CO₂ Emissions and Woody-Plant Range Expansions

When the atmosphere's CO₂ concentration is experimentally increased, the vast majority of Earth's plants lose less water to the atmosphere via transpiration, but they produce more biomass, the latter of which phenomena is generally more strongly expressed in woody perennial species than it is in annual herbaceous plants. As a result of increases in the atmosphere's CO₂ concentration, therefore, Earth's bushes, shrubs and trees would be expected to grow better and use water more efficiently, allowing them to expand their ranges into drier terrain more readily than would be expected of non-woody species. Simultaneously, increases in atmospheric CO₂ often make plants of all types actually prefer warmer temperatures (Idso and Idso, 1994), causing both woody and non-woody plants to grow more vigorously and expand their ranges during periods of concomitant global warming. Some of the evidence for these phenomena has been reviewed and archived in the Range Expansion section of the NIPCC's Topical Archive. The following presents the results of yet another paper on this subject.

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.

Additional References
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.