5. Carbon content estimation

It is becoming increasingly important to tie carbon content to forest inventory estimates. Regional and national estimates of ecosystem carbon content, and change in ecosystem carbon content over time, are important in global carbon cycling and its impact on atmospheric greenhouse gases and climate. International agreements are requiring improvements in the ability to assess forest carbon stocks and their change.

Carbon content is relatively easy to assess for the overstory and other vegetation types. In many cases, vegetative carbon is used as a surrogate for total ecosystem carbon since it is relatively easy to derive from existing information or ongoing inventory efforts. Total ecosystem carbon, which includes inorganic ecosystem components such as soil, is more difficult to assess, especially if the precision of the estimates must be quantified.

5.1 Carbon Content of Vegetation

The carbon content of vegetation is surprisingly constant across a wide variety of tissue types and species. Schlesinger (1991) noted that C content of biomass is almost always found to be between 45 and 50% (by oven-dry mass).

In many applications, the carbon content of vegetation may be estimated by simply taking a fraction of the biomass, say

where C is carbon content by mass, and B is oven-dry biomass.

For dead material, carbon content is a function of the state of decomposition. For material that can still be identified, such as fresh litter or standing dead trees, the above equation may be used to estimate C content if the mass of the material can be estimated, see section 5.2 below. For severely decomposed material, it may be necessary to determine C content of subsamples of material from a site, and then combine this with an estimate of mass of that class of material to derive an estimate of C content for that vegetative component.

Total carbon content of vegetation implies more than simply tree species. This involves other components of the plant community, such as herbs, shrubs, mosses, etc. The necessary strata must be determined for each situation. In some cases, it may make sense to obtain C content estimates for life forms such as epiphytes, while in other cases this is irrelevant. The approach follows the above for all classes of vegetation: first estimate the biomass using appropriate sampling methods and then apply the ratio to estimate C content.

5.2 Ecosystem Carbon Content

In addition to the carbon content of vegetation, it is necessary in many situations to estimate total ecosystem carbon content. This includes plant as well as non-plant biotic pools, and abiotic carbon pools as well.

For example, avian and mammalian biomass and carbon content is often ignored since it is such a small proportion of total ecosystem carbon, but it may be necessary in some cases to estimate arthropod biomass and carbon content in order to obtain a good estimate of total ecosystem C. Colonizing insects may comprise a significant portion of the total biomass of some systems, and abiotic materials incorporated into nests and colonies may also be a significant portion of total C.

A major abiotic carbon pool is the soil organic matter. This may in some cases be greater than the vegetative carbon. Dead plant material at the soil surface and in the upper soil horizons may also have a significant C content that should be considered in any estimate of ecosystem C content. This information is important in assessing fire risk as well as in estimating ecosystem carbon content, so the data may serve more than one purpose once collected. McKenzie et al. (2000) provide a compendium of methods for field data collection for carbon estimation in soil, litter, and coarse woody debris.

Carbon content of litter should usually be determined from field samples since some of the material may be extremely decomposed and the carbon content may differ from that of less decomposed material. Application of a ratio approach such as that described for vegetation can be use -see section 5.1 above- but will often underestimate C content of the litter layer due to the escape of carbonic gases during the process of decomposition.

Estimates of soil C may be obtained from field sampling, and this is the most precise and appropriate method to estimate site-specific carbon content. Field data collection should be used whenever precise estimates of soil C are needed, but it is important to consider temporal variation throughout a growing season in large studies that may require an extended sampling period. If a soil classification map for the area of concern exists, there may be information on carbon content for different soil types in the area. A given soil type may yet have different mean carbon content depending on the dominant vegetative cover and land use; soil under an agricultural field may have a very different C content than a similar soil under a mature forest. Estimates of soil C content may or may not be available for all conditions in an area of concern. Batjes (2000) provides access to an extensive database of global soil physical and chemical properties, including information that may be used to approximate soil C content in the absence of specific information. These estimates will be less precise than those obtained from field samples, but may be cost-efficient when high precision, site-specific estimates are not required. In many applications, it may be more cost effective, and ultimately result in higher precision in the final estimates, to use a greater number of less precise estimates of C content for individual sampling units than to measure C content of a subset of sampling units with high precision. The trade-offs here are a function of sampling design and cost, and must be evaluated in that context, see chapter on sample designs.