange of CO₂ at the surfaces of 3 shady feather moss and 3 exposed sphagnum moss sites in a black spruce forest during 35 days at the end of the 1995 growing season.  Mid-day gross photosynthesis was 0.5 to 1.0 mmol m^-2 s^-1 by feather moss and 0.5 to 2.5 mmol m^-2 s^-1 by sphagnum moss. Photosynthesis by sphagnum moss was reduced by approximately 70% at 0^oC, and reached a maximum rate at 8^oC. Nighttime respiration at moss temperatures of 0 to 15^oC was 1 to 2.5 mmol m^-2 s^-1 above feather moss and 0.5 to 1.5 mmol m^-2 s^-1 above sphagnum moss. The higher rates of respiration in the feather moss sites probably reflected a greater belowground input of carbon from spruce, and the lower rates of photosynthesis were probably associated with shading by the spruce canopy.  Photosynthesis by moss accounted for 10 to 50% of whole-forest gross uptake measured simultaneously by eddy covariance.  Respiration at the moss surface was 90 to 50% of whole-forest respiration, with a decreasing fraction on warm nights apparently because of a disproportionate rise in above-ground respiration.