Change in carbon stocks in dead organic matter

Dead organic matter (DOM) comprises dead wood and litter (See Table 1.1). Estimating the carbon dynamics of dead organic matter pools allows for increased accuracy in the reporting of where and when carbon emissions and removals occur. For example, only some of the carbon contained in biomass killed during a biomass burning is emitted into the atmosphere in the year of the fire. Most of the biomass is added to dead wood, litter and soil pools (dead fine roots are included in the soil) from where the C will be emitted over years to decades, as the dead organic matter decomposes. Decay rates differ greatly between regions, ranging from high in warm and moist environments to low in cold and dry environments. Although the carbon dynamics of dead organic matter pools are well understood qualitatively, countries may find it difficult to obtain actual data with national coverage on dead organic matter stocks and their dynamics.

In forest ecosystems, DOM pools tend to be largest following stand-replacing disturbances due to the addition of residual above-ground and below-ground (roots) biomass. In the years after the disturbance, DOM pools decline as carbon loss through decay exceeds the rate of carbon addition through litterfall, mortality and biomass turnover. Later in stand development, DOM pools increase again. Representing these dynamics requires separate estimation of age-dependent inputs and outputs associated with stand dynamics and disturbance-related inputs and losses. These more complex estimation procedures require higher Tier methods.

2.3.2.1 Land remaining in a land-use category

The Tier 1 assumption for both dead wood and litter pools for all land-use categories is that their stocks are not changing over time if the land remains within the same land-use category. Thus, the carbon in biomass killed during a disturbance or management event (less removal of harvested wood products) is assumed to be released entirely to the atmosphere in the year of the event. This is equivalent to the assumption that the carbon in non-merchantable and non-commercial components that are transferred to dead organic matter is equal to the amount of carbon released from dead organic matter to the atmosphere through decomposition and oxidation. Countries can use higher tier methods to estimate the carbon dynamics of dead organic matter. This section describes estimation methods if Tier 2 (or 3) methods are used.

Countries that use Tier 1 methods to estimate DOM pools in land remaining in the same land-use category, report zero changes in carbon stocks or carbon emissions from those pools. Following this rule, CO2 emissions resulting from the combustion of dead organic matter during fire are not reported, nor are the increases in dead organic matter carbon stocks in the years following fire. However, emissions of non-CO2 gases from burning of DOM pools are reported. Tier 2 methods for estimation of carbon stock changes in DOM pools calculate the changes in dead wood and litter carbon pools (Equation 2.17). Two methods can be used: either track inputs and outputs (the Gain-Loss Method, Equation 2.18) or estimate the difference in DOM pools at two points in time (Stock-Difference Method, Equation 2.19). These estimates require either detailed inventories that include repeated measurements of dead wood and litter pools, or models that simulate dead wood and litter dynamics. It is good practice to ensure that such models are tested against field measurements and are documented. Figure 2.3 provides the decision tree for identification of the appropriate tier to estimate changes in carbon stocks in dead organic matter.

Equation 2.17 summarizes the calculation to estimate the annual changes in carbon stock in DOM pools:

Equation 2.17

Annual change in carbon stocks in dead organic matter

Where:

ACdom = annual change in carbon stocks in dead organic matter (includes dead wood and litter), tonnes C yr-1

ACdw = change in carbon stocks in dead wood, tonnes C yr-1

AClt = change in carbon stocks in litter, tonnes C yr-

Figure 2.3 Generic decision tree for identification of appropriate tier to estimate changes in carbon stocks in dead organic matter for a land-use category

Figure 2.3 Generic decision tree for identification of appropriate tier to estimate changes in carbon stocks in dead organic matter for a land-use category

Acdom Figure

Note:

1: See Volume 1 Chapter 4, "Methodological Choice and Identification of Key Categories" (noting Section 4.1.2 on limited resources), for discussion of key categories and use of decision trees.

2: The two methods are defined in Equations 2.18 and 2.19, respectively.

