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TABLE 5-3
AVERAGE ANNUAL ACCUMULATION OF DRY MATTER AS BIOMASS IN
PLANTATIONSForest Type
TropicalTemperate

Annual Increment in Biomass(tonnes dm/hectare/year)
Acacia spp.

15.0

Eucalyptus spp.

14.5

Tectona grandis

8.0

Pinus spp.

11.5

Pinus caribaea

10.0

Mixed Hardwoods

6.8

Mixed Fast-Growing
Hardwoods

12.5

Mixed Softwoods

14.5

Douglas fir

6.0

Loblolly pine

4.0

Sources: Derived from Brown et al., 1986. Farnum et al., 1983.
Note: These are average accumulation rates over expected plantation lifetimes; actual rates
will vary depending on the age of the plantation. The data for the temperate species are
based on measurements in the United States. Data on other species, and from other regions,should be supplied by individual countries (as available). Additional temperate estimates byspecies and by country can be derived from data in ECE/FAO (1992), assuming that country
averages of net annual increment for managed and unmanaged stands are reasonableapproximations for plantations.

Biomass Loss
Two approaches can be used to estimate biomass harvest and other losses from managed
forests. Depending on the data collection and typical forestry practices in a given
country, it may be appropriate to use either approach alone, or use both if the two
approaches complement each other. This judgement must be made by national experts ineach country.
Commercial Harvest Statistics. The first, and obvious, approach is to use statistics onamounts of biomass actually removed from forests. In countries where commercial
harvests of various kinds make up a large majority of total biomass losses, and statistics
are well maintained, this may be the only approach needed. Country-specific estimates ofcommercial harvest statistics are provided in annual FAO Forest Products Yearbooks(1993b), and periodic Assessments (e.g., FAO, 1993a), and are also generally available
from national governments.
In using commercial harvest statistics, users must pay careful attention to the units
involved. Commercial harvest statistics are often provided for the commercial portion of
biomass only, in cubic metres (m3) of roundwood. If this is the case, values will need tobe converted to tons of dry biomass, and total biomass removed including slash. Somegeneral default values for converting volume data to tons are 0.65 t dm/m3 for deciduoustrees and 0.45 t dm/m3 for conifers. See Box 6 for more detailed information. To
account for the biomass lost beyond the commercial wood portion, expansion ratios canbe applied. Some general default values from the literature are 1.75 for undisturbed

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forests and 1.90 for logged forests.13 There is considerable variability in these conversion
values and expansion ratios, so it is highly desirable to use more specific locally available
data. Also, some commercial harvest data may be reported as equivalent total biomass(i.e., expansion ratios already applied). It is important to check carefully the informationin the original harvest data to ensure that expansion ratios are used only whereappropriate.14

BOX 6
VARIABILITY IN DENSITIES OF TREE SPECIESThere is considerable variation in average densities for different tree
species. While the broad average default values given in the text can beused for initial calculations, it is much better to use actual measured averagevalues if available, or literature values specific to the dominant species in aparticular forest. Dixon et al. (1991), for example, give densities for over
150 individual species, which range from 0.31 to 0.86 g/cm3. Other sourcesof wood densities include USDA Forest Service (1987), Cannell (1984),
Schroeder (1992), Dewar and Cannell (1992), UN ECE/FAO (1992),
Nabuurs and Mohren (1993) and Hamilton (1985).

Fuelwood Consumption Accounting. In many countries, however, commercial harvest
statistics will only partly account for wood removals. Significant quantities may beremoved from forests on an informal basis (i.e., they are never accounted for incommercial statistics). In these cases FAO statistics of fuelwood consumption can beused to supplement the commercial harvest data.
Any wood which was extracted from cleared forests and used for fuel will already havebeen accounted for in the forest and grassland conversion calculations above. This amountshould be subtracted from total wood consumed directly for fuel and for traditional
charcoal making, to determine the amount which must have come from remaining
managed forests. The result of this calculation can then be combined with any
commercial harvest amounts to produce a total amount of biomass lost from managed
forests.
There is an implicit assumption that slash is not accumulating. The instantaneous releaseof CO2 from the current year's slash that is explicit in Equation 1 (2) is a simple
mathematical device to treat slash oxidation from previous years under the assumptionthat the slash pool is not changing. The expansion ratio for slash in Equation 1 (2) could
be modified to address the destruction of belowground biomass left after harvest.
Treatment of carbon released from belowground biomass (e.g., roots) is discussed in the“Refinements in Calculations” section of this chapter.

