Report on the Environment
Carbon Storage in Forests
What are the trends in the ecological processes that sustain the Nation's ecological systems?
The above question pertains to all 'Ecological Processes' Indicators, however, the information on these pages (overview, graphics, references and metadata) relates specifically to "Carbon Storage in Forests". Use the right side drop list to view the other related indicators on this question.
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After carbon dioxide is converted into organic matter by photosynthesis, carbon is stored in forests for a period of time in a variety of forms before it is ultimately returned to the atmosphere through the respiration and decomposition of plants and animals, or harvested from forests for use in paper and wood products. A substantial pool of carbon is stored in woody biomass (roots, trunks, and branches). Another portion eventually ends up as organic matter in forest floor litter and the upper soil horizons. Carbon storage in forest biomass and forest soils is an essential physical and chemical attribute of stable forest ecosystems, and a key link in the global carbon cycle.
This indicator, developed by the U.S. Department of Agriculture (USDA) Forest Service, tracks decadal changes in net carbon storage rates in the pools of living and dead biomass in forests in the contiguous 48 states. The carbon pools for this indicator are estimated using USDA Forest Service Forest Inventory and Analysis (FIA) data from five historical periods (circa 1953, 1963, 1977, 1987, and 1997). These data cover forest classified as “timberland” under FIA data collection procedures—that is, forests capable of producing at least 20 cubic feet per acre per year of industrial wood and not withdrawn from timber utilization by statute or regulation. Timberland makes up roughly two-thirds of U.S. forest land. Alaska and Hawaii are not included because of limited historical data. The FIA program estimates carbon storage using on-the-ground measurements of tree trunk size from many forest sites; statistical models that show the relationship between trunk size and the weight of branches, leaves, coarse roots (greater than 0.1 inch in diameter), and forest floor litter; and estimates of forest land area obtained from aerial photographs and satellite imagery. Values are converted into carbon storage based on coefficients derived from previous field studies (Smith and Heath, 2002; Smith et al., 2003; Birdsey, 1996). Forest floor litter is composed of dead organic matter above the mineral soil horizons, including litter, humus, and fine woody debris. Larger branches and logs on the ground are counted as “down dead wood.” Organic carbon in soil is not included.
The change in carbon inventories from year to year—i.e., net storage—reflects increases in growth as well as decreases due to harvesting, land use change, and disturbances such as fire, insects, and disease. Overall, net carbon storage in forests of the contiguous 48 states has been positive since 1953 (Exhibit 6-13), indicating that over at least the last half-century, forests have served as a sink rather than a source of carbon. The average rate of net carbon storage in forests increased between the 1950s and the 1980s, peaking at 210 million metric tons of carbon per year (MtC/yr) from 1977 to 1986. The rate declined to 135 MtC/yr for the last period of record (1987-1996), with declining storage evident in live, dead, and understory pools. This decline is thought to be due to a combination of increased harvests relative to growth, more accurate data, and better accounting of emissions from dead wood (USDA Forest Service, 2004b). The rate of storage over this period is equivalent to approximately 9 to 10 percent of U.S. carbon dioxide emissions over a comparable period (U.S. EPA, 2005).
Carbon storage trends vary among regions of the country, depending on land use patterns and factors such as climate and soil quality. In three of the four major regions, net storage was positive throughout the period of record, with the North generally showing the largest net storage rates (Exhibit 6-14). The exception was the Pacific Coast region, which experienced net losses of forest carbon during two of the four reporting periods. Rates of net carbon storage appear to have decreased over time in the South; this trend is thought to be due to an increase in harvesting relative to growth (USDA Forest Service, 2004b). Some of the harvested carbon is sequestered in wood products.
- The data include only forest classified as “timberland,” which excludes about one-third of U.S. forest land cover. Historical data from Alaska and Hawaii are insufficient for inclusion in this indicator.
- Data are derived from state inventories that do not correspond exactly to the years identified in Exhibits 6-13 and 6-14.
- Carbon stored in forest soil is not included.
