Background
The authors note that many sagebrush-steppe ecosystems in the U.S. intermountain west have been experiencing increases in pinyon pine and juniper trees for some time now; and they say that several groups of scientists have suggested that "woodland expansion and infill will provide an offset for greenhouse gas emissions," citing the studies of Norris et al. (2001), Pacala et al. (2001), Asner et al. (2003), Hibbard et al. (2003), Canadell and Raupach (2008) and McKinley and Blair (2008).

What was done
Working in Underdown Canyon in the Shoshone Mountain Range on the Humboldt-Toiyabe National Forest in Nye and Lander Counties of Nevada (USA), Rau et al. developed a biomass, carbon (C) and nitrogen (N) spreadsheet model that uses tree cover, soil chemistry, soil physical properties and vegetation chemistry to estimate biomass, carbon and nitrogen accumulation on the landscape in response to woodland expansion, as well as the C and N losses associated with prescribed burning, in order to further explore this ongoing landscape-transforming phenomenon, which many believe is primarily driven by the ongoing rise in the air's CO2 content. See, in this regard Range Expansion (Woody Plants) in our Subject Index.

What was learned
The six scientists say their observations-based model indicates that in treeless sagebrush-steppe ecosystems, biomass accounts for less than 10% of total estimated ecosystem C and N to a soil depth of 53 cm; but they indicate that as tree cover increases to near-closed-canopy conditions, aboveground biomass may account for nearly 53% of total estimated ecosystem C and 13% of total estimated ecosystem N to a soil depth of 53 cm.

What it means
As the air's CO2 content rises, it is apparent that the phenomenon studied by Rau et al. provides its own negative feedback to CO2-induced global warming, as the CO2-driven expansion of trees onto sagebrush-steppe ecosystems greatly increases the amount of carbon that those ecosystems remove from the atmosphere and store in the plants that comprise them, and which they transfer to the soil beneath them for even more secure and voluminous storage.

References
Asner, G.P., Archer, S., Hughes, F.L., Ansley, J.R. and Wessman, C.A. 2003. Net changes in regional woody vegetation cover and carbon storage in Texas drylands, 1937-1999. Global Change Biology 9: 316-335.

Canadell, J.G. and Raupach, M.R. 2008. Managing forests for climate change mitigation. Science 320: 1456-1457.

Hibbard, D.A., Schimel, D.S., Archer, S., Ojima, D.S. and Parton, W. 2003. Grassland to woodland transitions: integrating changes in landscape structure and biogeochemistry. Ecological Applications 13: 911-926.

McKinley, D.C. and Blair, J.M. 2008. Woody plant encroachment by Juniperus virginiana in mesic native grassland promotes rapid carbon and nitrogen accrual. Ecosystems 11: 454-468.

Norris, D.M., Blair, J.M., Johnson, L.C. and McKane, R.B. 2001. Assessing changes in biomass, productivity, and C and N stores following Juniperus virginiana forest expansion into tallgrass prairie. Canadian Journal of Forest Research 31: 1940-1946.

Pacala, S.W., Hurtt, G.C., Baker, D., Peylin, P., Houghton, R.A., Birdsey, R.A., Heath, L., Sundquist, E.T., Stallard, R.F., Ciais, P., Moorcroft, P., Caspersen, J.P., Shevliakova, E., Moore, B., Kohlmaier, G., Holland, E., Gloor, M., Harmon, M.E., Fan, S.-M., Sarmiento, J.L., Goodale, C.L., Schimel, D. and Field, CB. 2001. Consistent land- and atmospheric-based U.S. carbon sink estimates. Science 292: 2316-2320.