Flooding Determines Seasonality in Sphagnum Moss Photosynthesis

Identifying causes of seasonality in Sphagnum mosses at the SPRUCE experiment at the Marcell Experimental Forest, Minnesota.

The Science

Sphagnum mosses form many of the world’s peat bogs, which store huge reservoirs of submerged carbon. These ecosystems are at risk in a changing climate. DOE researchers investigated how photosynthesis in Sphagnum mosses changes though the seasons at the Marcell Experimental Forest, Minn. Researchers were surprised to find that the peak in Sphagnum photosynthesis was delayed compared with the seasonal peak in sunlight strength and showed that the delayed peak was likely due to flooding of the Sphagnum and submergence by water suppressing photosynthesis.

The Impact

The influence of flooding on the seasonal cycle of Sphagnum photosynthesis is an advance in the understanding of these at-risk ecosystems that will help to improve model simulations under a changing environment.


Sphagnum mosses are the keystone species of peatland ecosystems. With rapid rates of climate change occurring in high latitudes, vast reservoirs of carbon accumulated over millennia in peatland ecosystems are potentially vulnerable to rising temperature and changing precipitation. DOE researchers investigated the seasonal drivers of Sphagnum photosynthesis—the entry point of carbon into wetland ecosystems. Continuous measurements of Sphagnum carbon exchange with the atmosphere show a seasonal cycle of Sphagnum photosynthesis that peaked in the late summer, well after the peak in photosynthetically active radiation. Statistical analysis of oscillations in the data showed that water table height was the key driver of weekly variation in Sphagnum photosynthesis in the early summer and that temperature was the primary driver of GPP in the late summer and autumn. A process-based model of Sphagnum photosynthesis was used to show the likelihood of seasonally changing maximum rates of photosynthesis and a previously unreported relationship between the water table and photosynthesising tissue area when the water table was at the Sphagnum surface. The model also suggested that variability in CO2 transport through the Sphagnum tissue to the site of photosynthetic fixation, caused by changing Sphagnum water content, had minimal effect on photosynthesis. Researchers came up with a list of four specific areas to improve the modeling of Sphagnum photosynthesis.

Principal Investigator(s)

Anthony P. Walker
Oak Ridge National Laboratory

Related Links


Support to Oak Ridge National Laboratory (ORNL) Terrestrial Ecosystem Science (TES) Scientific Focus Area from the Office of Biological and Environmental Research within the U.S. Department of Energy Office of Science.


Walker, A.P., et al. “Biophysical drivers of seasonal variability in Sphagnum gross primary production in a northern temperate bog.” JGR Biogeosciences 122(5) 1078–1097 (2017). [DOI:10.1002/2016JG003711]