The sequestering of carbon in the soils upon which woody plants grow has the potential to provide a powerful brake on the rate of rise of the air's CO2 content if the plant litter that is incorporated into those soils does not decompose more rapidly in a CO2-enriched atmosphere than it does in current ambient air. Hence, it is important to determine if this latter constraint is true or false; and in this summary we review this question with respect to litter produced by conifers.
Scherzel et al. (1998) exposed seedlings of two eastern white pine genotypes to elevated concentrations of atmospheric CO2 and O3 in open-top chambers for four full growing seasons, finding no changes in the decomposition rates of the litter of either genotype to the concentration increases of either of these two gases. Likewise, Kainulainen et al. (2003) could find no evidence that the litter of 22-year-old Scots pine trees that had been exposed to elevated concentrations of CO2 and O3 for three full years decomposed any faster or slower than litter produced in ambient air. In addition, Finzi and Schlesinger (2002) found that the decomposition rate of litter from 13-year-old loblolly pine trees was unaffected by elevated CO2 concentrations maintained for a period of two full years in a FACE study.
In light of these observations, plus the fact that Saxe et al. (1998) have determined that a doubling of the air's CO2 content leads to more than a doubling of the biomass production of coniferous species, it logically follows that the ongoing rise in the atmosphere's CO2 concentration is increasing carbon sequestration rates in the soils upon which conifers grow and, hence, is producing a significant negative feedback phenomenon that slows the rate of rise of the air's CO2 content, which would be assumed by many to be reducing the rate of global warming.
References
Finzi, A.C. and Schlesinger, W.H. 2002. Species control variation in litter decomposition in a pine forest exposed to elevated CO2. Global Change Biology 8: 1217-1229.
Kainulainen, P., Holopainen, T. and Holopainen, J.K. 2003. Decomposition of secondary compounds from needle litter of Scots pine grown under elevated CO2 and O3. Global Change Biology 9: 295-304.
Saxe, H., Ellsworth, D.S. and Heath, J. 1998. Tree and forest functioning in an enriched CO2 atmosphere. New Phytologist 139: 395-436.
Scherzel, A.J., Rebbeck, J. and Boerner, R.E.J. 1998. Foliar nitrogen dynamics and decomposition of yellow-poplar and eastern white pine during four seasons of exposure to elevated ozone and carbon dioxide. Forest Ecology and Management 109: 355-366.