Note that the data published in the 2002 State of the Nation’s Ecosystems Report as well as the 2003 and 2005 Web-Only Updates have been superseded by the 2008 Report and thus should be used with caution. For the most recent data, purchase the 2008 Report from Island Press.

The Indicator

This indicator seeks to track long-term changes in carbon sequestration in grasslands and shrublands. Measurements of this indicator through time can provide information on whether the ecosystem is a net source or a net sink of atmospheric carbon dioxide. An ecosystem accruing carbon is one contributing to a reduction in greenhouse gases. White et al. (2000) have estimated that grassland ecosystems worldwide store an amount of carbon that is about half of that stored by the world’s forests and roughly equivalent to that stored by agricultural systems.

An ecosystem not changing in carbon content, but also not producing high inorganic nitrogen exports, is likely a late-successional, mature system possessing high biotic diversity. Systems containing high amounts of carbon are often associated with high levels of ecosystem services (i.e., responsible for clean air and clean water).

The minimum data that are required for this indicator are percentage soil organic matter (SOM) in surface soil layers and carbon stored in plant material, estimated on an area basis. Soil measurements provide an excellent index of both potential soil fertility and nitrogen storage. Soil carbon storage is the net accumulation of (mostly dead) plant matter. It represents the net accumulation of carbon inputs (plant production) minus all sources of organic carbon loss. Changes in soil carbon storage can be caused by changes in climate, changes in atmospheric chemistry, or changes in the abundance and species composition of the vegetation. Plant carbon storage varies annually while soil carbon storage changes at longer time scales.

The Data Gap

Data are not currently available to provide systematic monitoring and reporting of soil and vegetation carbon. There are, of course, many research sites at which such information is collected. Soil carbon can be found at substantial depths, although routine sampling of soils to such depths is uncommon. A variety of available models can estimate total soil carbon storage from surface measurements of SOM and estimate plant carbon from above-ground vegetation measurements. However, there is a serious concern about the use of single-point estimates to represent large areas. Some procedures for establishing the representativeness of sites will be required. Intensive, long-term data are available from the Long Term Ecological Research (LTER) sites, including those in Alaska, Michigan, Minnesota, Kansas, Colorado, and New Mexico (there are two sites in New Mexico). Such sites could provide substantial validation for more widely dispersed measurements. Relatively long-term alpine and arctic tundra SOM data are available from LTER sites as well. See http://lternet.edu/sites/ for additional information and links to the LTER network.

References

Burke, I.C., C.M. Yonker, W.J. Parton, C.V. Cole, K. Flach, and D.S. Schimel. 1989. Texture, climate and cultivation effects on soil organic matter content in U.S. grassland soils. Soil Science Society of America 53: 800–805.

Esteban, G.J., and R.B. Jackson. 2000. The vertical distribution of soil organic carbon and its relation to climate and vegetation. Ecological Applications 10: 423–436.

Jenny, H. 1941. Factors of soil formation. New York: McGraw- Hill.

National Research Council. 2000. Ecological indicators for the nation. Washington, DC: National Academy Press.

Seastedt, T.R., C.C. Coxwell, D.S. Ojima, and W.J. Parton. 1994. Controls of plant and soil carbon in a semihumid temperate grassland. Ecological Applications 4:344–353.

White, R.P., S. Hurray, and M. Rohweder. 2000. Pilot analysis of global ecosystems: Grassland ecosystems. Washington, DC: World Resources Institute