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V. Terminology in assessing and reporting forest carbon change

Dieter Schoene


Assessing, reporting and accounting carbon stock changes in forests necessitates new terminology. However, although the measurement of carbon release and sequestration are new themes in forestry, they can build on established concepts of traditional forestry and on existing terms for these concepts. Thus, the "annual carbon stock change" of a growing stand corresponds to its "net annual increment"; "mean carbon storage" of a forest translates into its "average growing stock" in terms of timber. It will be essential to use the new terms consistently and to employ traditional forestry terms in this new context correctly.

More importantly, an opportunity may still exist at this early stage to standardize terms a priori instead of having to go through the effort of harmonizing and adjusting a profusion of ingrained terms ex post. The following is a first attempt in that direction.

The need for consistency is already evident. Examples of muddled use of traditional and new terms and the negative consequences abound. Thus, the 1996 Revised IPCC Guidelines prescribe a method for calculating "gross annual growth increment", meaning "net annual increment" of a forest; mistakenly, "tree density" is used as a synonym for "wood density", disregarding the fact that wood density can be expressed in several measures, which lead to different values. The term "biomass expansion factors" is at times used incorrectly as a synonym for "biomass conversion factors". Biomass expansion factors, derived from, and only valid for, specific measures of growing stock, are indiscriminately applied to growing stock, increment or fellings, leading to flawed carbon stock changes in the latter cases. Which parts of trees are tallied as growing stock varies as widely as the components of biomass included in carbon stocks.

Forestry terms used in carbon assessment

The 1996 Revised IPCC Guidelines for LULUCF formula derive carbon stock changes in forests by the basic formula:

Gross annual growth increment = average annual growth per ha in biomass * hectares of land in the respective category

Gross annual biomass loss = total harvest by category incl. fuelwood *expansion ratio to treat slash

Annual biomass changer = total annual growth increment - total annual biomass loss

Therefore, the following forestry terms play a role in carbon assessment. National definitions of these terms can differ substantially; however, as they are firmly anchored in the national systems, harmonization and adjustment may be the most realistic approach.

Growing stock:

Standing volume (TBFRA 2000): Volume of standing trees, living or dead, above stump measured over bark to top (0 cm). Includes all trees with diameter over 0 cm (dbh). Includes tops of stems, large branches, and dead trees lying on the ground, which can still be used for fibre or fuel. Excludes small branches, twigs and foliage.

Growing stock (TBFRA 2000): The living tree component of standing volume.

Growing stock (FRA 2000): Stem volume of all living trees more than 10 cm diameter at breast height (or above buttresses if these are higher), over bark, measured from stump to top of bole. Excludes all branches.

Growing stock (USDA Forest Service Handbook): The main part of growing stock trees between a one foot stump height and the growing stock top. Also called merchantable stem. The minimum growing stock top is 4.0 inches diameter outside bark, or the point at which the main stem breaks into branches if this occurs before it reaches the top diameter.

Proposal for harmonization: Volume of all living trees more than x cm diameter at breast height (or above buttress if these are higher) over or under bark measured from ground or stump height to a top of y cm, excluding or including branches to a minimum diameter of z cm. Excludes smaller branches, twigs, foliage, seeds, roots. Also called merchantable stem65.

Growing stock losses: Harvest, fellings, removals, natural losses

IPCC Guidelines: They do not differentiate fellings from removals and use the generic term "harvest" without defining it. Yet, an exact definition is important for carbon accounting, as evidenced from the following.

Fellings (FRA 2000): Average volume of all trees, living or dead, measured over bark to a minimum diameter of 10 cm dbh, that are felled during a given period (e.g. annually), whether or not they are removed from the forest or other wooded land. Includes silvicultural and pre-commercial thinnings and cleanings of trees more than 10 cm dbh left in the forest and natural losses of trees above 10 cm dbh.

Fellings (TBFRA 2000): Identical, but with diameter of 0 cm.

Removals (FRA 2000): (Annual) removals that generate revenue for the owner of the forest or other wooded land or trees outside the forest. They refer to "Volume Actually Commercialized", i.e. volume under bark actually cut and removed from the forest. This volume may include wood for industrial purposes (e.g. sawlogs, veneer logs, etc.) and for local domestic use (e.g. rural uses for construction). Includes removals of trees felled during an earlier period and removal of natural losses. Excludes bark, stumps, branches, treetops, harvesting waste and removals of fuelwood.

Removals (TBFRA 2000): Average annual fellings that are removed from the forest, other wooded land or other felling site during the given reference period.

Natural losses (TBFRA 2000): Average annual losses to the growing stock during the given reference period, measured to a minimum diameter of 0 cm dbh, due to mortality from causes other than cutting by man, e.g. natural mortality, diseases, insects, fire, windthrow or other physical damage.

The difference between the terms related to harvesting is relevant for carbon assessment.

