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Non-Vascular Plants -- Summary
As the atmospheric CO2 concentration continues to rise, nearly all of earth's plants will respond by exhibiting increased rates of photosynthesis and biomass production.  This holds true for plants utilizing C3, C4 and CAM photosynthetic physiologies.  It also holds true for grasses, forbs, bushes and trees, which are all vascular plants.  But what about non-vascular plants?  Will vegetation that lacks specialized cells for transporting photosynthetic products and water respond favorably to future increases in the air's CO2 content?  We here review the responses of a few such plants -- mainly mosses -- to atmospheric CO2 enrichment.

In the study of Van der Heijden et al. (2000a), the peat moss Sphagnum recurvum was grown for six months in controlled environments fumigated with air containing 350 and 700 ppm CO2.  Elevated CO2 consistently reduced rates of dark respiration by 40 to 60%, thus providing the CO2-enriched plants with greater carbon supplies to support their growth and other physiological processes, relative to control plants exposed to ambient air.  However, elevated CO2 only stimulated dry mass production in plants that were simultaneously subjected to the lowest of three nitrogen treatments.

In a similar four-month study conducted by Van der Heijden et al. (2000b), elevated CO2 increased total plant biomass in Sphagnum papillosum by 70%, while elevated nitrogen increased it by 53%.  However, neither elevated CO2 nor nitrogen induced a growth response in Sphagnum balticum.

In a mini-FACE study, Heijmans et al. (2001) maintained peat moss monoliths (dominated by Sphagnum magellanicum) in 1-m diameter circular plots maintained at atmospheric CO2 concentrations of 360 and 560 ppm for three growing seasons.  After three years, elevated CO2 was observed to have increased the height and dry weight of this Sphagnum species by 36 and 17%, respectively.  In contrast, Mitchell et al. (2002) established mini-FACE plots in a cutover bog dominated by Polytrichum strictum and Sphagnum fallax and reported that a 210-ppm increase in the air's CO2 concentration reduced the total biomass of these species by 17 and 14%, respectively.

These mini-FACE experiments suggest that atmospheric CO2 enrichment can increase or decrease the growth of certain non-vascular species.  Moreover, in a set of mini-FACE experiments conducted in bog environments located in four European countries, no CO2-induced growth effects on non-vascular plants were reported (Berendse et al., 2001; Hoosbeek et al., 2001), in spite of the fact that some of the tested bog communities contained the CO2-responsive Sphagnum papillosum and Sphagnum magellanicum species.  However, elevated nitrogen deposition reduced Sphagnum growth at two of the study sites (Berendse et al., 2001), while it enhanced the production of Polytrichum in one of them (Mitchell et al., 2002).

In a slightly different study, Hoorens et al. (2002) grew the peatland species Carex rostrata and Sphagnum recurvum with an additional 310 ppm CO2 for five months to study the impact of elevated CO2 on the decomposition of their biolitter.  While atmospheric CO2 enrichment had essentially no effect on biolitter quality and decomposition rates of Sphagnum; it reduced litter nitrogen content, but increased litter phenolic concentrations in Carex by 29 and 32%, respectively, which effectively reduced decomposition rates of Carex biolitter.

To summarize, Sphagnum, and other non-vascular plant species, appear to exhibit varied responses to elevated levels of atmospheric CO2, just as vascular plants exhibit a range of responses; and environmental factors and resource limitations can modify these behaviors.  In this regard, it is important to remember that most bog communities are characterized as being nutrient-poor.  Thus, the positive responses of non-vascular plants that have been reported have occurred under nutrient-poor conditions that often limit the potential positive effects of atmospheric CO2 enrichment in vascular plants.  Clearly, more research needs to be conducted on non-vascular plant responses to atmospheric CO2 enrichment to better understand how these unique plants and their associated ecosystems will respond to increasing atmospheric CO2 concentrations.

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.

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.

Hoorens, B., Aerts, R. and Stroetenga, M.  2002.  Litter quality and interactive effects in litter mixtures: more negative interactions under elevated CO2Journal of Ecology 90: 1009-1016.

Hoosbeek, M.R., van Breeman, N., Berendse, F., Grosvernier, P., Vasander, H. and Wallen, B.  2001.  Limited effect of increased atmospheric CO2 concentration on ombrotrophic bog vegetation.  New Phytologist 150: 459-463.

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.

Van der Heijden, E., Verbeek, S.K. and Kuiper, P.J.C.  2000a.  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.

Van der Heijden, E., Jauhiainen, J., Silvola, J., Vasander, H. and Kuiper, P.J.C.  2000b.  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.