- Carbon pools are not measured, but are estimated based on inventory-to-carbon coefficients developed with information from ecological studies. These coefficients may change over time as new ecological studies are conducted, which could change storage rate estimates.
These limitations are discussed in detail in Heath and Smith (2000) and Smith and Heath (2000, 2001).
Exhibits 6-13 and 6-14 were previously published in the data supplement to USDA Forest Service (2004b). The numbers depicted in these figures have not been published, but were provided by the USDA Forest Service (2004a). The physical measurements used as inputs in the carbon storage models can be obtained from the FIA database (USDA Forest Service, 2005) (http://fia.fs.fed.us/tools-data/).
Birdsey, R.A. 1996. Carbon storage for major forest types and regions in the conterminous United States. In: Sampson, R.N., and D. Hair, eds. Forests and global change, volume 2: Forest management opportunities for mitigating carbon emissions. Washington, DC: American Forests. pp. 1-25, 261-308.
Heath, L.S., and J.E. Smith. 2000. An assessment of uncertainty in forest carbon budget projections. Environ. Sci. Policy 3:73-82.
Smith, J.E., and L.S. Heath. 2002. Estimators of forest floor carbon for United States forests. Res. Pap. NE-722. Newtown Square, PA: USDA Forest Service, Northeastern Research Station. 37 pp.
Smith, J.E., and L.S. Heath. 2001. Identifying influences on model uncertainty: An application using a forest carbon budget model. Environ. Manage. 27:253-267.
Smith, J.E., and L.S. Heath. 2000. Considerations for interpreting probabilistic estimates of uncertainty of forest carbon. In: Joyce, L.A., and R. Birdsey, eds. The impact of climate change on America’s forests. General Technical Report RMRS-59. Fort Collins, CO: USDA Forest Service, Rocky Mountain Research Station. pp. 102-111.
Smith, J.E., L.S. Heath, and J.C. Jenkins. 2003. Forest volume-to-biomass models and estimates of mass for live and standing dead trees of U.S. forests. General Technical Report NE-298. Newtown Square, PA: USDA Forest Service, Northeastern Research Station. 57 pp.
USDA Forest Service. 2005. Forest Inventory and Analysis (FIA) database. Accessed 2005. http://fia.fs.fed.us/tools-data/
USDA Forest Service. 2004a. Data provided to ERG (an EPA contractor) by Linda Heath, USDA Forest Service. December 23, 2004.
USDA Forest Service. 2004b. National report on sustainable forests—2003. http://www.fs.fed.us/research/sustain/ (main site); http://www.fs.fed.us/research/sustain/one_pagers/indicator%2027.pdf (data supplement: summary); http://www.fs.fed.us/research/sustain/documents/Indicator%2027/c5i27.pdf (data supplement: graphics and metadata)
U.S. EPA (United States Environmental Protection Agency). 2005. Inventory of U.S. greenhouse gas emissions and sinks: 1990-2003. EPA/430/R-05/003.
|Carbon Storage in Forests|
|2.||ROE Question(s) This Indicator Helps to Answer|
|This indicator is used to help answer one ROE question: "What are the trends in the ecological processes that sustain the Nation's ecological systems?"|
This indicator describes net carbon storage rates in forests in the contiguous 48 states from 1953 to 1996. This indicator provides information about the function of the nation’s ecosystems, as storage of carbon in forests is a key component of the carbon cycle and can be considered a proxy for forest productivity.
This indicator is adapted from figures depicting net carbon storage from 1953 to 1996 previously published in the data supplement to the U.S. Department of Agriculture (USDA) Forest Service report, "National Report on Sustainable Forests—2003" (USDA Forest Service, 2004a). The data come from surveys and analyses conducted by the USDA Forest Service’s Forest Inventory Analysis (FIA) program.
Exhibits 6-13 and 6-14 were previously published in the data supplement to the USDA Forest Service report, "National Report on Sustainable Forests—2003" (USDA Forest Service, 2004a), available at http://www.fs.fed.us/research/sustain/2003SustainabilityReport/. The numbers depicted in these figures have not been published, but were provided by the USDA Forest Service. For more information, contact Linda Heath (email@example.com).
