0865-B1

Terms and Pathways for Assessing and Reporting Forest Carbon Change

Dieter Schoene^[1]

Abstract

Currently there are no generally accepted definitions of many forest- and carbon- related terms, and no standard ways of applying biomass expansion factors in carbon stock change assessments. However, a definite need for consistent terminology is emerging and will become more urgent, as Parties continue to report under the United Nations Framework Convention on Climate Change (UNFCCC), as preparations for the next Global Forest Resource Assessment proceed, and as the first commitment period of the Kyoto Protocol approaches. A concerted effort to harmonize terminology, define biomass expansion factors and undertake more reliable forest and carbon inventories will facilitate synergies in carbon assessment for UNFCCC, the Kyoto Protocol and Global Forest Resource Assessments, as well as streamline consistent, transparent international reporting.

Perspective

Measuring carbon release and sequestration of trees, stands and forests is a novel task in forestry. Assessing, reporting and accounting these carbon stock changes requires new tools, but, as importantly, also new terminology. Any new terms should be compatible with and build on established forestry concepts and definitions.

Parties to the UNFCCC currently assess carbon stock changes in forests according to the 1996 Revised Guidelines of the Intergovernmental Panel on Climate Change (IPCC/OECD/IEA, 1996). The lack of and need for consistent terminology is evident: Biomass expansion factors sometimes extend growing stock weight to total biomass weight, sometimes growing stock volume to biomass volume, and in other instances growing stock volume to biomass weight. To complicate matters, biomass components included in carbon stock differ even more than tree parts included in growing stock.

Under the Kyoto Protocol (KP) and its Marrakech Accord (MA), tons of carbon sequestered by trees will be traded on world markets. Unless assessments are accurate, comparable, and consistent over time, trading may involve fake merchandize. One goal of the new "Good Practice Guidance" (GPG), currently being developed by IPCC, is to avoid this. Clearly, it will be essential to define and use new terms consistently, and to employ traditional forestry terms correctly in the new context. In addition, a window of opportunity may exist at this stage to standardize terms a priori instead of undergoing the effort of harmonizing and adjusting a profusion of ingrained terms ex post (FAO, 2002).

The 1996 Guidelines and the new GPG together will apply to the first commitment period of the KP, estimating forest carbon stock changes as follows:

Approximately 50% of biomass is carbon. This formula links the following forestry terms to carbon assessment.

Conventional forestry terms related to carbon assessment

For the last Global Forest Resource Assessment (FRA) (UN-ECE/FAO, 2000; FAO, 2001), forest inventory experts from 32 countries elaborated common definitions in the so-called Kotka III process (Nyyssönen and Ahti, 1996). In reporting for FRA, countries adjusted their national data to match these agreed definitions. Therefore, these definitions are also considered in the following.

Growing stock

Table 1 reflects a sample of definitions^[2].

Table 1: Comparative matrix for definitional elements for growing stock

Based on this sample, a generalized definition useful for consistent carbon stock assessment could be:

Growing stock is the volume of all living trees more than x cm diameter at breast height (or above buttress if these are higher) measured under or over bark 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, all roots. For comparable, consistent carbon stock assessment, Parties should specify the elements indicated in italics.

Related terms are "average growing stock", taken either over the lifetime of a stand, or at a certain time over all stands of a forest, and "residual growing stock", excluding fellings.

Growing stock losses

The 1996 Guidelines contains the term "harvest" which is intuitive, but ambiguous for the purpose of carbon assessment. "Fellings" "removals" and "natural loss", as defined for the purpose of FRA differentiate aspects which are essential to carbon accounting (table 2).

Table 2: Comparative matrix for definitional elements of losses from growing stock

In a given year, timber which has fallen or has been felled in a previous year, may be removed. If such is the case, calculating carbon losses from removals overestimates current annual carbon drain. However, underestimates are more common: removals do not comprise biomass from cleanings, pre-commercial harvests, and small-diameter thinnings left on-site for economic reasons. These may constitute a considerable part of fellings in forests where younger ages predominate. Local firewood and on-site chipping of timber as fuel stand to play a growing role, yet fuelwood is often excluded from removals. Moreover, removals are measured under bark, while biomass expansion factors frequently apply to volume over bark. Applying these factors to growing stock under bark would underestimate biomass loss by 10 to 25 %.

Fig. 1: Relation between removals and fellings in TBFRA2000

The 1996 Guidelines refer to the FAO Forest Products Yearbook (FAO, 2000) for default "harvest" data. Yet, this annual report lists "roundwood" production, which consists of removals under bark, including fuel wood, whereas fellings would more closely relate to actual carbon drain. Fig. 1 provides a comparison of fellings and removals gleaned from the TBFRA 2000 (UN-ECE/FAO, 2000). For industrialized countries, the function allows converting removal data to fellings and vice versa, or accounting for bark.

Compared to "harvest", the terms "removals" and "fellings" and "natural loss" facilitate carbon stock change assessment; the term "fellings" is most applicable. The definitions in FRA and the Forest Products Yearbook, should be clarified and harmonized, particularly in regard to non-bolewood components, fuelwood and bark; harmonizing GPG definitions with those in FRA and the Forest Products Yearbook would streamline reporting.

Growing stock gains

Growth is used in forestry as a general term. Increment is preferred usage when applied to the increase of stand parameters over time (Davis, 1966), referring as a more technical term to the increase in girth, diameter, height, basal area, volume, quality or value over a specific period (Helms, 1998), to the growth-induced increase of diameter, basal area, height, and volume of trees or stands over a time period (Kramer and Akca, 1982).