13 Volume to mass conversions and expansion factors are taken from Brown et al.,1989 which reports on tropical forests. However, the values are in the range of thosereported by ECE/FAO (1992) for temperate forests.
14 If significant amounts of non-commercial biomass (slash) are burned on site during

harvest, then emissions from this burning should be treated as described for burningassociated with forest or grassland conversion in the next section. A portion of theburned carbon would be stored as unburned charcoal, and non-CO2 gases should also be
calculated.

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The amount of biomass removed from forests and other woody biomass stocks in theinventory year should be subtracted from the annual growth in these stocks for the same
year to arrive at the annual change in biomass stocks, positive or negative[Equation 1 (3)]. This result should be converted to a change in C (using the general
default value of 0.50 t C/t dm, if necessary), and to CO2 (using the ratio 44/12). A
positive value for CO2 in stocks is a CO2 removal from the atmosphere while a negativevalue is an emission. For reporting purposes, the sign should be changed to conform tothe convention that emissions are positive and removals are negative (i.e., negativeemissions).

5.2.3 Forest and Grassland Conversion
CO2 release
This category includes conversion of existing forests and natural grasslands to other land
uses, such as agriculture. The calculation of carbon fluxes due to forest and grassland
conversion is in many ways the most complex of the emissions inventory components.
Responses of biological systems vary over different time-scales e.g., biomass burning
occurs at less than one year scale, decomposition of wood at the decade scale, and loss ofsoil carbon at several decades scale. Thus, it is necessary to consider forest clearing
activity over three different time-scales and to sum the results to estimate the total fluxin the current year. Also, as with all categories of forest management and land-use
change activity, it is necessary to determine net CO2 flux.
Forests can be cleared to convert land to a wide variety of other uses, including
agriculture, highways, urban development, etc.15 In all cases there is a net carbon releaseto the atmosphere which should be accounted for in this calculation. The predominantcurrent cause of forest clearing is conversion to pasture and cropland in the tropics. Thisis accomplished by an initial cutting of undergrowth and felling of trees. The biomass may
then be combusted in a series of on-site burns or taken off site to be burned as fuel, or
perhaps used for forest products. A portion of the biomass remaining on site as slash is
not completely combusted and remains on the ground where it decomposes slowly.16
Some of the decay of remaining carbon left on the ground is probably accomplished bytermites, which produce both CO2 and CH4.17 However, the CH4 release from cleared,
unburned biomass is very difficult to quantify and is ignored for purposes of the basic
calculation, where all of the carbon in biomass which decays is assumed to be released as
CO2. Of the portion burned on site, a small fraction of the carbon remains as charcoal,
which resists decay for well over 100 years or more. There is a great deal of uncertainty

15 Conversion of tropical forests to pasture and cropland accounts for the largest

share of global forest clearing and resulting CO2 emissions. The discussion and default
information focus on this case, as it is most important that national inventories accountfor the largest contributions to emissions first. Forest clearing for other purposes (e.g.,urban development) should also be accounted for to the extent possible. As less default
information is provided for these cases, this will require national experts to provide input
data.
16 Decomposition rates of woody slash generally depend on several factors including

humidity, temperature, and chemical composition (e.g., nutrient content and secondary
chemicals).
17 This issue is discussed in the section on possible refinements to the methodology.