This indicator was constructed from aggregate figures of timber volume. Smith et al. (2001) present aggregate figures of timber volume by state and forest type. This includes historical data for nominal years 1953, 1963, 1977, 1987, and 1997. Smith et al. (2001) do not include either the full set of volume data (by measuring location or by county) or the raw diameter data used for volume calculations.
The USDA Forest Service’s online FIA database (USDA Forest Service, 2005), available at http://fia.fs.fed.us/tools-data/, contains detailed datasets by state from the late 1970s through 2003. These datasets include raw data such as tree diameter from individual FIA plots which were used to generate the 1997 aggregate figures in Smith et al. (2001). Older aggregate figures in Smith et al. (2001) were based on similar datasets, but these older datasets are not available online. Some historical data may also be obtained indirectly through FIA’s online "map-maker" system, which depicts historical data from individual sampling plots without revealing exact coordinates of sites, which are kept confidential to protect property rights.
The USDA Forest Service does not provide online access to the output of the models it used to calculate the amount of carbon contained within the various components of the forest (live trees, forest floor, etc.). These data may be obtained by contacting the USDA Forest Service (Linda Heath, firstname.lastname@example.org).
The USDA Forest Service’s FIA program developed this indicator from field measurements of tree diameter and other variables collected on forest classified as "timberland" throughout the contiguous 48 states (USDA Forest Service, 2004b). Timberland accounts for about two-thirds of forest land cover in the contiguous 48 states.
FIA’s current (2004) sampling plan consists of three analytical phases:
FIA Phase 2 and Phase 3 ground measurements are conducted at sites chosen following a grid or hexagon pattern to present results that are well distributed in space. FIA describes current (2004) field data collection methodology in procedural manuals available online (http://fia.fs.fed.us/library/field-guides-methods-proc/). The main variable of interest, diameter at breast height (DBH), is measured at 4.5 feet from the ground on the uphill side of the tree, which is a conventional forestry practice. FIA also followed a consistent methodology for measuring trees with multiple trunks (Linda Heath, USDA Forest Service, personal communication, 2004).
FIA has published fact sheets to support and explain its three-tiered sampling methodology. See a detailed description of phases at http://fia.fs.fed.us/library/fact-sheets/data-collections/Phase2_3.pdf (2 pp, 142K, About PDF) and of sampling and plot design at http://fia.fs.fed.us/library/fact-sheets/data-collections/Sampling and Plot Design.pdf (3 pp, 414K). FIA has also documented some specific sampling procedures (e.g., the size of sample plots) for data collected in the 1990s and stored them on its online database at http://fiatools.fs.fed.us/fido/index.html. Scientific support for FIA methods can be found in a special issue of the Journal of Forestry (vol. 7, no. 12, 1999).
The USDA Forest Service does not present full information about historic sampling design online. FIA sampling methodologies have varied from year to year and from region to region in terms of overall sample grids (Ken Stolte, USDA Forest Service, personal communication, 2004). However, current FIA sampling procedures provide useful information about the general principles by which the FIA program has operated through the years. Although FIA presents no documentation to confirm that the FIA program operated under these procedures when it conducted earlier measurements (data for this indicator date back to 1953), other sources confirm that the FIA has consistently used a similar multi-tiered approach. As noted in EPA’s 2003 Draft Report on the Environment (p. B-39), the FIA has historically monitored approximately 450,000 Phase 2 sites (roughly every 3 miles) and 125,000 Phase 3 sites. The measurements used to construct this indicator most likely would have come from Phase 2 sites.
