Putative CO2-induced global warming has long been predicted to turn boreal and tundra biomes into carbon sources extraordinaire. In fact, until just a few short years ago it was nearly universally believed that rising air temperatures would lead to the thawing of extensive areas of permafrost and the subsequent decomposition of their vast stores of organic matter, which, it was thought, would release much of the peatlands' tightly-held carbon, enabling it to make its way back to the atmosphere as CO2. Improved soil drainage and increased aridity were also envisioned to help the process along, possibly freeing enough carbon at a sufficiently rapid rate to rival the amount released to the atmosphere as CO2 by all anthropogenic sources combined. The end result was claimed to be a tremendous positive feedback to the ongoing rise in the air's CO2 content, which was envisioned to produce a greatly amplified atmospheric greenhouse effect that would lead to catastrophic global warming. In a slight variation of an old maxim, however, this scenario was simply too bad to be true. Why? Because it did not even begin to deal with the incredible complexity of the issue, several important neglected aspects of which have been briefly described by Weintraub and Schimel (2005).
One of the first cracks in the seemingly sound hypothesis was revealed by the study of Oechel et al. (2000), wherein long-term measurements of net ecosystem CO2 exchange rates in wet-sedge and moist-tussock tundra communities of the Alaskan Arctic indicated that these ecosystems were gradually changing from carbon sources to carbon sinks. The ultimate transition occurred between 1992 and 1996, at the apex of a regional warming trend that culminated with the highest summer temperature and surface water deficit of the previous four decades.
How did it happen, this dramatic and unexpected biological transformation? The answer of the scientists who documented the phenomenon was that it was really nothing special: no more, as they put it, than "a previously undemonstrated capacity for ecosystems to metabolically adjust to long-term changes in climate." Yes, just as people can change their behavior in response to environmental stimuli, so can plants. And this simple ecological acclimation process is only one of several newly-recognized phenomena that have caused scientists to radically revise the way they think about global change in Arctic regions.
Another important study in this area was conducted by Camill et al. (2001), who investigated (1) changes in peat accumulation across a regional gradient of mean annual temperature in Manitoba, Canada, (2) net aboveground primary production and decomposition for 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. In direct contradiction of earlier thinking on the subject, but in confirmation of the more recent findings of Camill (1999a,b), the researchers discovered that aboveground biomass and decomposition "were more strongly controlled by local succession than regional climate." In other words, they determined that over a period of several years, natural changes in plant community composition generally "have stronger effects on carbon sequestration than do simple increases in temperature and aridity." In fact, their core-derived assessments of peat accumulation over the past two centuries demonstrated that rates of biological carbon sequestration 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 have risen by 60-72%.
In another relevant study, Griffis and Rouse (2001) drew upon the findings of a number of experiments conducted over the past quarter-century at a subarctic sedge fen near Churchill, Manitoba, Canada, in order to develop an empirical model of net ecosystem CO2 exchange there. The most fundamental finding of this endeavor was that "carbon acquisition is greatest during wet and warm conditions," such as is generally predicted for the world as a whole by today's most advanced climate models. However, since regional climate change predictions are not very dependable, the two scientists investigated the consequences of a 4°C increase in temperature accompanied by both a 30% increase and decrease in precipitation; and "in all cases," as they put it, "the equilibrium response showed substantial increases in carbon acquisition." One of the reasons behind this finding, as explained by Griffis and Rouse, is that "arctic ecosystems photosynthesize below their temperature optimum over the majority of the growing season," so that increasing temperatures enhance plant growth rates considerably more than they increase plant decay rates.
In summing up their findings, Griffis and Rouse reiterate the fact that "warm surface temperatures combined with wet soil conditions in the early growing season increase above ground biomass and carbon acquisition throughout the summer season." Indeed, they note that "wet spring conditions can lead to greater CO2 acquisition through much of the growing period even when drier conditions persist [our italics]." They thus conclude that if climate change plays out as described by current climate models, i.e., if the world becomes warmer and wetter, "northern wetlands should therefore become larger sinks for atmospheric CO2."
