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Criterion 5 Indicator 27
Rationale
Contribution of Forest Ecosystems to the Total Global Carbon Budget, Including Absorption and Release of Carbon
 
Human activities have changed and are continuing to change the chemical composition of the atmosphere. One of the major changes in the atmosphere is the concentration of carbon dioxide. During the process of photosynthesis, woody plants absorb carbon dioxide, store the carbon in wood, and release oxygen, thereby mitigating the increase of carbon dioxide in the atmosphere. Because of their large, low-cost, carbon storage potential, forests provide one of the few opportunities to absorb (sequester) carbon. However, forests may also release (emit) carbon. Knowledge about change in carbon, called carbon flux, provides important information in regulating atmospheric carbon. For more information on carbon as a policy issue, see Heath and Joyce (1997).

Can This Indicator Be Quantified
Carbon flux can be estimated from forest inventory data. Carbon content in forest inventories can be estimated using factors contained in Birdsey (1992), starting with dry weight biomass. Although carbon flux is expected from all ecosystem components, we concentrate on changes in live and dead biomass, either standing or recently harvested. Much of the carbon flux is in the biomass component. The growth component represents the absorption of carbon. Decomposition of mortality, standing dead trees counted as mortality in a previous inventory measurement cycle, and logging residue result in emissions. Estimates are presented in Table 27-1. The removal of wood by harvests is counted in Indicator 28.  
Table 27-1. Contribution of forest tree biomass by component to the total global carbon budget. A negative flux indicates carbon is being emitted to the atmosphere; a positive flux indicates carbon is being sequestered from the atmosphere and stored in the forest.
 
Component Carbon flux (million metric tons/year)
Growth19.7
Mortality-2.5
Logging residue-1.5
Total tree carbon flux15.7
 
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The growth component is simply a function of average annual gross growth. Logging residue was estimated based on an average harvest for the years 1993-1997, taken from harvest estimates provided by the Oregon Department of Forestry. An average ratio of total biomass to merchantable biomass per tree was estimated across all trees, and this ratio was applied to the average harvest to estimate logging residue consistent with the data. Logging residue and annual mortality were assumed to decompose in the same year they appeared.

Data Source and Availability
 
These estimates are based on forest inventory data in the Westwide forest inventory data base (Woudenberg and Farrenkopf, 1995). The database contains plot and tree records of an inventory of sample plots throughout the western United States. The sample is designed to produce estimates of total volume by state on productive forestland within a designated sampling error. The summary statistics in this indicator are based on a pre-release of data for Oregon (Woudenberg and Farrenkopf, 1995. Tree measurements are not always taken on reserved forestland and on forestland of low productivity. Overall, tree data were not collected on approximately 1,585,662 hectares of forestland.

Reliability of Data
   
Data on privately-owned lands in western Oregon were collected in 1985, and are somewhat dated. New inventory data are scheduled for release soon. The tree data for private lands in eastern Oregon were collected in 1992 on plots established previously. Data for National Forest and Bureau of Land Management forests were collected in 1995. As the new data for privately-owned lands are released and analyzed, the results will become increasingly reliable, especially for aboveground carbon. At the national level, a preliminary uncertainty estimate of carbon flux (90-percent probability) is approximately ±19% (Heath and Smith, in review).

Scale
   
One inventory plot represents on average 1,312 hectares. Area estimates are augmented to match land areas from the 1980 census. The minimum area for classification of forest land is 0.4048 hectares.

Recommended Action for Data Collection
 
For a comprehensive carbon estimate, inventory data needs to be collected on all forestland. Currently, data are not collected on about 10% of the forestland base. Collection of non-living tree data would also improve estimates. Some data on coarse woody debris and decay class of standing dead trees are already collected, but the data are not included in the standard database. A consistent compilation of the inventory data would also aid in estimating this indicator.

Definitions
   
Forestland – Land at least 10% stocked by forest trees of any size, including land that formerly had such tree cover and that will be regenerated. Strips of forest must have a crown width of 36.58 meters to qualify as forest land.
 
Logging residue – Biomass left on the ground after trees have been harvested. In this indicator, logging residue was estimated as a ratio of the recorded harvest. The ratio is the average ratio of total gross biomass, oven dry-weight, and merchantable biomass oven-dry weight for all 12.5 cm dbh or larger trees in the database.
 
Productive forestland – Forestland that is capable of producing more than 1.4 cubic meters per hectare per year of industrial wood in natural stands.
 
Reserved forestland Forestland that is withdrawn from harvest by statute or administrative regulation.
 

Selected References
   
Birdsey, R.A. 1992. Carbon storage and accumulation in United States forest ecosystems. USDA Forest Service, Washington Office, General Technical Report, WO-59. 51 p.
 
Heath, L.S., and L.A. Joyce. 1997. Carbon sequestration in forests as a national policy issue. In: Communicating the role of silviculture in managing the national forests: Proceedings of the National Silviculture Workshop, p. 29-36. US DA Forest Service, Northeastern Forest Experiment Station, General Technical Report NE-GTR-238. 205 p.
 
Heath, L.S., and J.E. Smith. In review. An assessment of uncertainty in forest carbon budget projections.
 
Woudenberg, S.W., and T.O. Farrenkopf. 1995. The Westwide forest inventory data base: user’s manual. USDA Forest Service, Intermountain Research Station, General Technical Report INT-GTR-317. 67 p.