Until the late 1990s, FIA inventories were conducted in different years in different states. Hence, the years identified in the exhibits for this indicator are nominal; for example, "1977" represents the most recent inventory data that were available for each state as of 1977. Over the last decade, FIA has transitioned to a new study design that aims to map the entire nation every five years, with a portion of every state surveyed in every year. The sampling design does not include any special effort to represent sensitive populations or ecosystems. This indicator is designed to offer a very general representation of overall trends in forest growth. While particular forest types are identified during data collection, it is with the intention of ensuring that the statistical methods applied in data transformation are appropriate to the species in question (e.g., ratios of carbon storage to trunk diameter in oak-hickory forest versus scrub pine forest). Nonetheless, the availability of forest type data presents an opportunity for analysis of growth within specific forest types, including types that might be particularly sensitive to environmental stressors.
This indicator was created using models that transform direct measurements and observations (e.g., tree diameter) into estimates of the amount of carbon stored in several different types of reservoirs: aboveground live standing trees, aboveground standing dead trees, understory vegetation, down dead wood (including stumps), forest floor litter, belowground live trees (roots), and belowground dead wood. Smith et al. (2001) explain how these reservoirs are defined. The indicator does not include organic carbon in soil.
USDA Forest Service (2004b) cites several analytical methods. The USDA Forest Service estimates carbon storage from measurements of tree diameter using a series of statistical models that are supported by scientific peer-reviewed literature. First, statistical models are used to relate trunk size and the weight and volume of branches, leaves, and coarse roots. For this step, the USDA Forest Service uses species-specific "taper models." While models have varied over time and from one FIA region to another (Linda Heath, USDA Forest Service, personal communication, 2004), Hansen (2003) provides a good general reference for these models. Next, the USDA Forest Service uses live-tree volumes to estimate various carbon reservoirs, following equations derived from ecological studies described by Smith et al. (2003). The USDA Forest Service estimates carbon in the forest floor using additional models that account for forest area, type, and age (Smith and Heath, 2002). Carbon in understory vegetation is estimated using equations given in Birdsey (1996). In general, the process of converting forest inventory data to carbon through biometric models is supported by forest scientists as a standard analytical approach (Barford et al., 2001; IPCC, 1997).
This indicator requires spatial generalization based on a nationwide survey, which the survey has been designed to support. Aerial imagery helps ensure that measurements are extrapolated to the appropriate forest area and that generalizations account for the appropriate forest type. Projections are not extended beyond the spatial bounds of the inventory. Thus, FIA does not attempt to provide data for Alaska, Hawaii, or forest not classified as timberland. This indicator does not require temporal extrapolation or generalization, aside from the fact that data are grouped roughly by decade.
|9.||Quality Assurance and Quality Control|
Several of the fact sheets and field guides in FIA’s online library contain information about quality assurance and quality control (QA/QC) of data (http://fia.fs.fed.us/library/). One document in particular, http://fia.fs.fed.us/library/fact-sheets/data-collections/QA.pdf (2 pp, 174K), provides a thorough discussion of many aspects of the QA/QC process as it relates to field measurements. QA/QC results for the northeast region are available at http://www.fs.fed.us/ne/fia/methodology/p2/NEQAresults/index.html.
There are no established reference points, thresholds, or ranges for this indicator.
|11.||Comparability Over Time and Space|
This indicator can be used to evaluate general national trends, but it is limited somewhat by differences in methodology over time and space.
Historical survey design and sampling methods are not well documented and are likely to have changed over time. FIA does not measure every plot during every calendar year. Historically, FIA has inventoried different states in different years, and it may only inventory the same state every fifth or tenth year (source: various FIA documents linked from FIA’s Web site at http://fia.fs.fed.us/). In some cases, the interval may even be as great as 20 years (Ken Stolte, USDA Forest Service, personal communication, 2004). Another factor to consider is that while USDA Forest Service (2004a) classifies data as belonging to five base years (i.e., 1953, 1963, 1977, 1987, 1997), not all measurements were actually made in these years. For example, the 1953 dataset uses the most recent measurements that were available in 1953, some of which were substantially older.
FIA methods have varied over time and between regions, including criteria such as minimum tree size for measurement and the degree to which sample design includes relatively inaccessible areas (Linda Heath, USDA Forest Service, personal communication, 2004). The carbon storage indicator is probably best suited to remain an indicator of general spatial and temporal trends, rather than a specific numerical indicator of forest growth rates.