In a somewhat different type of study, Mauquoy et al. (2002) analyzed three cores obtained from a raised peat bog in the UK (Walton Moss) and a single core obtained from a similar bog in Denmark (Lille Vildmose) for macro- and micro-fossils (pollen), bulk density, loss on ignition, carbon/nitrogen ratios and humification, while they were 14C dated by accelerator mass spectrometry. Among a variety of other things, it was determined, in their words, that "the lowest carbon accumulation values for the Walton Moss monoliths between ca. cal AD 1300 and 1800 and between ca. cal AD 1490 and 1580 for Lille Vildmose occurred during the course of Little Ice Age deteriorations," which finding they describe as being much the same as the observation "made by Oldfield et al. (1997) for a Swedish 'aapa' mire between ca. cal AD 1400 and 1800." They also report that "carbon accumulation before this, in the Medieval Warm Period, was higher, as was also the case following the Little Ice Age, as the earth transitioned to the Modern Warm Period. Consequently, whereas climate alarmists claim that warming will hasten the release of carbon from ancient peat bogs, these real-world data actually demonstrate that just the opposite is likely to be true.
In a somewhat similar study, but one that concentrated more on the role of nitrogen than of temperature, Turunen et al. (2004) derived recent (0-150 years) and long-term (2,000-10,000 years) apparent carbon accumulation rates for several ombrotrophic peatlands in eastern Canada with the help of 210Pb- and 14C-dating of soil-core materials. This work revealed that the average long-term apparent rate of C accumulation at 15 sites was 19 ± 8 g C m-2 yr-1, which is comparable to long-term rates observed in Finnish bogs by Tolonen and Turunen (1996) and Turunen et al. (2002). Recent C accumulation rates at 23 sites, on the other hand, were much higher, averaging 73 ± 17 g C m-2 yr-1, which results, in their words, are also "similar 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)." Noting that recent rates of C accumulation are "strikingly higher" than long-term rates, Turunen et al. suggested that increased N deposition "leads to larger rates of C and N accumulation in the bogs, as has been found in European forests (Kauppi et al., 1992; Berg and Matzner, 1997), and could account for some of the missing C sink in the global C budget."
Returning to the role of temperature, Payette et al. (2004) quantified the main patterns of change in a subarctic peatland on the eastern coast of Canada's Hudson Bay, which were caused by permafrost decay between 1957 and 2003, based on detailed surveys conducted in 1973, 1983, 1993 and 2003. This work revealed there was continuous permafrost thawing throughout the period of observation, such that "about 18% of the initial frozen peatland surface was melted in 1957," while thereafter "accelerated thawing occurred with only 38%, 28% and 13% of the original frozen surface still remaining in 1983, 1993 and 2003, respectively." This process, in their words, was one of "terrestrialization" via the establishment of fen/bog vegetation, which nearly always results in either no net loss of carbon or actual carbon sequestration. As a result, Payette et al. concluded that "contrary to current expectations, the melting of permafrost caused by recent climate change does not [our italics] transform the peatland to a carbon-source ecosystem." Instead, they say that "rapid terrestrialization exacerbates carbon-sink conditions and tends to balance the local carbon budget."
In a study of experimental warming of Icelandic plant communities designed to see if the warming of high-latitude tundra ecosystems will result in significant losses of species and reduced biodiversity, as climate alarmists often claim will occur, Jonsdottir et al. (2005) conducted a field experiment to learn how vegetation might respond to moderate warming at the low end of what is predicted by most climate models for a doubling of the air's CO2 content. Specifically, they studied the effects of 3-5 years of modest surface warming (1-2°C) on two widespread but contrasting tundra plant communities, one of which was a nutrient-deficient and species-poor moss heath and the other of which was a species-rich dwarf shrub heath. At the conclusion of the study, no changes in community structure were detected in the moss heath. In the dwarf shrub heath, on the other hand, the number of deciduous and evergreen dwarf shrubs increased more than 50%, bryophytes decreased by 18% and canopy height increased by 100%, but with the researchers reporting that they "detected no changes in species richness or other diversity measures in either community and the abundance of lichens did not change."
Although Jonsdottir et al.'s study was a relatively short-term experiment as far as ecosystem studies go, its results are encouraging, as they indicate that a rise in temperature need not have a negative effect on the species diversity of high-latitude tundra ecosystems and that it may actually have a positive influence on plant growth, which consequences are a far, far cry from the ecological disasters that climate alarmists typically claim will occur in response to rising temperatures.