By necessity, elements and thresholds included in definitions of increment must correspond to those for growing stock. Therefore, Parties should specify the elements indicated there also for increment. In addition to definitions provided in FRA, table 3 provides common increment terms, which may also facilitate carbon stock change assessment.

Table 3: Comparative matrix for definitional elements for growing stock gains

Terms in forest biomass and carbon stock change assessment

The MA prescribes as carbon pools: above- and below ground biomass, litter, dead wood, and soil organic matter. Table 4 examines carbon pools which occur both in MA and FRA 2000.

Table 4: Biomass terms in FRA 2000 and in the Marrakech Accord

There are inconsistencies within FRA in regard to stumps and fine roots. More importantly, biomass-related terms in FRA do not match carbon pools in the MA, e.g. concerning coarse woody debris. Neither the MA nor the 1996 Guidelines define carbon pools with sufficient precision. Gaps remain related to foliage, dead wood, stumps, fine roots and woody and non-woody understory vegetation.

The GPG should clarify which elements are covered by above- and below-ground biomass and decide, if deadwood, non-woody and woody understory vegetation should be included in carbon stock change assessments. A future FRA might consider reporting tree biomass, coarse woody debris and understory vegetation separately or not at all, concurring with the GPG.

Some additional terms for biomass and carbon assessment are proposed in Table 5

Table 5: Additional biomass- and carbon-related terms

Biomass expansion and conversion 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. Biomass expansion factors for growing stock, increment and fellings differ in most instances. Figures 2 and 3 provide examples. In the past, even though virtually all published expansion factors apply to growing stock, they have been applied indiscriminately to growing stock, increment, fellings or removals. Increment- and felling expansion factors tend to differ strongly from GEF particularly for young stands.

Figure 2: Growing stock and increment expansion factors for Scotch pine of different ages. Based on Schoene and Schulte, 1999.

Figure 3: Deriving conifer FEF from GEF: Factors for adjusting GEF as a function of minimum top and quadratic mean stand diameter. Based on Kramer and Akca, 1982.

Figure 4 provides a systematic overview of the most important ways of employing biomass expansion factors (BEF) as growing stock expansion factors (GEF), increment expansion factors (IEF), or fellings expansion factors (FEF). The precise nomenclature for biomass expansion factors chosen follows recently proposed patterns (Snowdon et al, 2000; Wirth and Schumacher, 2002). For convenience, in the context of growing stock, one might simply speak of weight-weight, volume-volume, or volume-weight factors.

Figure 4: System and nomenclature of biomass expansion factors^[3]

Besides the expansion factors, conversion factors play a role in carbon inventories (Table 6)

Table 6: Conversion factors in biomass and carbon inventories

Applying definitions and biomass expansion factors

As definitions for growing stock, increment, fellings and losses differ, variations in reported 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 losses during felling.

On the other hand, these expansion factors do differ as a function of species, age, tree diameter, stand height, growing stock, geographical region, site quality, stand structure, nitrogen inputs, and climate change (Schoene and Schulte, 1999; Gracia et al, 2002; Snowdon et al, 2000; Brown et al, 1999). As a first option for applying expansion factors, locally derived factors could be applied locally only. They would then differ between countries, but will compensate automatically for different definitions. However, few specific GEF values are known and IEF 's remain virtually uncharted. To fill the gaps, major national efforts would have to be undertaken.

Deriving BEF's regionally in a concerted effort might appear as a second, more efficient option. As a prerequisite, merchantable volumes reflecting different national definitions would then have to be adjusted to an agreed, common definition. However, given the sensitivity of expansion factors to site conditions and geography, this international approach is not promising either. Parties are on the horns of a dilemma when assessing carbon change via increment and IEF's: They either accept highly uncertain carbon assessments, or high costs in deriving reliable expansion factors.

To some extent, biomass expansion factors represent only a makeshift tool for utilizing forest inventories for the purpose of carbon inventories. They are useful, whenever national capacities permit only rudimentary forest inventories, as in many developing countries (Saket, 2002). These countries are also ineligible for credits from forest management. Therefore, applying highly uncertain "default" expansion factors to equally uncertain increment data is then the only feasible option for 'guesstimating' carbon stock and their changes, although efforts to close information gaps for FRA and UNFCCC are underway (Saket et al, 2002), and should receive renewed impetus through the climate change regime.

Many developed countries, on the other hand, will earn carbon credits from their forests. With better capacities, they might consider eliminating large uncertainties related to increment by assessing net growing stock change via successive or continuous forest inventories. In the future, longer commitment periods, and a "rolling" mechanism for compensating over- or under-estimates of carbon change would facilitate such a choice. Given the prospective efforts in deriving even the most essential BEF's, developed countries might also consider establishing and applying allometric equations or tables for tree biomass directly to sample tree parameters in efficient, successive forest and carbon assessments, without the detour of biomass expansion factors.

Conclusions

Carbon stock changes are reported or accounted globally, but measured only indirectly via growing stock or tree biomass of forests. Therefore, established forestry terms should be employed correctly and consistently. Consistent biomass- and carbon- related terminology is as essential. As the latter is still in a nascent state, an opportunity for standardizing terms now exists, ensuring comparable, consistent assessment and reporting for FRA, UNFCCC and KP. The pathway for carbon stock change assessment might be updated for subsequent commitment periods for higher accuracy and precision.

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