Volume 8, Number 29: 20 July 2005
CO2-induced global warming has long been predicted to turn tundra ecosystems into carbon sources extraordinaire. Only a few years ago it was nearly universally believed that higher temperatures would lead to the thawing of extensive regions of permafrost and the exposure and decomposition of their vast stores of organic matter, releasing once tightly-held carbon and allowing it to make its way back to the atmosphere as CO2, from whence and in what form it originally came. Today, however, this long-held belief is being seriously questioned.
A case in point is the article of Weintraub and Schimel (2005) - Nitrogen Cycling and the Spread of Shrubs Control Changes in the Carbon Balance of Arctic Tundra Ecosystems - which was recently published in BioScience. In it, the two ecologists report that "shrubs are growing in predominance in tundra communities in response to warming," citing the work of Sturm et al. (2001) and Stow et al. (2004) in Alaska and the work of Sturm et al. (2005) in northern Canada and Russia. Furthermore, since shrubs, in their words, "are the woodiest plants in the tundra," they say this transformation "may increase ecosystem carbon storage, because wood has the highest C:N [carbon/nitrogen] ratio of any plant tissue and decomposes slowly." However, they also note that "whether net ecosystem carbon storage increases or decreases will depend on the balance of (a) carbon losses from soil organic matter and (b) carbon storage in plant pools due to higher primary productivity and changes in plant community composition."
In elucidating these ideas via an analysis of the results of a number of recent pertinent studies, Weintraub and Schimel note that "the dominant shrub in the Alaskan arctic tundra, Betula nana, is spreading in response to the changing arctic climate, especially in tussock communities (Hobbie, 1996; Hobbie and Chapin, 1998)." And because these shrubs "trap and hold snow," they report that "the soil underneath them is better insulated in the winter." As a result, as they describe it, the consequent enhanced insulation "elevates temperatures in the active layer of the soil enough that there can be dramatic increases in microbial activity over the course of the winter, due to the increase in the unfrozen water content of the soil (Sturm et al., 2005)," which phenomenon "enhances winter N mineralization (Schimel et al., 2004)." They further note that the consequent higher nutrient availability in the spring has been shown to favor early-season photosynthesis and shrub growth (Bliss and Matveyeva, 1992; Chapin et al., 1995; Schimel et al., 1996; Arft et al., 1999; Michaelson and Ping, 2003; Sturm et al., 2005), which enhances shrub dominance and promotes increased ecosystem carbon storage by increasing the amount of carbon stored per unit of nitrogen and by lowering rates of soil organic matter decomposition in shrub-impacted soils.
Alternatively, the two ecologists note that "faster N cycling in shrub soil, due in part to the high quality of shrub litter, may help to promote higher rates of decomposition of preexisting soil organic matter as shrubs encroach into other tundra communities ... where soil microbes are severely N limited." In addition, they say that with increasing shrub dominance, "carbon losses from the soil are likely to increase in the winter because of higher rates of carbon mineralization in snow-insulated shrub soils," and that "over time, higher winter soil temperatures under shrubs may also contribute to the mineralization of carbon that is currently frozen in permafrost."
So which set of phenomena predominates? The experiment of Oechel et al. (2000) suggests that phenomena that promote carbon sequestration ultimately prevail, as do the studies of Camill et al. (2001), Griffis and Rouse (2001) and Turunen et al. (2004).
In the first of these studies, long-term measurements of net ecosystem CO2 exchange in wet-sedge and moist-tussock tundra communities of the Alaskan Arctic revealed a gradual shifting of their carbon balances from a state of net carbon release to one of net carbon capture, with the ultimate transition occurring between 1992 and 1996, at the apex of a regional warming trend that culminated with the local climate experiencing the highest average summer temperature of the previous four decades.
In the second study - which dealt with (1) changes in peat accumulation across a regional gradient of mean annual temperature in Manitoba, Canada, (2) net aboveground primary production and decomposition of major functional plant groups of the region, and (3) soil cores from several frozen and thawed bog sites that were used to determine long-term changes in organic matter accumulation following the thawing of boreal peatlands - it was determined that aboveground biomass and decomposition were more strongly controlled by local plant succession than by regional climate. In fact, the core-derived assessments of peat accumulation over the past two centuries demonstrated that carbon sequestration rates can almost double following the melting of permafrost, in harmony with the findings of Robinson and Moore (2000) and Turetsky et al. (2000), who found rates of organic matter accumulation in other recently-thawed peatlands to rise by 60-72% in newly-warmed climatic regimes.
In the third study, which was based upon the findings of several experiments conducted over the past quarter-century at a sedge fen near Churchill, Manitoba, it was found that the plants of that ecosystem typically functioned in thermal conditions that were well below the temperature at which they photosynthesized most effectively, so that warming there would enhance photosynthesis and result in more carbon being removed from the atmosphere via increased plant growth than is released to the air by increased organic matter decay.
