CO2 Emissions and Woody-Plant Range Expansions
Spencer, C.N., Matzner, S.L., Smalley, J., Bukrey, M., Onberg, J. and Chapman, M. 2009. Forest expansion and soil carbon changes in the Loess Hills of Eastern South Dakota. The American Midland Naturalist 161: 273-285.
Spencer et al. (2009) evaluated the status of remnant prairie patches in the Loess Hills of southeastern South Dakota (USA), using three parallel approaches: aerial photograph analysis, vegetation surveys and stable carbon isotope analysis of soil organic matter. In reporting their findings, they say that "time series analysis of aerial photos indicated that forest cover expanded by 37.5% between 1941 and 2000," while "vegetation surveys revealed several distinct community types ranging from forested ravines supporting basswood, American elm and black walnut trees to upland prairie remnants and oak savannas that now include encroaching eastern red cedar trees, and/or a dense understory of prickly ash, ironwood and other woodland species moving up from the ravines." Last of all, they state that "carbon isotope values (δ13C) in soil cores decreased as much as 5-7‰ towards the soil surface consistent with increased prevalence of C3 forest species over C4 prairie species in recent years."
So dramatic is the story told by their data, the six scientists say they "expect the existing prairie remnants to be completely replaced by forest within the next several decades," and they note that "similar community changes have been documented in a number of other prairie sites throughout the Midwest (Collins and Wallace, 1990; Briggs and Gibson, 1992; Gehring and Bragg, 1992)."
Although this prairie-to-woodland transformation of the landscape has often been considered to be undesirable, Spencer et al. state that "forest encroachment may result in an increase in soil organic carbon and/or biomass pools," noting that the increase in carbon storage in the 0-1 cm layer of the soils of the grassland and wooded sites they studied "extrapolates to an overall increase in soil carbon storage from 4.83 mg ha-1 for grassland sites to 6.58 mg ha-1 for wooded sites," or an increase of fully 35%. And, of course, what they call "an obvious consequence of forest encroachment into grasslands" also results in a large "increase in above-ground plant biomass and carbon storage," and they remind us that these changes in landscape carbon storage have "implications for atmospheric CO2 concentrations and climate change," which are equally obviously of a positive nature, as they tend to slow the rate-of-rise of the air's CO2 content and reduce the ultimate degree of CO2-induced global warming.
It is good to know, in this regard, that another of Earth's natural bio-thermostats is "alive and well," and that it is continuing to perform its valuable climate-moderating function.
Briggs, J.M. and Gibson, D.J. 1992. Effect of fir on tree spatial patterns in a tallgrass prairie landscape. Bulletin of the Torrey Botanical Club 119: 300-307.
Collins, S.L. and Wallace, L.L. (Eds.) 1990. Fire in North American Tallgrass Prairies. University of Oklahoma Press, Norman, Oklahoma, USA, 175p.
Gehring, J.L. and Bragg, T.B. 1992. Changes in prairie vegetation under eastern red cedar in an eastern Nebraska bluestream prairie. The American Midland Naturalist 128: 209-217.
Idso, K.E. and Idso, S.B. 1994. Plant responses to atmospheric CO2 enrichment in the face of environmental constraints: a review of the past 10 years' research. Agricultural and Forest Meteorology 69: 153-203.