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about the fraction of carbon which remains unburned in charcoal under these conditions
and also about the ultimate fate of this charcoal.18 The remainder is released
instantaneously to the atmosphere. For biomass removed for fuelwood, the fate is very
similar. A small fraction of the carbon remains in unburned charcoal which effectively
provides long term storage, while the majority of the carbon is released to theatmosphere.
For conversion of grasslands to crop or pasture lands, the default assumption is that
there is no change in aboveground biomass between the pre-conversion natural grasslandand the post-conversion crops or pasture. This assumption can be varied if there are
locally available data that show a net change (see Box 7).
Forest and grassland conversion also results in CO2 emissions through soil disturbance,
particularly when the conversion is to cultivated or tilled lands. When forests areconverted to croplands, a fraction of the soil carbon may be released as CO2, primarily
through oxidation of organic matter. This can be a long term process which continues
for many years after the change in land use occurs. The calculations in Section 5.3 allow
for estimation of loss in soil carbon due to land conversions.
Calculations
Emissions of CO2 due to forest and grassland conversions are calculated through asequence of steps treating:•the net change in aboveground biomass carbon•

the portion of this change that is burned in the first year (either on- or off-site)
versus the amount left to decay over a longer time period•

for the burned portion, loss to the atmosphere versus long-term storage incharcoal•current emissions from decay of biomass cleared over the previous decade•current releases of carbon from soils due to conversions (decomposition of soilorganic matter).

18 The portion of burned carbon that remains on the ground as charcoal is highly

uncertain. Measurements following burning of a forest for conversion to pasture indicatethat 2.6 per cent of the pre-burn aboveground carbon, or 8.5 per cent of the burned
carbon, is converted to charcoal (Fearnside et al., 1990a). According to Fearnside et al.
(1990b), pastures are typically burned two to three times over about a 10-year period.
Under such a scenario, the latter burns probably result in combustion of some of thecharcoal formed during the first burn and formation of additional charcoal. Fearnside etal. (1990b) estimate that about 4.6 percent of the pre-burn aboveground carbon, or
10.1 percent of the burned carbon, is converted to charcoal under this scenario. Basedon results of observations in the Brazilian Amazon (Fearnside et al., 1990a) and in a
Florida pine forest (Comery, 1981), Crutzen and Andreae (1990) adopt charcoal valuesof 5 percent of the pre-burn aboveground carbon and 10 percent of the burned carbon
for clearing in the tropics. Recent estimates of charcoal produced suggest that thesevalues may be too high and will be re-assessed in future revisions to the Guidelines.

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BOX 7
ABOVEGROUND BIOMASS IN GRASSLANDS

Conversion of a grassland to cultivated land may result in net CO2
emissions to the atmosphere due to soil disturbance and resultant oxidationof soil carbon. In the simple default calculations, it is assumed that there isno net reduction in standing biomass because aboveground biomass
densities of grasslands are approximately the same as that of croplands andpasture. Therefore any changes in this aboveground pool due to the landuse change are likely to be generally small in comparison with other changes
in carbon stocks in terrestrial systems.
Consequently, changes inaboveground biomass are ignored in the basic calculation. As with all
default assumptions, users are encouraged to vary this one if they believe it
is inaccurate for their conditions. Some grasslands can contain significantly
more or less standing biomass than the default estimate of 10 tonnes dry
matter/hectare for grasslands. If national experts have data locally available
and differences are significant, these values should be used. In this case, theassumption of no net change in aboveground biomass would not be valid.
The net change in aboveground biomass in this situation would bedetermined with exactly the same procedure as used in the forest clearingcase.
Net change in aboveground biomass
First, the amount of aboveground biomass that is cleared in the emissions Inventory
Year19 is calculated by multiplying the annual forest area (or savannas, grasslands, etc., if
appropriate) converted to pasture or cropland or other land uses by the net change inaboveground biomass. This calculation is carried out for each relevant forest/grassland
type and, if appropriate, by region within a country.20 The net change is the difference
between the density (t dm/ha) of aboveground biomass on that forest/grassland prior toconversion, and the density of aboveground living biomass (t dm/ha) remaining as living
vegetation, after clearing. The after clearing value includes the biomass that regrows on
the land in the year after conversion and any original biomass which was not completely
cleared.
Tables 5-4 to 5-6 provide a range of values for aboveground biomass in forests prior toclearing, which can be used as default data if more appropriate and accurate data are not