Last of all, in the fourth study, it was found that recent rates of carbon accumulation in several ombrotrophic peatlands in eastern Canada were strikingly higher than long-term rates, which result is similar, in the words of the scientists involved in the research, "to results from Finland (Tolonen and Turunen, 1996; Pitkanen et al., 1999) and for boreal Sphagnum dominated peat deposits in North America (Tolonen et al., 1988; Wieder et al., 1994; Turetsky et al., 2000)."
In conclusion, it would appear that all of these many observations suggest that Arctic tundra ecosystems tend to sequester much more carbon in warm times than in cold times, and that old fears of runaway global warming fueled by warming-induced increases in CO2 emissions from Arctic tundra ecosystems are nothing more than that, i.e., old fears that have no basis in fact.
Sherwood, Keith and Craig Idso
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Griffis, T.J. and Rouse, W.R. 2001. Modelling the interannual variability of net ecosystem CO2 exchange at a subarctic sedge fen. Global Change Biology 7: 511-530.
Hobbie, S.E. 1996. Temperature and plant species control over litter decomposition in Alaskan tundra. Ecological Monographs 66: 503-522.
Hobbie, S.E. and Chapin, F.S. 1998. The response of tundra plant biomass, above-ground production, nitrogen, and CO2 flux to experimental warming. Ecology 79: 1526-1544.
Oechel, W.C., Vourlitis, G.L., Hastings, S.J., Zulueta, R.C., Hinzman, L. and Kane, D. 2000. Acclimation of ecosystem CO2 exchange in the Alaskan Arctic in response to decadal climate warming. Nature 406: 978-981.
Pitkanen, A., Turunen, J. and Tolonen, K. 1999. The role of fire in the carbon dynamics of a mire, Eastern Finland. The Holocene 9: 453-462.
Robinson, S.D. and Moore, T.R. 2000. The influence of permafrost and fire upon carbon accumulation in high boreal peatlands, Northwest Territories, Canada. Arctic, Antarctic and Alpine Research 32: 155-166.
Schimel, J.P., Bilbrough, C. and Welker, J.M. 2004. Increased snow depth affects microbial activity and nitrogen mineralization in two arctic tundra communities. Soil Biology and Biochemistry 36: 217-227.
Stow, D.A., Hope, A., McGuire, D., Verbyla, D., Gamon, J., Huemmrich, F., Houston, S., Racine, C., Sturm, M., Tape, K., Hinzman, L., Yoshikawa, K., Tweedie, C., Noyle, B., Silapaswan, C., Douglas, D., Griffith, B., Jia, G., Epstein, H., Walker, D., Daeschner, S., Petersen, A., Zhou, L., and Myneni, R. 2004. Remote sensing of vegetation and land-cover change in arctic tundra ecosystems. Remote Sensing of Environment 89: 281-308.
Sturm, M., Racine, C.R. and Tape, K. 2001. Increasing shrub abundance in the Arctic. Nature 411: 546-547.
Sturm, M., Schimel, J.P., Michaelson, G.J., Welker, J.M., Oberbauer, S.F., Liston, G.E., Fahnestock, J.T. and Romanovsky, V.E. 2005. Winter biological processes could help convert arctic tundra to shrubland. BioScience 55: 17-26.
Tolonen, K., Davis, R.B. and Widoff, L. 1988. Peat accumulation rates in selected Maine peat deposits. Maine Geological Survey, Department of Conservation Bulletin 33: 1-99.
Tolonen, K. and Turunen, J. 1996. Accumulation rates of carbon in mires in Finland and implications for climate change. The Holocene 6: 171-178.
Turetsky, M.R., Wieder, R.K., Williams, C.J, and Vitt, D.H. 2000. Organic matter accumulation, peat chemistry, and permafrost melting in peatlands of boreal Alberta. Ecoscience 7: 379-392.
Turunen, J., Roulet, N.T., Moore, T.R. and Richard, P.J.H. 2004. Nitrogen deposition and increased carbon accumulation in ombrotrophic peatlands in eastern Canada. Global Biogeochemical Cycles 18: 10.1029/2003GB002154.
Weintraub, M.N. and Schimel, J.P. 2005. Nitrogen cycling and the spread of shrubs control changes in the carbon balance of Arctic tundra ecosystems. BioScience 55: 408-415.
Wieder, R.K., Novak, M., Schell, W.R. and Rhodes, T. 1994. Rates of peat accumulation over the past 200 years in five Sphagnum-dominated peatlands in the United States. Journal of Paleolimnology 12: 35-47.