Background
The authors set the stage for their study by noting that "Arctic tundra ecosystems currently store approximately 14% of the Earth's terrestrial carbon (Oberbauer et al., 1991)," reporting further that "over the past 10,000 years climatic conditions have favored C sequestration in the Arctic (Billings, 1987; Gorham, 1991; Oechel and Billings, 1992)."  They then note that "continued warming is expected to promote plant growth and sequestration of carbon from the atmosphere," cautioning, however, that "should ecosystem respiration exceed gross ecosystem production [yielding a negative net ecosystem production (NEP)], a portion of the large amount of C stored in these arctic ecosystems will be released to the atmosphere, serving to intensify the ongoing warming."

What was done
In wet sedge tundra within the Long-Term Ecological Research site in the northern foothills of the Brooks Range, Alaska, USA, the authors established the following treatments, among others: control (CT), warming with field greenhouses (GH) and warming together with N + P fertilization (GHNP), which they maintained for a total of 13 years while making various measurements on the plants and soils of each treatment.

What was learned
Peak growing season measurements made in the 13th year of the study indicated that NEP, defined as "the net gain of CO2 by the whole ecosystem, plants plus soil," was approximately 90% greater in the GH and GHNP treatments compared to the control, while aboveground biomass in the GH and GHNP treatments was approximately 40% and 65% greater, respectively, than that of the control treatment.

What it means
The authors say that "the long-term responses of biomass and C fluxes that were measured in this study were generally consistent with results of similar experiments in a wide range of tundra types (e.g., Henry et al., 1986; Chapin et al., 1995; Jonasson et al., 1996, 1999; Press et al., 1998; Robinson et al., 1998)."  They thus conclude that "global warming and the associated increase in nutrient mineralization have the potential to alter arctic ecosystem processes and states."  We additionally note that the warming-induced change is in the direction of greater carbon sequestration, providing a negative feedback that reduces the rate of CO2-induced global warming.

References
Billings, W.D.  1987.  Carbon balance of Alaskan tundra and taiga ecosystems: past, present, and future.  Quaternary Science Reviews 6: 165-177.

Chapin, F.S., Shaver, G.R., Giblin, A.E., Nadelhoffer, K.J. and Laundre, J.A.  1995.  Responses of arctic tundra to experimental and observed changes in climate.  Ecology 76: 694-711.

Gorham, E.  1991.  Northern peatlands: role in the carbon cycle and probable responses to climatic warming.  Ecological Applications 1: 182-195.

Henry, G.H.R., Freedman, B. and Svoboda, J.  1986.  Effects of fertilization on three tundra plant communities of a polar desert oasis.  Canadian Journal of Botany 64: 2502-2507.

Jonasson, S., Lee, J.A., Callaghan, T.V., Havstrom, M. and Parsons, A.  1996.  Direct and indirect effects of increasing temperatures on subarctic ecosystems.  Ecological Bulletin 45: 180-191.

Jonasson, S., Michelsen, A., Schmidt, I.K. and Nielsen, E.V.  1999.  Responses in microbes and plants to changes in temperature, nutrient and light regimes in the Arctic.  Ecology 80: 1828-1843.

Oberbauer, S.F., Tenhunen, J.D. and Reynolds, J.F.  1991.  Environmental effects on CO2 efflux from water track and tussock tundra in Arctic Alaska, USA.  Alpine Research 23: 162-169.

Oechel, W. and Billings, W.D.  1992.  Effects of global warming on the carbon balance of arctic plants and ecosystems.  In: Chapin III, F.S., Jeffries, R., Shaver, G., Reynolds, J. and Svoboda, J. (Eds.).  Physiological Ecology of Arctic Plants: Implications for Climate Change.  Academic Press, New York, NY, pp. 139-168.

Press, M.C., Potter, J.A., Burke, M.J.W., Callaghan, T.V. and Lee, J.A.  1998.  Responses of a subarctic dwarf heath community to simulated environmental change.  Journal of Ecology 86: 315-327.

Robinson, C.H., Wookey, P.A., Lee, J.A., Callaghan, T.V. and Press, M.C.  1998.  Plant community responses to simulated environmental change at a high arctic polar semi-desert.  Ecology 79: 856-866.