Earth's peatlands contain a vast amount of sequestered carbon, about as much, in fact, as that contained in the entire atmosphere. Hence, they are vital elements of the planet's carbon cycle, and it is important to determine how the CO2 assimilation rates of the plants that produce them may change in response to the ongoing rise in the air's CO2 content. In this Summary, therefore, we assemble the results of pertinent studies of moss that we have reviewed on our website. 

Van der Heijden et al. (2000b) grew the peat moss Sphagnum recurvum in hydroponic culture at three different levels of nitrogen nutrition within controlled environment chambers maintained at atmospheric CO2 concentrations of 350 and 700 ppm for a period of six months. The extra 350 ppm of CO2 initially stimulated photosynthetic rates by about 30%; but the effect disappeared after only three days. Simultaneously, however, the elevated CO2 treatment reduced rates of dark respiration throughout the entire study by 40 to 60%. Thus, the CO2-enriched mosses always had more soluble sugars at their disposal than did the mosses growing in ambient air; but the only significant increase in moss dry weight (17%) occurred in the lowest of the three nitrogen treatments.

In a similar hydroponic experiment employing two levels (high and low) of nitrogen nutrition, Van der Heijden et al. (2000a) grew two other peat moss species (S. balticum and S. papillosum) within controlled environment chambers maintained at atmospheric CO2 concentrations of 360 and 720 ppm for four months. In this study, the elevated CO2 increased total plant dry mass in S. papillosum by 70%, as did high nitrogen by 53%, while together the two treatments stimulated total growth by 68%. However, neither elevated CO2 nor high nitrogen, alone or in combination, had any significant effects on the growth of S. balticum.

In a totally different type of study, Heijmans et al. (2001) removed intact monoliths from a heathland bog comprised of a nearly pure stand of S. magellanicum and exposed them to atmospheric CO2 concentrations of 360 and 560 ppm for three growing seasons in a mini-FACE experiment; while in a separate experiment they subjected similar monoliths to various rates of nitrogen deposition. At the end of the 3-year period, the elevated CO2 had increased moss height and biomass by 36 and 17%, respectively. In contrast, the high nitrogen treatment reduced moss height and biomass by 43 and 32%, respectively.

In a multi-site comparative study, Berendse et al. (2001) established mini-FACE plots in Sphagnum-dominated peat bogs at four sites across Western Europe (Finland, Sweden, Switzerland and the Netherlands), where they exposed various mosses (S. balticum, S. papillosum, S. magellanicum and S. fallax) to three consecutive growing seasons of atmospheric CO2 concentrations of 350 or 560 ppm. Surprisingly, the extra 210 ppm of CO2 did not significantly affect Sphagnum biomass production at any of the four sites, nor did it alter the cover of the tall moss Polytrichum strictum at any of the locations. Even more surprising, Mitchell et al. (2002) report that the 210 ppm increase in the air's CO2 content at the Swiss FACE site actually reduced the biomass production of P. strictum and S. fallax by 17 and 14%, respectively.

In a still different type of study, Csintalan et al. (2005) measured the net CO2 assimilation rates of five well-hydrated forest-floor moss species growing in the vicinity of a natural CO2-emitting spring near Lajatico (Toscana, Italy), where atmospheric CO2 concentrations averaged about 700 ppm over the daylight period; but they made their measurements at a CO2 concentration of 350 ppm, in order to determine the nature of any photosynthetic acclimation that may have occurred in response to the plants' multi-generational exposure to an atmospheric CO2 concentration essentially twice that of the current atmosphere, comparing their results with those obtained from similarly-treated control plants growing outside the influence of the CO2 vent.

So what did they find? The five researchers report that "the CO2 assimilation was higher in the native CO2 vent species." Judging as best we can from the graphical representation of their results, the net CO2 assimilation rates of the plants that had been exposed to the nominally-doubled atmospheric CO2 concentration for their entire lives (but that were measured at a CO2 concentration of 350 ppm) were 42% greater (Ctenidium molluscum), 44% greater (Hypnum cupressiforme), 49% greater (Pseudoscleropodium purum), 80% greater (Pleurochaete squarrosa), and 85% greater (Platygyrium repens) than the net CO2 assimilation rates of plants of the same species that were both grown and measured at 350 ppm CO2. Contrary to the results of many shorter-term elevated-CO2-exposure experiments, therefore (see Acclimation in our Subject Index) - where downward (but typically not complete) acclimation of photosynthesis has typically been observed - Csintalan et al. determined that "native mosses showed upward acclimation," indicative of an extremely positive long-term photosynthetic adjustment of the five species to life-long (indeed, multi-generational) atmospheric CO2 enrichment.

Last of all, in a radically different type of study, Tuba et al. (1998) grew a desiccation-tolerant moss in open-top chambers maintained at 350 and 700 ppm for a period of five months, after which the plants were allowed to dry to a nearly completely desiccated state. During this dry-down, the moss maintained positive carbon gains 14% longer in the CO2-enriched air than in ambient air, leading to a 69% increase in total carbon assimilation over the desiccation period in the high-CO2 treatment.

So what is the take-home message of these diverse observations? It would appear to be that we cannot yet be totally confident of what a high-CO2 world of the future will bring in the way of benefit and/or hindrance to moss physiology, in light of the variable and sometimes opposite findings of different studies with both identical and different moss species. More work and, it would appear, very careful work must be conducted on these important plants. Enough has been learned already, however, to be cautiously optimistic about their carbon-sequestering prowess in a future high-CO2 atmosphere. 


References 

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.

Csintalan, Z., Juhasz, A., Benko, Z., Raschi, A. and Tuba, Z. 2005. Photosynthetic responses of forest-floor moss species to elevated CO2 level by a natural CO2 vent. Cereal Research Communications 33: 177-180.

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.

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.

Tuba, Z., Csintalan, Z., Szente, K., Nagy, Z. and Grace, J. 1998. Carbon gains by desiccation-tolerant plants at elevated CO2. Functional Ecology 12: 39-44.

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

Van der Heijden, E., Verbeek, S.K. and Kuiper, P.J.C. 2000b. Elevated atmospheric CO2 and increased nitrogen deposition: effects on C and N metabolism and growth of the peat moss Sphagnum recurvum P. Beauv. Var. mucronatum (Russ.) Warnst. Global Change Biology 6: 201-212.