In a given year, substantial amounts of timber, which have been felled in previous years, may be removed. If such is the case, calculating carbon losses from removals may result in an overestimate of carbon drain66. Removals disregard biomass from pre-commercial harvests, small-diameter thinnings and cleanings, which may constitute a substantial fraction of fellings in young forests. On-site chipping of timber as fuel for heating and energy plants plays a growing role, yet fuelwood is excluded from removals. In this case carbon loss is underestimated.

The 1996 IPCC Good Practice Guidelines refer to the FAO Forest Products Yearbook for default "harvest" data. Yet, this annual report refers to removals, whereas the term fellings would be the one more closely related to carbon drain.

Proposal: Accept TBFRA separate definitions for removals and fellings and employ fellings in carbon stock change assessment wherever possible.

Growing stock gains: Growth, increment

The 1996 IPCC Guidelines use "gross annual growth increment" and calculate it from "average annual growth per ha in biomass", without defining the terms and without specifying treatment of mortality. Growth is a well-established general term in forestry and used when speaking of growth in general. Increment is more specific and preferred usage when applied to the increase stand parameters over time (Davis, 1966, p. 64). It is a technical term, referring to the increase in girth, diameter, height, basal area, volume, quality or value over a specific period (Helms, 1998), and is defined below.

Gross annual increment (TBFRA 2000): Average annual volume of increment over the reference period of all trees measured to a minimum diameter breast height of 0 cm. Includes the increment of trees which have been felled or have died during the reference period.

Net annual increment (TBFRA 2000): Average annual volume over the given reference period of gross increment less that of natural losses of all trees to a minimum dbh of 0 cm.

Proposal for harmonization: Procedure consistent with that proposed for growing stock (by necessity).

Terms in biomass stock and change assessment

Woody biomass (TBFRA 2000): The mass of woody parts (wood, bark, branches, twigs, stumps and roots) of trees, alive and dead, shrubs and bushes, measured to a minimum diameter of 0 cm dbh. Includes above-stump woody biomass, stumps and roots; excludes foliage. Also excludes non-woody vegetation.

Above-ground woody biomass (TBFRA 2000): The mass of the woody part (stem, bark, branches, twigs) of trees, alive or dead, shrubs and bushes, excluding stumps and roots.

Stumps and roots (TBFRA 2000): Parts of the whole tree volume, which excludes the above-stump woody biomass. Height of stump is taken to be that at which tree would be cut under normal felling practice in that country. Excludes small roots.

Woody biomass (FRA2000): As above. Includes stumps and roots, no explicit exclusion of small and fine roots

These terms from FRA 2000 are useful, but do not represent all necessary terms for biomass and carbon assessment. Some additional concepts are defined in the following, based as a first approach on the USDA Forest Service Handbook (cited in Wharton and Griffith, 1997).

Forest ecosystem biomass: Woody and non-woody biomass of a forest ecosystem.

Growing stock biomass (GB): The oven-dry weight (including or excluding bark) of growing stock. Also called merchantable stem biomass.

Timber67 biomass (TB): Growing stock biomass plus branches, twigs, foliage, seeds, coarse and fine roots. Differentiation into above-ground or above-stump biomass and below-ground or below-stump biomass.

Timber biomass volume (TBV): Volume of growing stock plus volume of branches, twigs, foliage, seeds, roots.

Fellings biomass (FB): Timber biomass of fellings.

Timber biomass increment (TBI): Oven-dry weight of annual (gross/net) increment plus net increment of branches, twigs, foliage, roots. (Smaller than net primary productivity of biomass from timber trees, which includes turnover of foliage, branches and fine roots.)

Proposal: Besides woody biomass, growing stock biomass and timber biomass might be used in carbon inventories and introduced separately into TBFRA to increase transparency, since these are the biomass and carbon pools directly calculated from growing stock. Non-timber woody and non-woody vegetation biomass and carbon stores should not be included in the biomass expansion or conversion factors since added variability and error may occur. The term "timber" might be replaced by "tree" or "stand".

TBFRA biomass definitions may include foliage and fine roots, as they are usually included in biomass studies and biomass expansion factors. They can represent a significant fraction of the biomass of young stands. Fine roots in particular represent a high fraction of primary productivity of these stands and are not included in soil organic matter.

When referring to carbon in the respective biomass, the expression "carbon in ..." might be used, e.g. carbon in fellings biomass, carbon in timber biomass or carbon in growing stock.

Defining conversion and biomass expansion factors

Currently, "biomass expansion factor" is a generic term used rather inconsistently to describe a multiplication factor, which expands growing stock or growing stock biomass to account for non-merchantable biomass components of the forest ecosystem. There is no consensus on the exact definitions of either the base for the expansion or the non-merchantable components to be included, or on the units of measurement of the product. The following exemplifies current practice.