The changes in carbon stocks in the dead wood and litter pools for an area remaining in a land-use category between inventories can be estimated using two methods, described in Equation 2.18 and Equation 2.19. The same equation is used for dead wood and litter pools, but their values are calculated separately.

Equation 2.18

Annual change in carbon stocks in dead wood or litter (Gain-Loss Method)

Where:

AC dom = annual change in carbon stocks in the dead wood/litter pool, tonnes C yr-1 A = area of managed land, ha

DOMin = average annual transfer of biomass into the dead wood/litter pool due to annual processes and disturbances, tonnes d.m. ha-1 yr-1 (see next Section for further details).

DOMout = average annual decay and disturbance carbon loss out of dead wood or litter pool, tonnes d.m. ha-1 yr-1

CF = carbon fraction of dry matter, tonne C (tonne d.m.)-1

The net balance of DOM pools specified in Equation 2.18, requires the estimation of both the inputs and outputs from annual processes (litterfall and decomposition) and the inputs and losses associated with disturbances. In practice, therefore, Tier 2 and Tier 3 approaches require estimates of the transfer and decay rates as well as activity data on harvesting and disturbances and their impacts on DOM pool dynamics. Note that the biomass inputs into DOM pools used in Equation 2.18 are a subset of the biomass losses estimated in Equation 2.7. The biomass losses in Equation 2.7 contain additional biomass that is removed from the site through harvest or lost to the atmosphere, in the case of fire.

The method chosen depends on available data and will likely be coordinated with the method chosen for biomass carbon stocks. Transfers into and out of a dead wood or litter pool for Equation 2.18 may be difficult to estimate. The stock difference method described in Equation 2.19 can be used by countries with forest inventory data that include DOM pool information, other survey data sampled according to the principles set out in Annex 3A.3 (Sampling) in Chapter 3, and/or models that simulate dead wood and litter dynamics.

Equation 2.19

Annual change in carbon stocks in dead wood or litter (Stock-Difference

Method)

Acdom =

T

AC = annual change in carbon stocks in dead wood or litter, tonnes C yr-1

A = area of managed land, ha

DOMti = dead wood/litter stock at time ti for managed land, tonnes d.m. ha-1

DOMt2 = dead wood/litter stock at time t2 for managed land, tonnes d.m. ha-1

T = (t2 - t1) = time period between time of the second stock estimate and the first stock estimate, yr

CF = carbon fraction of dry matter (default = 0.37 for litter), tonne C (tonne d.m.)-1

Note that whenever the stock change method is used (e.g., in Equation 2.19), the area used in the carbon stock calculations at times t1 and t2 must be identical. If the area is not identical then changes in area will confound the estimates of carbon stocks and stock changes. It is good practice to use the area at the end of the inventory period (t2) to define the area of land remaining in the land-use category. The stock changes on all areas that change land-use category between t1 and t2 are estimated in the new land-use category, as described in the sections on land converted to a new land category.

INPUT OF BIOMASS TO DEAD ORGANIC MATTER

Whenever a tree is felled, non-merchantable and non-commercial components (such as tops, branches, leaves, roots, and noncommercial trees) are left on the ground and transferred to dead organic matter pools. In addition, annual mortality can add substantial amounts of dead wood to that pool. For Tier 1 methods, the assumption is that the carbon contained in all biomass components that are transferred to dead organic matter pools will be released in the year of the transfer, whether from annual processes (litterfall and tree mortality), land management activities, fuelwood gathering, or disturbances. For estimation procedures based on higher Tiers, it is necessary to estimate the amount of biomass carbon that is transferred to dead organic matter. The quantity of biomass transferred to DOM is estimated using Equation 2.20.