19 For simplicity of explanation, the discussion refers to the Inventory Year as thoughdata for a single year were the desired input. However, as noted in the overview, forland use and forestry emissions estimates, it is recommended that data averaged over
several years be used in place of annual data. E.g., forest inventories are often done atfive or ten year periods - this would then be the period over which data would beaveraged to obtain the annual flux.
20 Defining regions will require balancing data

availability, biological and land-use
heterogeneity, and practical considerations such as the available time and effort.
Furthermore, developing adequate land use and land-use change data is a central issue. In
the case of land clearing, these data would likely be obtained from a combination of
departments of land management, agriculture, and forestry. These data will come at avariety of scales in time and space, and producing consistent records will be a challenging
task to all countries. In time, new internationally-based remote sensing programmes
could greatly facilitate this task; this is discussed in the technical appendix.

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available in a given country.21 For aboveground biomass after clearing, it is necessary to
account for any vegetation (i.e., crops, pasture, or forest) that replaces the vegetationthat was cleared. A reasonable figure for crops or pasture is 10 tonnes of dry biomass
per hectare (Houghton et al., 1987; see also Box 7). Higher estimates of biomass of
replacement crops is expected when perennials plants such as coffee, tea, cocoa, coconut,etc., are established. Where replacement by perennials is common (e.g., in many Asian
countries), every effort should be made to obtain representative values.
The
recommended default assumption is that all of the original aboveground biomass isdestroyed during clearing. If locally available data indicate that some fraction of theoriginal biomass is left living after clearing, this should be added to the after clearing value.

TABLE 5-4
AVERAGE ABOVEGROUND BIOMASS ESTIMATESFOR TROPICAL FORESTS BY CLIMATIC ZONE(TONNES DM/HA)
Tropical forests
Wet

R > 2000

Moist
with short
dry season

Moist
with
long dryseason2000>R>1000

Dry

MontaneMoist

Montane Dry

R<1000R>1000

R<1000300

14060-90a20-55 a105

40

Continental

225

185

100

75

190

no dataInsular275

175

no datalittle to noforest exists

255

no forest exists

America295

no data

90

105

150

50

AfricaAsia:

R= annual rainfall in mm/yr
Sources: Estimates were derived from a model in a geographic information system and calibrated with reliable forest inventory
data (Iverson et al., 1994) or from direct measurements (a P. Frost, pers. comm., 1996). Multi-date inventories were brought
to a common year of about 1980. The estimates do not distinguish between primary or secondary forests but represent
values averaged over the whole forested area in a given climatic zone in a given tropical region. These average values can
include forests in all successional states, from mature or undisturbed to young secondary. Additional country-specificbiomass estimates are presented in Table 5-5. Data are from Brown et. al. (1993) for Asia; Brown and Gaston (1995) for
Africa; and S. Brown (pers. comm., 1995) for America.
REGIONAL DEFAULT ESTIMATES FOR BIOMASS DENSITY MAY BE USED AS AN INITIAL STARTING POINT OR FORCOMPARISON PURPOSES. HOWEVER, IN ANY COUNTRY FOR WHICH FOREST CONVERSION OR REGROWTH IS
A SIGNIFICANT SOURCE OR SINK, LOCAL EXPERTS AND MEASUREMENTS SHOULD BE CONSULTED TO DEVELOPMORE ACCURATE VALUES REFLECTING LOCAL CONDITIONS.

21 As in the case of land-use data, developing appropriate biomass data is a challenging

task. In theory, it can be obtained directly by destructive sampling but this is unrealistic
for adequate coverage for even small countries. An alternative approach is to use
inventory data where one exploits volumetric data on merchantable timber and uses a
sequence of expansion factors to convert this to total stemwood, total above groundbiomass, and total biomass. See the references to Tables 5-3 and 5-4.

