What was done
The authors removed intact peat moss monoliths from a heathland bog and exposed them to atmospheric CO2 concentrations of 360 and 560 ppm for three growing seasons in a mini-FACE experiment designed to study the effects of elevated CO2 on species growth and composition.  The monoliths were dominated by the moss Sphagnum magellanicum Brid., which comprised about 95% of each mini-ecosystem, but also contained up to eleven species of vascular plants.  In a separate experiment, the authors subjected similar monoliths to various rates of nitrogen deposition to study the effects of nitrogen concentration on growth in these heathland communities.

What was learned
After three years of treatment exposure, elevated CO2 increased Sphagnum height and biomass by 36 and 17%, respectively.  However, elevated CO2 did not significantly impact the aboveground biomass of vascular plants within the monoliths.  Moreover, atmospheric CO2 enrichment did not cause any changes in monolith species composition, thus allowing the diversity and relative proportions of vascular and non-vascular plants to be maintained.

In contrast to these results, high nitrogen deposition reduced Sphagnum height and biomass by 43 and 32%, respectively.  In addition, high nitrogen deposition increased non-vascular plant aboveground biomass by 42% and increased vascular plant presence at the cost of Sphagnum.

What it means
As the atmospheric CO2 concentration increases, it is likely that Sphagnum magellanicum will respond by exhibiting increases in its height and biomass, which should enhance the amount of carbon sequestration occurring in boggy heathlands.  Although some vascular plants persist in such areas, it is unlikely that increased Sphagnum growth will reduce plant biodiversity there, as elevated CO2 maintained species diversity in this study.

If atmospheric nitrogen deposition continues to increase, this Sphagnum species may respond by reducing its biomass production and presence within bogs due to the expansion of vascular plants.  However, this response is as yet uncertain, for the authors were unable to study the interactive effects of elevated CO2 and nitrogen on these monoliths, and additional responses may yet be discovered.  However, if vascular plants increase their abundance in boggy habitats, it is likely that carbon sequestration in such areas will increase, due to the typically higher rates of photosynthesis associated with vascular vs. non-vascular plants.