Equation 2.20

Annual carbon in biomass transferred to dead organic matter

DOMin = {Lmortality + Lslash + (Ldisturbance * fBLol )}

Where:

DOMin = total carbon in biomass transferred to dead organic matter, tonnes C yr-1

Lmortality = annual biomass carbon transfer to DOM due to mortality, tonnes C yr-1 (See Equation 2.21)

Lslash = annual biomass carbon transfer to DOM as slash, tonnes C yr-1 (See Equations 2.22)

Ldisturbances = annual biomass carbon loss resulting from disturbances, tonnes C yr-1 (See Equation 2.14)

fBLol = fraction of biomass left to decay on the ground (transferred to dead organic matter) from loss due to disturbance. As shown in Table 2.1, the disturbance losses from the biomass pool are partitioned into the fractions that are added to dead wood (cell B in Table 2.1) and to litter (cell C), are released to the atmosphere in the case of fire (cell F) and, if salvage follows the disturbance, transferred to HWP (cell E).

Note: If root biomass increments are counted in Equation 2.10, then root biomass losses must also be counted in Equations 2.20, and 2.22.

Examples of the terms on the right hand side of Equation 2.20 are obtained as follows: Transfers to dead organic matter from mortality, Lmortanty

Mortality is caused by competition during stand development, age, diseases, and other processes that are not included as disturbances. Mortality cannot be neglected when using higher Tier estimation methods. In extensively managed stands without periodic partial cuts, mortality from competition during the stem exclusion phase, may represent 30-50% of total productivity of a stand during its lifetime. In regularly tended stands, additions to the dead organic matter pool from mortality may be negligible because partial cuts extract forest biomass that would otherwise be lost to mortality and transferred to dead organic matter pools. Available data for increment will normally report net annual increment, which is defined as net of losses from mortality. Since in this text, net annual growth is used as a basis to estimate biomass gains, mortality must not be subtracted again as a loss from biomass pools. Mortality must, however, be counted as an addition to the dead wood pool for Tier 2 and Tier 3 methods.

The equation for estimating mortality is provided in Equation 2.21:

Equation 2.21 Annual biomass carbon loss due to mortality

Where:

Lmortality = annual biomass carbon loss due to mortality, tonnes C yr-1 A = area of land remaining in the same land use, ha

Gw = above-ground biomass growth, tonnes d.m. ha-1 yr-1 (see Equation 2.10) CF = carbon fraction of dry matter, tonne C (tonne d.m.)-1 m = mortality rate expressed as a fraction of above-ground biomass growth

When data on mortality rates are expressed as proportion of growing stock volume, then the term Gw in Equation 2.21 should be replaced with growing stock volume to estimate annual transfer to DOM pools from mortality.

Mortality rates differ between stages of stand development and are highest during the stem exclusion phase of stand development. They also differ with stocking level, forest type, management intensity and disturbance history. Thus, providing default values for an entire climatic zone is not justified because the variation within a zone will be much larger than the variation between zones.

Annual carbon transfer to slash, Ls/as/

This involves estimating the quantity of slash left after wood removal or fuelwood removal and transfer of biomass from total annual carbon loss due to wood harvest (Equation 2.12). The estimate for logging slash is given in Equation 2.22 and which is derived from Equation 2.12 as explained below:

Equation 2.22 Annual carbon transfer to slash

Lsiah = [{ • BCEFr • (1 + R)}-{H • £>}]• CF

Where:

Lslash = annual carbon transfer from above-ground biomass to slash, including dead roots, tonnes C yr-1 H = annual wood harvest (wood or fuelwood removal), m3 yr-1

BCEFr = biomass conversion and expansion factors applicable to wood removals, which transform merchantable volume of wood removal into above-ground biomass removals, tonnes biomass removal (m3 of removals)-1. If BCEFR values are not available and if BEF and Density values are separately estimated then the following conversion can be used:

o D is basic wood density, tonnes d.m. m-3

o Biomass Expansion Factors (BEFR) expand merchantable wood removals to total aboveground biomass volume to account for non-merchantable components of the tree, stand and forest. BEFR is dimensionless.

R = ratio of below-ground biomass to above-ground biomass, in tonne d.m. below-ground biomass (tonne d.m. above-ground biomass)-1. R must be set to zero if root biomass increment is not included in Equation 2.10 (Tier 1)

CF = carbon fraction of dry matter, tonne C (tonne d.m.)-1

Fuelwood gathering that involves the removal of live tree parts does not generate any additional input of biomass to dead organic matter pools and is not further addressed here.