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TABLE 5-5
ABOVEGROUND BIOMASS ESTIMATES FOR VARIOUS
TROPICAL FOREST TYPES BY COUNTRY(TONNES DM/HA)Country

Forest Type

Climatic Zone

Aboveground
Biomass

AfricaBenin

Closed forest
Tree savanna

Dry
Dry

175
96

Botswanaa

Mixed tree savanna

Dry-long dry season

19

Burkina Faso
(National)

Degraded treesavannaDry- long dry season

20

CameroonPrimary

Very moist

310

Gambia(National)

Gallery forest
Closed woodland
Open woodland
Tree savanna

Moist- dry season
Dry
Dry
Dry

140
97
50
28

Ghana

Closed forest

Moist-short dry season

395

Guinea(National)

Mixed; closed
Open, secondary

Moist-long & short dry

135

Mozambique

Dense forest
Dense forest
Dense forest

Moist- long dry
Moist- long dryDry- long dry season

120
130
70

Zambiaa

Woodland-miombo
Woodland-miombo

Moist-long dry season
Dry-long dry season

91
81

Zimbabwea

Woodland-miombo

Dry-long dry season

29

Bangladesh

Closed -large crowns
Closed -small crowns
Disturbed closed
Disturbed open

Very moist
Very moist
Very moist
Very moist

206-210150
190
85

Cambodia

Dense
Semi-dense
Secondary
Open
Open
Well to poorly stocked
Evergreen
Deciduous

Moist-short dry
Moist-short dry
Moist-short dry
Moist-short dry
Moist-long dry
Moist-long dry

295
370
190
160
70
100-155

Moist-long dry

120

High to low volume
ClosedForest fallow

Dry

44-81

Dry

16

Asia

India

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TABLE 5-5 (CONT.)ABOVEGROUND BIOMASS ESTIMATES FOR VARIOUS
TROPICAL FOREST TYPES BY COUNTRY(TONNES DM/HA)Country

Forest Type

Climatic Zone

Aboveground
Biomass

Asia - (cont)
Malaysia-Peninsular(National)

Superior/moderate hill
Poor hill
Upper hill
Disturbed hill
Logged hill
Forest fallow
Freshwater swamp
Disturbed freshwater swamp
Logged freshwater swamp

Very moist
Very moist
Very moist
Very moist
Very moist
Very moist
Very moist
Very moist
Very moist

245-310
180
275
200
180
140
220
285
185

Malaysia- Sarawak

Mixed dipterocarps-dense
stocking, flat to undulating
terrain
Mixed dipterocarps-dense
stocking, mountainous
Mixed dipterocarps- medium
stocking, flat to mountainous

Very moist

325-385

Very moist

330-405

Very moist

280-330

MyanmarEvergreen
Mixed deciduous
Indaing forest

Moist-short dry
Moist-short dry
Moist-short dry

60-20045-135
10-65

Philippines

Old-growth dipterocarp
Logged dipterocarp

Very moist
Very moist

370-520300-370

Sri Lanka

Evergreen-high yield
Evergreen-medium yield
Evergreen-low yield
Evergreen-logged
Secondary

Very moist
Very moist
Very moist
Very moist
Very moist435-530
365-470
190-400
255
280

ThailandDegraded dry evergreen

Moist-long dry

America -

5.28

85

All forests are located in the wet/very moist climatic zone except whereindicated.

Bolivia

Closed forest

230

BrazilClosed forest

315

Ecuador

Closed forest

182

French Guyana

Closed forest
Riparian forest
Savanna forest

309
275
205

Guatemala

Closed forest

242

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TABLE 5-5 (CONT.)ABOVEGROUND BIOMASS ESTIMATES FOR VARIOUS
TROPICAL FOREST TYPES BY COUNTRY(TONNES DM/HA)Country

Forest Type

Climatic Zone

Aboveground
Biomass

America - (cont.)

All forests are located in the wet/very moist climatic zone except where indicated.

GuyanaClosed forest
Logged forest
Wallaba forest-seasonal
Mixed forest
Low mixed forest
Liana forest
Wallaba forest
Wallaba forest on white sands

254
190
145
275
192
125
148
405

Nicaragua

Orifino forest
Lowland mixed
Mature forest
Secondary

240
235
240
183

PanamaHigh density-mixed
Low density-mixed
Campnosperma forest -high density
Campnosperma forest -low density
High density-mixed
Low density-mixed

Peru

Primary
Lightly logged
Heavily logged
Late secondary
Young secondary
Flooded secondary
Low forest

210
192
125
140
20
195
155

SurinamUpland forest
Small crown-upland
Savanna forest
Riparian forest
Liana forest
Wallaba forest

255
136
195
217
120
250

Venezuela

Semi-deciduous-dry
Closed forest

78
230

239-366
169-245860
470
186-252
118-143

Source: All biomass estimates were derived from either reliable forest inventory data for subnational to national
forest areas (sources of inventories and details of methods used to convert to biomass are given in Brown, (1996)
or from direct measurements, (a P. Frost, pers. comm., 1996).