Biomass expansion factors have been applied indiscriminately to growing stock, increment, fellings or removals of individual trees, entire stands, individual age classes or entire forests to derive stocks and changes of ecosystem biomass or its components (e.g. leaves, roots, above- or below-ground compartments).

Figure1 provides a systematic overview of the most important ways of employing or deriving biomass expansion factors.

Figure 1. Pathways of deriving and applying biomass expansion and conversion factors and proposal for consistent terminology

The nomenclature for biomass expansion factors chosen in Figure 1 follows recently proposed patterns (Snowdon et al., 2000; Wirth and Schumacher, 2002). Besides the above expansion factors, several conversion factors facilitate carbon inventories:

Specific densities68 (D) convert volumes to biomass oven-dry weight:

DG = wood density, usually defined as the ratio of oven-dry weight of wood over its green volume (Ilic et al., 2000, p.2; Reyes et al., 1992)

DI = wood density of increment, not necessarily identical to DG

DF = density of fellings, not necessarily identical to DG or DI

DTBV = density of timber biomass volume, likely to deviate from DG, DI,, DF

Carbon fractions (C%) convert biomass to carbon:

CG % = carbon fraction of timber biomass, not necessarily identical to the carbon content of timber biomass increment (CI%) or to the carbon content of the fellings biomass (CF%).

Parties define and measure growing stock, increment or fellings differently. Therefore, variations in biomass expansion factors may not reflect different biomass allocation in trees, but rather different minimum top and breast-height diameters, stump heights, measurement points, volume formulae, or dissimilar treatment of bark, defects, branches and harvest losses. Frequently, merchantable volumes can be adjusted to account for these different definitions by using established conversions or expert judgement (Kramer, 1982; Koehl, 2000). Where this cannot be accomplished, the expansion factors themselves may compensate for these different definitions, and will then differ for a legitimate reason. There are, therefore, two possible approaches for carbon stock change assessment:

Proposal: Currently there are no generally accepted ways of deriving or applying biomass expansion and conversion factors in carbon assessments or generally accepted definitions of terms and concepts. On the other hand, a definite need for consistent terminology is emerging and will become more urgent as the GPG enter into force and the first Commitment Period approaches. The framework for expansion and conversion factors, proposed here elaborating some approaches found in current literature, might be considered.


Davis, K.P. 1966. Forest management. New York McGraw-Hill. 519 pp. (p. 64).

Helms, J.A., ed. 1998. The dictionary of forestry. Bethesda, USA. Society of American Foresters. 210 pp.

Ilic, J., Boland, D., McDonald, M., Downes, G. & Blakemore, P. 2000. Wood density phase 1 - State of knowledge. National Carbon Accounting System Technical Report No.18. Canberra. Australian Greenhouse Office,. p. 2.

Johnson,W. & Sharpe, D. 1982. The ratio of total to merchantable forest biomass and its application to the global carbon budget. Can. Jour. of For. Research 13: 372-383.

Koehl, M. 2000. Reliabilitry and comparability of TBFRA 2000 results. In TBFRA 2000. Geneva, UN-ECE/FAO. pp. 27-61.

Kramer, H. 1982. Nutzungsplanung in der Forsteinrichtung. Frankfurt. Sauerländer Verlag.128 pp.

Reyes, Gisel, Brown, S., Chapman, J. & Lugo, A. 1992. Wood densities of tropical tree species. General technical report SO-88. Southern Experiment Station, New Orleans, LA. USDA Forest Service. 15 pp.

Snowdon, P., Eamus, D., Gibbons, P., Khanna, P., Keith, H., Raison, J. & Kirschbaum, M. 2000. Synthesis of allometrics, review of root biomass and design of future woody biomass sampling strategies. Canberra, Australian Greenhouse Office. 114 pp.

Wharton, E.H. & Griffith, D.M. 1997. Estimating total forest biomass in Maine, 1995. Radnor, PA. USDA Forest Service. 50 pp.

Wirth, C. & Schumacher, J. 2002. Biomass functions for Norway spruce in Central Europe. Presentation at the COST E21 Meeting, Besalu.

65 Various measures of growing stock can be adjusted and converted rather easily, based on known mensurational relations among tree components, e.g. taper curves, form classes, etc.

66 On the other hand, removals are measured under bark. Many biomass expansion factors expand volume over bark. Applying the latter would underestimate biomass drain by 5-25 percent.

67 Timber, as used here, is defined in the USDA Forest Survey Handbook as all living trees 5 inches DBH and larger, including growing stock trees and cull trees. The term is not quite appropriate, as "timber" apparently has a dual meaning: a) forest crops and stands containing timber; b) wood, other than fuelwood, potentially usable for lumber. Perhaps one might consider replacing timber by the more neutral term "tree" or "stand", e.g. tree biomass, stand biomass.

68 When it comes to carbon in wood products, the term density has to be defined very clearly, as oven-dry, green, basic density and specific gravity may be considered.

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