Q. How can shifting marginal agricultural lands to new
uses sequester carbon and mitigate climate change?
A. In much of the United States—particularly
the
Southern Seaboard, Mississippi Portal, Northern Crescent,
and Fruitful Rim—today's agricultural lands were
once dominated by forest ecosystems. In other regions,
such as the Northern Great Plains and the Prairie Gateway,
native grasses once covered most of the lands now in farm
production.
Cultivation has significantly reduced soil carbon levels
in much of these areas, between 20 and 60 percent for
lands that have been in production more than 20 years
(see studies cited in Lal et al. (1998)
and Lewandrowski et al. (2000)).
Because the vast majority of U.S. cropland has been in
production for several decades, soil carbon levels have
largely stabilized. The potential exists to rebuild soil
carbon levels by shifting marginal agricultural lands
back into forests
and grasses.
In areas that are well suited to growing trees, afforestation
and reforestation of marginal agricultural lands offer
the most promise as a terrestrial carbon sink. Published
estimates of the amount of carbon that could be stored,
or sequestered, through forest expansion vary by region,
species of tree established, and whether conversions are
from cropland or pasture. Average annual sequestration
rates of between 1.5 and 2.0 metric tons per acre over
the first 15 years of tree growth are common in many parts
of the Southeast (Birdsey et al., 1996).
Similar estimates for Northern and Pacific Coast areas
generally fall between 0.6 and 1.4 metric tons per acre.
In areas that were once prairie or are otherwise poorly
suited to forests, conversion of croplands to grasses
may be a more economically efficient carbon sink than
conversion to trees. Grassland ecosystems generally contain
less above-ground carbon per acre than forest systems
but often have higher levels of soil carbon. This is because
soil carbon in grasslands is mostly a function of root
mortality and because the roots of grasses are thin, compact,
and can extend to a depth of a meter of more. Estimates
of annual carbon accumulation in soils that have been
shifted from crop production to grasses can be as high
as 1 metric ton per acre but generally fall between 0.25
and 0.39 metric ton per acre—the key variable being
the type of tillage that was practiced prior to conversion
to grasses (Eve and Sperow, 2000).
The economic potential of developing terrestrial carbon
sinks as offsets to national greenhouse gas emissions
could become clearer after the 6th Conference of Parties
(COP6) to the Framework Convention
on Climate Change. In preparation for COP6, the Intergovernmental
Panel on Climate Change (IPCC) recently completed a detailed
study on the role of land use, land-use change, and
forestry in combating climate change (Watson
et al., 2000).
References
- Birdsey, R.A. 1996. “Regional
Estimates of Timber Volume and Forest Carbon for Fully
Stocked Timberland, Average Management After Cropland
or Pasture Reversion to Fores,” Appendix 3 in Forests
and Global Change. Vol. 2. Forest Management Opportunities
for Mitigating Carbon Emissions (R.N. Sampson and
D. Hai,r ed,.). American Forests. Washington, DC.
- Eve, M., and M. Sperow. 2000. Natural
Resources Ecology Laboratory, Colorado State University,
Fort Collibns, CO. Personal communication.
- Lal, R., J.M. Kimble, R.F. Follet,
and C.V. Cole. 1998. The Potential of U.S. Cropland
to Sequester Carbon and Mitigate the Greenhouse Effect.
Ann Arbor Press, Chelsea MI.
- Lewandrowski, J., H. McDowell,
R. House, and M. Peters. 2000. Mitigating Greenhouse
Gas Emissions: Implications of the Kyoto Protocol for
U.S. Agriculture and U.S. Agricultural Policy. World
Resources Review. Vol. 12, No. 1, pp. 126-148.
- Intergovenmental Panel on Climate
Change (IPCC). 2000. Land Use, Land-Use Change, and
Forestry. Special Report of the Intergovernmental
Panel on Climate Change, Watson, R.T., I.R. Noble, B.
Bolin, N.H. Ravindranath, D.J. Verardo, and D.J. Dokken
(eds.). Cambridge University Press.
|
