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published in the 2002 State of the Nation’s Ecosystems Report as well as the 2003 and 2005 Web-Only Updates have been superseded by the 2008 Report and thus should be used with caution.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

Metric tons of carbon are to be reported for the following components: biomass (total live tree material above ground including coarse roots), soil (soil organic matter), coarse woody debris, and dead plant material   on the forest floor (forest floor ). The weight of organic materials in plants is approximately 50% carbon.

Coarse woody debris consists of dead mass, including standing dead trees and branches and logs lying on the ground greater than 3.0 inches in diameter. Forest floor includes all dead organic matter above the mineral soil horizons, including litter, humus, small twigs, and fine woody debris (branches and twigs less than 3.0 inches in diameter lying on the forest floor).   Carbon contained in live branches and leaves, dead trees, and   forest floor, are estimated from, and are therefore less precise than, data for harvestable wood.   Soil carbon should be measured to a depth of one meter, and would likely be based on discrete samples, such as those collected by the Natural Resources Conservation Service (NRCS), and forest characteristics such as forest type and biomass.

Determining the amount of carbon stored in a forest is challenging because it can change dramatically within a few days following a natural event, such as a fire, or timber harvest. Carbon lost during long-term agricultural use of soils can be restored by reforestation. In fact, the steady increase of carbon stored in eastern forests is a reflection of the re-establishment of forests on abandoned agricultural lands.

Carbon storage does not necessarily end when harvest occurs. Some wood products are used in long-term applications such as housing. Other products (e.g., newspapers) may end up in landfills, thus storing carbon for long periods of time. However, conditions in landfills can be conducive for biological degradation, thus causing some of this stored carbon to be lost over time as methane, carbon dioxide, and other byproducts. In some cases the methane is captured and utilized or burned, but otherwise is lost to the atmosphere. Methane is a greenhouse gas that is much more potent than carbon dioxide on a per-carbon basis.

The Data

Data currently available are for standing live trees, including coarse live roots.

Data Sources: The information presented here is based on data from the USDA Forest Service Forest Inventory and Analysis (FIA) program (see technical note for forest area and ownership ),   in particular the field estimates of the size of trees of various species, coupled with statistical models of the relationships between tree stem volume and the other components of carbon storage.   Although extensive, the field measurements used as the basis for this indicator do not include national parks and wilderness areas or slower-growing forests. Expansion to these areas is occurring with the application of FIA's sampling annualized design.

Data Collection Methodology and Data Manipulation: Carbon storage is estimated using on-the-ground measurements of tree trunk size from many forest sites and statistical models that show the relationship between trunk size and the weight of branches, leaves, and coarse roots (>0.1 inch in diameter). Such data are combined with estimates of forest land area obtained from aerial photographs and satellite imagery. Data for Alaska and Hawaii are not included in this data series. Note that there are 1.1 English tons per metric ton. In most international discussions, carbon storage is reported in metric tons.

Data Access: Data for the estimates of carbon in forests used here were obtained directly from the USDA Forest Service. Additional information about carbon storage in forests can be obtained at http://www.fs.fed.us/ne/global/research/carbon/forcarb.html .

The Data Gap

Data for standing live trees, including coarse live roots, were used. Data for carbon stored in forest soils, dead trees, plant litter on the forest floor, and forest products were not presented in this report, because they were judged not to be adequate for national reporting.

Data on soil carbon are scarce, and the influences of management activities on soil carbon are still poorly known. More intensive measurements of soil carbon are occurring with the application of FIA's annualized design, and are planned by NRCS.

Some forests have not yet been fully inventoried, notably in parts of Alaska and for pinyon-juniper forests throughout the western United States . Where data were available, they were employed in the estimation process; where data were not available, assumptions were used. Data on these areas are now being collected and will be incorporated into future estimates.

Estimates of carbon storage in the soil and forest floor currently available were developed using models based on data from specific forest ecosystem studies. With the application of FIA's annualized design, data are being collected on other components of the forest besides harvestable wood.   Soil and forest floor are being sampled as carbon.   Coarse woody debris data can be converted to carbon in a way analogous to the use of live tree data.    

The amount of product in landfills is based on studies conducted by the Forest Service Forest Products Laboratory and other sources. The Forest Service developed conversion factors to translate products in use and materials in landfills to carbon-equivalents. These conversion models account for all steps in the transformation of cut timber into products and through use to disposal. The models are run separately for each region of the United States and for different kinds of harvest (e.g., pulpwood, sawtimber). More information on these models can be found in Skog and Nicholson (1998).

2003 Web Site Update: Data are for standing live trees including coarse live roots. Data for this update were obtained directly from the USDA Forest Service (personal communication from L.S. Heath and J.E. Smith; see also Heath et al. 2003). Note that the time points and actual values have changed somewhat from the 2002 Report. The differences are quite minor as shown in the comparison graphic.

References

Heath, L.S., J.E. Smith, and R.A. Birdsey. 2003. Carbon trends in U.S. forest lands: A context for the role of soils in forest carbon sequestration. P. 35-45, in Kimble, J.M., L.S. Heath, R.A. Birdsey, and R. Lal, eds. 2003. The Potential of U.S. Forest Soils to Sequester Carbon and Mitigate the Greenhouse Effect. Lewis Publishers, Boca Raton , FL. 429 pp.

Skog, K.E., and G.A. Nicholson. 1998. Carbon cycling through wood products: the role of wood and paper products in carbon sequestration. Forest Products Journal 48: 75-83.