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Effect of Global Warming on Winter Forest-Soil Respiration
Reference
Monson, R.K., Lipson, D.L., Burns, S.P., Turnipseed, A.A., Delany, A.C., Williams, M.W. and Schmidt, S.K. 2006. Winter forest soil respiration controlled by climate and microbial community composition. Nature 439: 711-714.

What was done
The authors studied the effect of changes in snow cover on soil carbon cycling during the course of the natural climatic variations experienced over a six-year period at the Niwot Ridge Ameriflux site in the U.S. Rocky Mountains (401'58"N, 10532'47"W), where they used (1) eddy covariance measurements to assess the net CO2 exchange of a subalpine montane forest ecosystem, (2) special chambers for measuring soil respiration beneath snow, (3) various instruments to characterize a number of snow properties, and (4) standard techniques for identifying soil microbial populations and assessing their growth kinetics and substrate-use.

What was learned
Monson et al. found that "microbes collected from under the snowpack could grow exponentially at 0C," and that "their growth rates increased rapidly with increasing temperature," so that anything that decreased the temperature of their environment decreased forest soil respiration in winter, as they typically observed to be the case. In this regard, they also observed that "a shallower snowpack has less insulation potential, causing colder soil temperatures," and they report that "long-term monitoring of mountain snow packs in the western USA and Europe have shown trends towards decreasing depth, with several mountain ranges experiencing 50-75% decreases, and these have been attributed to positive temperature anomalies (Laternser and Schneebeli, 2003; Scherrer et al., 2004; Mote et al., 2005)."

What it means
Because rising air temperatures lead to decreases in snow depth, because decreases in snow depth lead to colder soil temperatures, and because colder soil temperatures lead to reduced microbial respiration, it can be appreciated that global warming may actually lead to greater quantities of carbon being trapped in the soils of montane forest ecosystems. As Monson et al. describe it, "decreases in the winter snow pack will generally cause decreases in the loss of respired CO2 from the soils of forest ecosystems, thus enhancing the potential for soil carbon sequestration." Hence, their work reveals another negative feedback mechanism whereby earth's biosphere acts to resist changes in temperature due to thermal pressures exerted by independent climate forcing factors.

References
Laternser, M. and Schneebeli, M. 2003. Long-term snow climate trends of the Swiss Alps (1931-99). International Journal of Climatology 23: 733-750.

Mote, P.W., Hamlet, A.F., Clark, M.P. and Lettenmaier, D.T. 2005. Declining mountain snow pack in Western North America. Bulletin of the American Meteorological Society 86: 39-49.

Scherrer, S.C., Appenzeller, C. and Laternser, M. 2004. Trends in Swiss alpine snow days - The role of local and large-scale climate variability. Geophysical Research Letters 31: 10.1029/2004GL020255.

Reviewed 31 May 2006