A useful discussion of assumptions in this indicator appears in Birdsey and Heath (2001).
|12.||Sources of Uncertainty|
Content under review.
|13.||Sources of Variability|
Content under review.
This indicator reports rates of change over time, but no detailed statistical trend analysis has been performed on this indicator.
Limitations to this indicator include the following:
Barford, C.C., S.C. Wofsy, M.L. Goulden, J.W. Munger, E.H. Pyle, S.P. Urbanski, L. Hutyra, S.R. Saleska, D. Fitzjarrald, and K. Moore. 2001. Factors controlling long– and short-term sequestration of atmospheric CO2 in a mid-latitude forest. Science 294:1688-1691.
Birdsey, R.A., and L.S. Heath. 2001. Forest inventory data, models, and assumptions for monitoring carbon flux. In: Lal, R., ed. Soil carbon sequestration and the greenhouse effect. SSSA Special Publication No. 57. Madison, WI: Soil Science Society of America, Inc. pp. 125-135.
Birdsey, R.A. 1996. Carbon storage for major forest types and regions in the conterminous United States. In: Sampson, R.N., and D. Hair, eds. Forests and global change, volume 2: forest management opportunities for mitigating carbon emissions. Washington, DC: American Forests. pp. 1-25, 261-308.
Birdsey, R.A. 1992. Carbon storage and accumulation in United States forest ecosystems. General Technical Report WO–59. Washington, DC: USDA Forest Service.
Hansen, M. 2003. Volume and biomass estimation in FIA: National consistency vs. regional accuracy. In: McRoberts, R.E., et al. Proceedings of the third annual forest inventory and analysis symposium; 2001 October 17–19; Traverse City, MI. General Technical Report NC-230. St. Paul, MN: USDA Forest Service, North Central Research Station. pp. 109-120. http://ncrs.fs.fed.us/pubs.
Heath, L.S., and J.E. Smith. 2000. An assessment of uncertainty in forest carbon budget projections. Environ. Sci. Policy 3:73-82.
IPCC (Intergovernmental Panel on Climate Change). 1997. Revised 1996 guidelines for national greenhouse gas inventories, vol. 1-3. Paris: IPCC/OECD/IEA. http://www.ipcc-nggip.iges.or.jp/public/gl/invs1.html.
Smith, J.E., L.S. Heath, and J.C. Jenkins. 2003. Forest volume–to–biomass models and estimates of mass for live and standing dead trees of U.S. forests. General Technical Report NE-298. Newtown Square, PA: USDA Forest Service, Northeastern Research Station.
Smith, J.E., and L.S. Heath. 2002. Estimators of forest floor carbon for United States forests. Res. Pap. NE–722. Newtown Square, PA: USDA Forest Service, Northeastern Research Station.
Smith, W.B., J.S. Vissage, D.R. Darr, and R.M. Sheffield. 2001. Forest resources of the United States, 1997. General Technical Report NC–219. St. Paul, MN: USDA Forest Service, North Central Research Station. http://www.ncrs.fs.fed.us/pubs/gtr/gtr_nc219.pdf (198 pp, 1.2MB).
Smith, J.E., and L.S. Heath. 2000. Considerations for interpreting probabilistic estimates of uncertainty of forest carbon. In: Joyce, L.A., and R. Birdsey, eds. The impact of climate change on America’s forests. General Technical Report RMRS–59. Fort Collins, CO: USDA Forest Service, Rocky Mountain Research Station. pp. 102–111.
USDA (United States Department of Agriculture) Forest Service. 2005. Forest Inventory and Analysis (FIA) database. Accessed 2005. http://fia.fs.fed.us/tools-data/.
USDA Forest Service. 2004a. National report on sustainable forests–2003. http://www.fs.fed.us/research/sustain/2003SustainabilityReport/.
USDA Forest Service. 2004b. Data report: A supplement to the national report on sustainable forests–2003. FS–766. http://www.fs.fed.us/research/sustain/2003SustainabilityReport/contents.htm.