In a study that included an entirely new element of complexity, Cole et al. (2002) constructed 48 small microcosms from soil and litter they collected near the summit of Great Dun Fell, Cumbria, England. Subsequent to "defaunating" this material by reducing its temperature to -80°C for 24 hours, they thawed and inoculated it with native soil microbes, after which half of the microcosms were incubated in the dark at 12°C and half at 18°C for two weeks, in order to establish near-identical communities of the soils' natural complement of microflora in each microcosm. The former of these temperatures was chosen to represent mean August soil temperature at a depth of 10 cm at the site of soil collection, while the latter was picked to be "close to model predictions for soil warming that might result from a doubling of CO2 in blanket peat environments."
Next, ten seedlings of Festuca ovina, an indigenous grass of blanket peat, were planted in each of the microcosms, while 100 enchytraeid worms were added to each of half of the mini-ecosystems, producing four experimental treatments: ambient temperature, ambient temperature plus enchytraeid worms, elevated temperature, and elevated temperature plus enchytraeid worms. Then, the 48 microcosms -- sufficient to destructively harvest three replicates of each treatment four different times throughout the course of the 64-day experiment -- were arranged in a fully randomized design and maintained at either 12 or 18°C with alternating 12-hour light and dark periods, while being given distilled water every two days to maintain their original weights.
So what did the researchers learn? First of all, they found that elevated temperature reduced the ability of the enchytraeid worms to enhance the loss of carbon from the microcosms. At the normal ambient temperature, for example, the presence of the worms enhanced dissolved organic carbon (DOC) loss by 16%, while at the elevated temperature expected for a doubling of the air's CO2 content they had no effect on DOC. In addition, Cole et al. note that "warming may cause drying at the soil surface, forcing enchytraeids to burrow to deeper subsurface horizons;" and since the worms are known to have little influence on soil carbon dynamics below a depth of about 4 cm (Cole et al., 2000), the researchers concluded that this additional consequence of warming would further reduce the ability of enchytraeids to enhance carbon loss from blanket peatlands. In summing up their findings, therefore, Cole et al. concluded that "the soil biotic response to warming in this study was negative," in that it resulted in a reduced loss of carbon to the atmosphere.
But what about the effects of elevated CO2 itself on the loss of DOC from soils? Freeman et al. (2004) note that riverine transport of DOC has increased markedly in many places throughout the world over the past few decades (Schindler et al., 1997; Freeman et al., 2001; Worrall et al., 2003); and they suggest that this phenomenon may be related to the historical increase in the air's CO2 content.
The researchers' first piece of evidence for this conclusion came from a 3-year study of monoliths (11-cm diameter x 20-cm deep cores) taken from three Welsh peatlands -- a bog that received nutrients solely from rainfall, a fen that gained more nutrients from surrounding soils and groundwater, and a riparian peatland that gained even more nutrients from nutrient-laden water transported from other terrestrial ecosystems via drainage streams -- which they exposed to either ambient air or air enriched with an extra 235 ppm of CO2 within a solardome facility. This study revealed that the DOC released by monoliths from the three peatlands was significantly enhanced -- by 14% in the bog, 49% in the fen and 61% in the riparian peatland -- by the additional CO2 to which they were exposed, which is the order of response one would expect from what we know about the stimulation of net primary productivity due to atmospheric CO2 enrichment, i.e., it is low in the face of low soil nutrients, intermediate when soil nutrient concentrations are intermediate, and high when soil nutrients are present in abundance. Consequently, Freeman et al. concluded that the DOC increases they observed "were induced by increased primary production and DOC exudation from plants," which conclusion logically follows from their findings.
Nevertheless, and to further test their hypothesis, they followed the translocation of labeled 13C through the plant-soil systems of the different peat monoliths for about two weeks after exposing them to ~99%-pure 13CO2 for a period of five hours. This exercise revealed that (1) the plants in the ambient-air and CO2-enriched treatments assimilated 22.9 and 35.8 mg of 13C from the air, respectively, (2) the amount of DOC that was recovered from the leachate of the CO2-enriched monoliths was 0.6% of that assimilated, or 0.215 mg (35.8 mg x 0.006 = 0.215 mg), and (3) the proportion of DOC in the soil solution of the CO2-enriched monoliths that was derived from recently assimilated CO2 (the 13C labeled CO2) was ten times higher than that of the control.