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TABLE 5-6
DRY MATTER IN ABOVEGROUND BIOMASS IN TEMPERATE AND BOREAL FORESTS(TONNES DM/HA)
Temperate Forests

Boreal Forests

Coniferous

220 - 295

Broadleaf

175 - 250

Mixed broadleaf/coniferous

40 - 87

Coniferous

22 - 113

Forest-tundra

8 - 20Source:Temperate forest estimates from Whittaker and Likens (1973) and Houghton et al. (1983). Total biomass
estimates were converted to aboveground biomass by multiplying by 0.83 (Leith and Whittaker, 1975). Boreal
forests biomass estimates are from Bazilevich, (1993); Finnish Forest Research Institute, (1995); Kokorin andNazarov, (1995a); and Isaev et al. (1993). Alternative estimates of aboveground biomass per hectare, by country,
for coniferous species and non-coniferous species, can be derived using statistics provided in ECE/FAO (1992).
Most temperate and boreal countries have their own national estimates of biomass densities for forests whichshould be used. These default values are very rough estimates and are provided for comparison only.

Immediate emissions from burning
The biomass that is cleared has one of three immediate fates:
1.

a portion may be burned on site;2.

a portion may be removed from the conversion site and used as fuelwood or for
products;

3.

a portion is converted to slash, and decays on site to CO2 over a decade or so.
Some estimates in the literature suggest that a global average of about 50 per centof the cleared biomass is burned in the first year with the remaining 50 per cent left
to decay (e.g., Houghton, 1991; and Crutzen and Andreae, 1990). This value could
be used as a default for first order calculations if the user does not have access tomore appropriate local information. It is important to recognise that this average isdominated by practices in tropical America which has the largest current rate of
deforestation. There are certainly wide variations in burning practices between andwithin regions. It is highly recommended that, for final inventories, users provide
their own values reflecting practices and burning conditions in the regions ofinterest, rather than using the global default value. To calculate the gross amount ofcarbon released in the current year to the atmosphere it is necessary to consider
the burned portions and the decaying portion over different time horizons.

When a forest is cleared for pasture or agriculture use not all trees are cut; some ofthem are left standing live. The carbon in these remaining trees needs to be consideredin emission calculations.
To estimate the CO2 released by the burning of cleared aboveground vegetation,
estimate (a) the fraction of the affected biomass that is subjected to burning (theremaining, disturbed biomass is slash) and (b) the fraction of the burned biomass that isoxidised. The fraction of burned biomass which does not oxidise remains as charcoal.
The amount of biomass oxidised is converted to carbon units to estimate the carbon flux

