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Soil Carbon Response of Grasslands to Woody Plant Invasions
McCarron, J.K., Knapp, A.K. and Blair, J.M.  2003.  Soil C and N responses to woody plant expansion in a mesic grassland.  Plant and Soil 257: 183-192.

In a study of the soil and plant carbon contents of pristine grasslands and adjacent grasslands that have experienced invasions of woody shrubs over the past 30-100 years, Jackson et al. (2002) claim that this landscape-altering phenomenon has been greatly overrated as a carbon sink and, therefore, as a means of slowing the rate of rise of the air's CO2 content and reducing the potential for global warming.  In fact, they claim that woody-plant invasion-induced losses of soil organic carbon at mesic sites are large enough to offset concomitant increases in plant biomass carbon.  In reviewing their paper, however, we take strong issue with this conclusion, demonstrating that their data do not provide unambiguous support for this contention.

What was done
In a study that cites the implications of Jackson et al.'s work, McCarron et al. measured the effects of shrub encroachment in a mesic grassland on soil CO2 flux, extractable inorganic N, and N mineralization beneath isolated C3 shrub communities (islands) of Cornus drummondii C.A. Mey and surrounding undisturbed native tallgrass prairie at the Konza Prairie Biological Station in northeast Kansas, U.S.A., during the 1999 and 2000 growing seasons.

What was learned
The authors report that "a shift in plant community composition from grassland to shrubland resulted in a 16% decrease in annual soil CO2 flux with no differences in total soil C or N or inorganic N."  They also note that their results "are consistent with two other recent studies [Norris (2000) and Smith (2001)] in a nearby tallgrass prairie that assessed the effects of juniper (Juniperus virginiana) forest invasion on C and N cycling."

What it means
The studies of McCarron et al. (2003), Norris (2000) and Smith (2001) all refute the ill-founded conclusion of Jackson et al. (2002) [see also our Editorial of 21 August 2002], as they clearly demonstrate that the invasion of mesic grasslands by woody plants leaves soil carbon stores essentially unaltered, while greatly boosting aboveground inventories of sequestered carbon.  Hence, the invasion of mesic grasslands by shrubs and trees clearly enhances the biological sequestration of carbon in these widespread and globally-dispersed ecosystems that cover vast areas of the earth's surface.

Jackson, R.B., Banner, J.L., Jobbagy, E.G., Pockman, W.T. and Wall, D.H.  2002.  Ecosystem carbon loss with woody plant invasion of grasslands.  Nature 418: 623-626.

Norris, M.  2000.  Biogeochemical Consequences of Land Cover Change in Eastern Kansas.  In: Division of Biology, Kansas State University, Manhattan, Kansas, USA.

Smith, D.  2001.  Changes in Carbon Cycling as Forests Expand into Tallgrass Prairie: Mechanisms Driving Low Soil Respiration Rates in Juniper Forests.  In: Division of Biology, Kansas State University, Manhattan, Kansas, USA.

Reviewed 14 January 2004