This latter observation suggests that the amount of DOC recovered from the leachate of the ambient-air monoliths was only about a tenth as much as that recovered from the leachate of the CO2-enriched monoliths, which puts the former amount at about 0.022 mg. Hence, what really counts, i.e., the net sequestration of 13C experienced by the peat monoliths over the two-week period (which equals the amount that went into them minus the amount that went out), comes to 22.9 mg minus 0.022 mg = 22.878 mg for the ambient-air monoliths and 35.8 mg minus 0.215 mg = 35.585 mg for the CO2-enriched monoliths. In the end, therefore, even though the CO2-enriched monoliths lost ten times more 13C via root exudation than did the ambient-air monoliths, they still sequestered about 55% more 13C overall, primarily in living-plant tissues.
In light of this impressive array of pertinent findings, it would appear that continued increases in the air's CO2 concentration and temperature would not result in massive losses of carbon from earth's peatlands. Quite to the contrary, these environmental changes -- if they persist -- would likely work together to enhance carbon capture by these particular ecosystems.
References
Berg, B. and Matzner, E. 1997. Effect of N deposition on decomposition of plant litter and soil organic matter in forest systems. Environmental Reviews 5: 1-25.
Camill, P. 1999a. Patterns of boreal permafrost peatland vegetation across environmental gradients sensitive to climate warming. Canadian Journal of Botany 77: 721-733.
Camill, P. 1999b. Peat accumulation and succession following permafrost thaw in the boreal peatlands of Manitoba, Canada. Ecoscience 6: 592-602.
Camill, P., Lynch, J.A., Clark, J.S., Adams, J.B. and Jordan, B. 2001. Changes in biomass, aboveground net primary production, and peat accumulation following permafrost thaw in the boreal peatlands of Manitoba, Canada. Ecosystems 4: 461-478.
Cole, L., Bardgett, R.D. and Ineson, P. 2000. Enchytraeid worms (Oligochaeta) enhance mineralization of carbon in organic upland soils. European Journal of Soil Science 51: 185-192.
Cole, L., Bardgett, R.D., Ineson, P. and Hobbs, P.J. 2002. Enchytraeid worm (Oligochaeta) influences on microbial community structure, nutrient dynamics and plant growth in blanket peat subjected to warming. Soil Biology & Biochemistry 34: 83-92.
Freeman, C., Evans, C.D., Monteith, D.T., Reynolds, B. and Fenner, N. 2002. Export of organic carbon from peat soils. Nature 412: 785.
Freeman, C., Fenner, N., Ostle, N.J., Kang, H., Dowrick, D.J., Reynolds, B., Lock, M.A., Sleep, D., Hughes, S. and Hudson, J. 2004. Export of dissolved organic carbon from peatlands under elevated carbon dioxide levels. Nature 430: 195-198.
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.
Jonsdottir, I.S., Magnusson, B., Gudmundsson, J., Elmarsdottir, A. and Hjartarson, H. 2005. Variable sensitivity of plant communities in Iceland to experimental warming. Global Change Biology 11: 553-563.
Kauppi, P.E., Mielikainen, K. and Kuusela, K. 1992. Biomass and carbon budget of European forests. Science 256: 70-74.
Mauquoy, D., Engelkes, T., Groot, M.H.M., Markesteijn, F., Oudejans, M.G., van der Plicht, J. and van Geel, B. 2002. High-resolution records of late-Holocene climate change and carbon accumulation in two north-west European ombrotrophic peat bogs. Palaeogeography, Palaeoclimatology, Palaeoecology 186: 275-310.
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.
Payette, S., Delwaide, A., Caccianiga, M. and Beauchemin, M. 2004. Accelerated thawing of subarctic peatland permafrost over the last 50 years. Geophysical Research Letters 31: 10.1029/2004GL020358.
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.
Schindler, D.W., Curtis, P.J., Bayley, S.E., Parker, B.R., Beaty, K.G. and Stainton, M.P. 1997. Climate-induced changes in the dissolved organic carbon budgets of boreal lakes. Biogeochemistry 36: 9-28.
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.
Turunen, J., Tomppo, E., Tolonen, K. and Reinikainen, A. 2002. Estimating carbon accumulation rates of undrained mires in Finland: Application to boreal and subarctic regions. The Holocene 12: 69-80.
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.
Worrall, F., Burt, T. and Shedden, R. 2003. Long term records of riverine dissolved organic matter. Biogeochemistry 64: 165-178.
Last updated 24 May 2006