Background
Anisimov et al. (2007) reported that regional rates of warming in the western North American Arctic had reached 0.1°C per year over the prior 35 years; and Deslippe et al. indicate that this warming had been associated with "marked changes in terrestrial ecosystems, including increased microbial activity leading to increased plant N availability (Chapin, 1983; Nadelhoffer et al., 1992; Aerts, 2006) and faster carbon turnover in Arctic soils (Hobbie and Chapin, 1998; Shaver et al., 2006)," which has in turn led to an "expansion of shrubs (Hobbie, 1996; Sturm et al., 2001)," partially due to the fact that "beneath shrub thickets, increased local snow-trapping in winter, increased soil insulation, higher winter and spring-time soil temperatures, and increased rates of nutrient mineralization lead to local conditions that further favor shrub growth and expansion onto tussock tundra (Sturm et al., 2005; Weintraub and Schimel, 2005)." And they add that ectomycorrhizal fungi of the Arctic, which are known to positively respond to warming, "have been shown to be important determinants of plant response to ecosystem change through their dual role as drivers of decomposition processes (Read and Perez-Moreno, 2003) and as the main nutrient harvesting structures of plants (Smith and Read, 1997)."

What was done
In an attempt to better understand these several interacting phenomena, Deslippe et al. passively warmed a number of patches of heathland-type tundra communities -- where the dominant deciduous dwarf shrub, Betula nana (Bog birch), occupies hollows between sedge-tussocks in mixture with other mid-canopy plants -- by installing 1.5-m-high wooden frames with 2.5-m x 5-m bases to which they attached 0.15-mm sheets of polyethylene, which also allowed for air circulation from the base of the walls. Over a period of 18 years, these structures led to mean annual temperature increases of 2.09, 1.76, 1.60 and 1.26°C within the vegetative canopy and the soil at depths of 10 cm, 20 cm and 40 cm, respectively.

What was learned
The four Canadian researchers report that "the most dramatic response [they] observed was the large and significant increase in the proportion of [ectomycorrhizal fungal] clones affiliated with the Cortinariaceae, and Cortinarius in particular in the warming treatment," the latter of which fungi "is adapted to N-limited conditions, and thrives on high carbon inputs from its host to fuel the production of biomass and extracellular enzymes that act on soil organic matter," thereby finding and providing for their B. nana hosts the extra N they require for accelerated growth. And as a result, they further report that these phenomena led to a warming-induced "55% increase in above ground biomass of B. nana plants."

What it means
Deslippe et al. conclude that "warming profoundly alters nutrient cycling in tundra, and may facilitate the expansion of B. nana through the formation of mycorrhizal networks of larger size."

References
Aerts, R. 2006. The freezer defrosting: global warming and litter decomposition rates in cold biomes. Journal of Ecology 94: 712-724.

Anisimov, O.A., Vaughan, D.G., Callaghan, T.V. et al. 2007. Polar regions (Arctic and Antarctic). In: Parry, M.L., Canziani, O.F., Palutikof, J.P., van der Linden, P.J. and Hanson, C.E. (Eds.) Climate Change 2007: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom, pp. 653-685.

Chapin III, F.S. 1983. Direct and indirect effects of temperature on Arctic plants. Polar Biology 2: 47-52.

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 III, F.S. 1998. The response of tundra plant biomass, above-ground production, nitrogen, and CO2 flux to experimental warming. Ecology 79: 1526-1544.

Nadelhoffer, K.J., Giblin, A.E., Shaver, G.R. and Linkins, A.E. 1992. Microbial processes and plant nutrient availability in Arctic soils. In: Chapin III, F.S., Jefferies, R.L., Reynolds, J.F., Shaver, G.R. and Svoboda, J. (Eds.) Arctic Ecosystems in a Changing Climate: An Ecophysiological Perspective. Academic Press, San Diego, USA, pp. 281-300.

Read, D.J. and Perez-Moreno, J. 2003. Mycorrhizas and nutrient cycling in ecosystems -- a journey towards relevance? New Phytologist 157: 475-492.

Shaver, G.R., Giblin, A.E., Nadelhoffer, K.J., Thieler, K.K., Downs, M.R., Laundre, J.A. and Rastetter, E.B. 2006. Carbon turnover in Alaskan tundra soils: effects of organic matter quality, temperature, moisture and fertilizer. Journal of Ecology 94: 740-753.

Smith, S.E. and Read, D.J. 1997. Mycorrhizal Symbiosis. Academic Press, London, United Kingdom.

Sturm, M., Racine, C.R. and Tape, K. 2001. Increasing shrub abundance in the Arctic. Nature 411: 546-547.

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 5: 408-415.