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Contrasting Effects of Elevated CO2 and Nitrogen Supply on Peatland Productivity
Reference
Berendse, F., Van Breemen, N., Rydin, H., Buttler, A., Heijmans, M., Hoosbeek, M.R., Lee, J.A., Mitchell, E., Saarinen, T., Vasander, H. and Wallen, B.  2001.  Raised atmospheric CO2 levels and increased N deposition cause shifts in plant species composition and production in Sphagnum bogs.  Global Change Biology 7: 591-598.

Background
Peatlands contain about the same amount of carbon as the atmosphere.  Thus, the reclamation and regeneration of peatlands plays a key role in the global carbon cycle.

In regenerating cutover bogs across Central Europe, bare peat is typically colonized by Polytrichum strictum or Eriophorum vaginatum.  Over time, these species alter the microclimate and allow the flourishing of Sphagnum species, which ultimately out-compete Polytrichum and Eriophorum, increasing water storage and peat accumulation.

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.  Therefore, factors that reduce the height advantage held by Polytrichum tend to hasten the domination of Sphagnum and promote the sequestration of carbon in European peatlands.

What was done
The authors established mini-FACE plots in Sphagnum-dominated bogs at four sites across Western Europe (Finland, Sweden, Switzerland, and the Netherlands) and performed two three-season-long parallel experiments in them.  In one experiment, plots were exposed to atmospheric CO2 concentrations of either 350 or 560 ppm, while in the other they were subjected to either normal or elevated deposition rates of NH4NO3.  The site in Finland was co-dominated by S. balticum and S. papillosum, the site in Sweden was dominated by S. magellanicum, the one in Switzerland by S. fallax, and the one in the Netherlands by S. magellanicum.

What was learned
Elevated CO2 did not significantly affect Sphagnum biomass production at any of the four sites.  In contrast, elevated N deposition significantly reduced Sphagnum biomass production at two of the four sites (Switzerland and the Netherlands).  Similarly, elevated CO2 did not impact the cover of vascular plants, nor did it alter the cover of the tall moss Polytrichum strictum at any of the locations.  Elevated N, however, significantly enhanced the cover of Polytrichum strictum at the Swiss site and that of vascular plants at the Dutch site.  Also, there was a negative linear relationship between the cover of P. strictum and Sphagnum biomass production under elevated N deposition at the Swiss site and the cover of vascular plants and Sphagnum biomass production under elevated N deposition at the Dutch site.

What it means
The data described in this paper suggest that future increases in the air's CO2 content will have little to no effect on Sphagnum biomass production in Central European bogs.  Other studies reviewed on our website, however, have shown just the opposite, i.e., that atmospheric CO2 enrichment increases biomass production in both S. papillosum (Van der Heijden et al., 2000) and S. magellanicum (Heijmans et al., 2001), which were the dominant species at three of the four study sites of the current paper.  This aspect of the three studies, therefore, is unsettled; but it is clear, at least, that elevated CO2 does not decrease the biomass production of Sphagnum species.

With respect to increasing N deposition, some bog communities will likely exhibit some reductions in Sphagnum biomass production, while others will exhibit no reductions.  The decisive factor may be each site's N deposition history.  For example, N deposition has typically been much higher in Switzerland (1.8 gN m-2y-1) and the Netherlands (3.9 gN m-2y-1) than in Sweden (0.8 gN m-2y-1) and Finland (0.4 gN m-2y-1); and bog communities in the first two countries may be saturated with nitrogen, such that further increases in nitrogen actually depress Sphagnum growth by favoring the growth of the Polytrichum strictum or any of a number of vascular plants.

Although these parallel studies thus provide some insight into the effects of elevated CO2 and N deposition on the growth and dynamics of Sphagnum-dominated bogs, they do not provide us with the complete picture that would be revealed by simultaneously enriching them with both CO2 and nitrogen, which is something that must be done to help us better understand how global climate change may affect these important ecosystems.

References
Heijmans, M.M.P.D., Berendse, F., Arp, W.J., Masselink, A.K., Klees, H., De Visser, W. and Van Breemen, N.  2001.  Effects of elevated carbon dioxide and increased nitrogen deposition on bog vegetation in the Netherlands.  Journal of Ecology 89: 268-279.

Van der Heijden, E., Jauhiainen, J., Silvola, J., Vasander, H. and Kuiper, P.J.C.  2000.  Effects of elevated atmospheric CO2 concentration and increased nitrogen deposition on growth and chemical composition of ombrotrophic Sphagnum balticum and oligo-mesotrophic Sphagnum papillosumJournal of Bryology 22: 175-182.


Reviewed 11 September 2002