Inventories using higher Tier methods can also estimate the amount of logging slash remaining after harvest by defining the proportion of above-ground biomass that is left after harvest (enter these proportions in cells B and C of Table 2.1 for harvest disturbance) and by using the approach defined in Equation 2.14. In this approach, activity data for the area harvested would also be required.

2.3.2.2 Land conversion to a new land-use category

The reporting convention is that all carbon stock changes and non-CO2 greenhouse gas emissions associated with a land-use change be reported in the new land-use category. For example, in the case of conversion of Forest Land to Cropland, both the carbon stock changes associated with the clearing of the forest as well as any subsequent carbon stock changes that result from the conversion are reported under the Cropland category.

The Tier 1 assumption is that DOM pools in non-forest land categories after the conversion are zero, i.e., they contain no carbon. The Tier 1 assumption for land converted from forest to another land-use category is that all DOM carbon losses occur in the year of land-use conversion. Conversely, conversion to Forest Land results in buildup of litter and dead wood carbon pools starting from zero carbon in those pools. DOM carbon gains on land converted to forest occur linearly, starting from zero, over a transition period (default assumption is 20 years). This default period may be appropriate for litter carbon stocks, but in temperate and boreal regions it is probably too short for dead wood carbon stocks. Countries that use higher Tier methods can accommodate longer transition periods by subdividing the remaining category to accommodate strata that are in the later stages of transition.

The estimation of carbon stock changes during transition periods following land-use conversion requires that annual cohorts of the area subject to land-use change be tracked for the duration of the transition period. For example, DOM stocks are assumed to increase for 20 years after conversion to Forest Land. After 20 years, the area converted enters the category Forest Land Remaining Forest Land, and no further DOM changes are assumed, if a Tier 1 approach is applied. Under Tier 2 and 3, the period of conversion can be varied depending on vegetation and other factors that determine the time required for litter and dead wood pools to reach steady state.

Higher Tier estimation methods can use non-zero estimates of litter and dead wood pools in the appropriate land-use categories or subcategories. For example, settlements and agro-forestry systems can contain some litter and dead wood pools, but because management, site conditions, and many other factors influence the pool sizes, no global default values can be provided here. Higher Tier methods may also estimate the details of dead organic matter inputs and outputs associated with the land-use change.

The conceptual approach to estimating changes in carbon stocks in dead wood and litter pools is to estimate the difference in C stocks in the old and new land-use categories and to apply this change in the year of the conversion (carbon losses), or to distribute it uniformly over the length of the transition period (carbon gains) Equation 2.23:

AC™,, = annual change in carbon stocks in dead wood or litter, tonnes C yr-1

Where:

AC™,, = annual change in carbon stocks in dead wood or litter, tonnes C yr-1

Co = dead wood/litter stock, under the old land-use category, tonnes C ha-1 Cn = dead wood/litter stock, under the new land-use category, tonnes C ha-1 Aon = area undergoing conversion from old to new land-use category, ha

Ton = time period of the transition from old to new land-use category, yr. The Tier 1 default is 20 years for carbon stock increases and 1 year for carbon losses.

Inventories using a Tier 1 method assume that all carbon contained in biomass killed during a land-use conversion event (less harvested products that are removed) is emitted directly to the atmosphere and none is added to dead wood and litter pools. Tier 1 methods also assume that dead wood and litter pool carbon losses occur entirely in the year of the transition.

Countries using higher Tier methods can modify Co in Equation 2.23 by first accounting for the immediate effects of the land-use conversion in the year of the event. In this case, they would add to Co the carbon from biomass killed and transferred to the dead wood and litter pools and remove from Co any carbon released from dead wood and litter pools, e.g., during slash burning. In that case Co in Equation 2.23 would represent the dead wood or litter carbon stocks immediately after the land-use conversion. Co will transit to Cn over the transition period, using linear or more complex dynamics. A disturbance matrix (Table 2.1) can be defined to account for the pool transitions and releases during the land-use conversion, including the additions and removals to Co.

