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Contrasting Effects of Elevated CO2 and Nitrogen Supply on the Growth Responses of Two Moss Species
Mitchell, E.A.D., Butler, A., Grosvernier, P., Rydin, H., Siegenthaler, A. and Gobat, J.-M.  2002.  Contrasted effects of increased N and CO2 supply on two keystone species in peatland restoration and implications for global change.  Journal of Ecology 90: 529-533.

It has been calculated that earth's peatlands contain approximately the same amount of carbon as the atmosphere.  Thus, the regeneration of peatlands can play a key role in the global carbon cycle.  In the course of this process in cutover bogs across Central Europe, bare peat is typically colonized by Polytrichum strictum or Eriophorum vaginatum.  Over time, these species change the microclimates of the bogs in ways that promote the growth of Sphagnum species, which ultimately out-compete the original colonizing species and increase water storage and peat accumulation within the bogs.  The competition between these species is fierce; and Polytrichum generally maintains the upper hand for quite some time, due to its typical height advantage over Sphagnum.  Thus, any perturbations to bog systems that reduce this advantage of Polytrichum may enhance the recovery and domination of the systems by Sphagnum.

What was done
The authors established mini-FACE plots in a cutover bog in the Jura Mountains of Switzerland and performed two parallel experiments, each of which lasted three growing seasons.  In one experiment, the plots were exposed to atmospheric CO2 concentrations of 350 and 560 ppm to study the effects of elevated CO2 on the regeneration of the two dominant bog species at the study site - Polytrichum strictum and Sphagnum fallax.  In the other experiment, the plots were subjected to normal and elevated deposition rates of NH4NO3 to study the effects of elevated N deposition on the regeneration of this same pair of non-vascular species.

What was learned
Elevated CO2 reduced the height differential between the taller Polytrichum and the shorter Sphagnum by 28%; but it also reduced total biomass production by 17 and 14% in Polytrichum and Sphagnum, respectively.  In contrast, elevated N increased the height differential by 42%.  Moreover, elevated N reduced the total production of Sphagnum by 48% while it increased the production of Polytrichum by 74%.

What it means
If the air's CO2 content rises by itself without a concomitant increase in N deposition, recovering peatlands will likely experience faster domination by Sphagnum species.  Even though elevated CO2 reduced total moss biomass production, for example, there was no great difference between Sphagnum and Polytrichum in this regard.  Hence, the CO2 effect on height differential should drive the system to favor the conversion of bogs from Polytrichum to Sphagnum domination, which should enhance their ability to sequester carbon at a faster rate.

If, however, atmospheric nitrogen deposition rises along with the increase in the air's CO2 content, it is likely that Polytrichum will be highly favored over Sphagnum.  Since nitrogen emissions are much more easily controlled than CO2 emissions, however; it is reasonable to consider the effects of CO2 alone in a future world where all possible is done to protect the environment.  The authors thus conclude that their results "are clear enough to suggest a positive effect of elevated CO2 on bog regeneration, at least with the combination of Sphagnum fallax and Polytrichum strictum and under the climatic and ecological conditions of the studied site, which we believe are representative for many natural regeneration patterns in Central European cutover bogs."

Reviewed 4 September 2002