Countries using a Tier 1 approach can apply the Tier 1 default carbon stock estimates for litter, and if available dead wood pools, provided in Table 2.2, but should recognize that these are broad-scale estimates with considerable uncertainty when applied at the country level. Table 2.2 is incomplete because of the paucity of published data. A review of the literature has identified several problems. The IPCC definitions of dead organic matter carbon stocks include litter and dead wood. The litter pool contains all litter plus fine woody debris up to a diameter limit of 10 cm (see Chapter 1, Table 1.1). Published litter data generally do not include the fine woody debris component, so the litter values in Table 2.2 are incomplete.

There are numerous published studies of coarse woody debris (Harmon and Hua, 1991; Karjalainen and Kuuluvainen, 2002) and a few review papers (e.g., Harmon et al., 1986), and but to date only two studies are found to provide regional dead wood carbon pool estimates that are based on sample plot data. Krankina et al. (2002) included several regions in Russia and reported coarse woody debris (> 10 cm diameter) estimates of 2 to

7 Mg C ha-1. Cooms et al. (2002) reported regional carbon pools based on a statistical sample design for a small region in New Zealand. Regional compilations for Canada (Shaw et al., 2005) provide estimates of litter carbon pools based on a compilation of statistically non-representative sample plots, but do not include estimates of dead wood pools. Review papers such as Harmon et al. (1986) compile a number of estimates from the literature. For example, their Table 5 lists a range of coarse woody debris values for temperate deciduous forests of 11 - 38 Mg dry matter ha-1 and for temperate coniferous forests of 10 - 511 Mg dry matter ha-1. It is, however, statistically invalid to calculate a mean from these compilations as they are not representative samples of the dead wood pools in a region.

While it is the intent of these IPCC Guidelines to provide default values for all variables used in Tier 1 methodologies, it is currently not feasible to provide estimates of regional defaults values for litter (including fine woody debris < 10 cm diameter) and dead wood (> 10 cm diameter) carbon stocks. Litter pool estimates (excluding fine woody debris) are provided in Table 2.2. Tier 1 methodology only requires the estimates in Table 2.2 for lands converted from Forest Land to any other land-use category (carbon losses) and for lands converted to Forest Land (carbon gains). Tier 1 methods assume that litter and dead wood pools are zero in all non-forest categories and therefore transitions between non-forest categories involve no carbon stock changes in these two pools.

Table 2.2

Tier 1 default values for litter and dead wood carbon stocks

Forest type

Broadleaf

Needleleaf

Broadleaf

Needleleaf

Climate

deciduous

evergreen

deciduous

evergreen

Litter carbon stocks

Dead wood carbon stocks

of mature forests

of mature forests

(tonnes C ha-1)

(tonnes C ha-1)

Boreal, dry

25 (10 - 58)

(6 - 86)

n.a.b

n.a

Boreal, moist

(11 - 117)

55 (7 - 123)

n.a

n.a

Cold Temperate, dry

28 (23 - 33)a

27 (17 - 42) a

n.a

n.a

Cold temperate, moist

(5 - 31)a

(10 - 48) a

n.a

n.a

Warm Temperate, dry

28.2 (23.4 - 33.0)a

20.3 (17.3 - 21.1)a

n.a

n.a

Warm temperate, moist

(2 - 31) a

(6 - 42)a

n.a

n.a

Subtropical

(2 - 3)

4.1

n.a

n.a

Tropical

(1 - 3)

5.2

n.a

n.a

Source:

Litter: Note that these values do not include fine woody debris. Siltanen et al., 1997; and Smith and Heath, 2001; Tremblay et al., 2002; and Vogt et al.,1996, converted from mass to carbon by multiplying by conversion factor of 0.37 (Smith and Heath, 2001).

Dead Wood: No regional estimates of dead wood pools are currently available - see text for further comments

a Values in parentheses marked by superscript "a" are the 5th and 95th percentiles from simulations of inventory plots, while those without superscript "a" indicate the entire range.

b n.a. denotes 'not available'

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