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from burning.22 A reasonable average for converting from dry biomass to carbon
content is to multiply dry biomass by 0.50.23 Of the portion of cleared biomass which isburned, some of this may be burned in the field to facilitate clearing, and some may beremoved and used as fuel. The portion which is burned in the field is used subsequently
for calculating the non-CO2 trace gas emissions from open burning of cleared biomass, inthe next section. The amount removed for fuel is important for calculations of fuel wood
extracted from forest and other woody biomass stocks as described earlier in these basic
calculations.
Emissions from decay
The aboveground biomass which remained on site but was not burned is estimated tooxidise in roughly a decade, and this historical release associated with land clearing must
be considered. The 10-year period is a recommended default value, as a reasonablehistorical horizon in light of the twin realities of data availability and biological dynamics(see Houghton, 1991; and Crutzen and Andreae, 1990). This can be varied if the user has
data or a strong rationale to suggest that a longer or shorter average decay time is more
representative of local conditions. The "committed" flux calculation simply accounts forthe current oxidising of material left unburned during the specified historical decay
period.
The decay phenomenon can be simply characterised for emissions estimation purposes.
Each year, some portion of the cleared aboveground biomass is left as slash, and weassume that 10 per cent of this decomposes each year, based on the default 10 year
period. Therefore, the total carbon being released to the atmosphere in the Inventory
Year is a function of the land clearing rate for each of the past 10 years, and the portionof the aboveground carbon remaining on site but not combusted each year. The current
year emissions from decay of biomass cleared in a historical year would be 10 per cent of
the total decay. The total current emissions from decay of historically cleared biomass
would then be the sum of the current estimated emissions from biomass cleared in eachof the ten historical years.
To simplify the calculations, the methodology uses decade average values for the land
clearing and portion left to decay. Working with average values, one would divide thetotal emissions from decay by 10 to get the contribution of one "average" historical year's
clearing to current emissions, then multiply by 10 to account for ten historical years'
clearing which could be expected to affect current emissions. Obviously the division by
10 and multiplication by 10 cancel each other and can be ignored. Therefore, the flux in
the Inventory Year from aboveground vegetation decay due to current and historical land
clearing is simply expressed in Equation 2.

22 As discussed elsewhere in these Guidelines this method counts the carbon actuallyemitted as CO and CH4 as though it were carbon dioxide. Later in this chapter, theseemissions of CO and CH4 will also be estimated separately.
23 The range most cited is 0.43-0.58; hence it is suggested that 0.5 is the appropriate

default assumption.

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EQUATION 2
average annual land clearing over the period (default of 10 years)x
the average quantity of aboveground dry biomass per hectare remaining onsite as slash but not burned (either oxidised or converted to charcoal)x
carbon content of dry biomass=
flux in the Inventory Year from historical land clearing of the abovegroundvegetation

Soil carbon release
For calculating the annual CO2 flux associated with the loss of soil organic carbon
following forest clearing or grassland conversion, the methodology is described in Section
5.3 for all types of transitions, and will not be described further here.
Burning of Forests: Non-CO2 Trace gases
Where there is open burning associated with forest clearing (or other land-use change),
it is important to estimate the emissions of methane (CH4), carbon monoxide (CO),nitrous oxide (N2O), and oxides of nitrogen (NOx, i.e., NO and NO2). The approach isessentially the same as that used for non-CO2 trace gases for all burning of unprocessedbiomass, including savanna burning and field burning of crop residues. For these activities
there is a common approach in the proposed methodology in that crude estimates oftrace gas emissions can be based on ratios to the total carbon released by burning. The
carbon trace gas releases (CH4 and CO) are treated as direct ratios to total carbon
released. To handle nitrogen trace gases, ratios of nitrogen to carbon in biomass areused to derive total nitrogen released from burning, and then emissions of N2O and NOx
are based on ratios to total nitrogen release. Table 5-7 provides suggested default values
for trace gas emission ratios.24 These are presented with ranges which emphasise theiruncertainty. However, the basic calculation methodology requires that users select a best
estimate value.25

24 The emission ratios used in this section are derived from Crutzen and Andreae,(1990), Delmas, (1993) and Lacaux et al., (1993). They are based on measurements in awide variety of fires, including forest and savanna fires in the tropics and laboratory fires
using grasses and agricultural wastes as fuel. Research will need to be conducted in the
future to determine if more specific emission ratios, e.g., specific to forest fires, can beobtained. Also, emission ratios vary significantly between the flaming and smouldering
phases of a fire. CO2, N2O, and NOx are mainly emitted in the flaming stage, while CH4
and CO are mainly emitted during the smouldering stage (Lobert et al., 1990). The
relative importance of these two stages will vary between fires in different ecosystems
and under different climatic conditions, and so the emission ratios will vary. As inventory
methodologies are refined, emission ratios should be chosen to represent as closely as
possible the ecosystem type being burned, as well as the characteristics of the fire.
25 Emissions inventory developers are encouraged to provide estimates of uncertainty

along with these best estimate values where possible, or to provide some expression of

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All of the crude biomass burning calculations have two steps: 1) estimating total carbon
released, and 2) applying emission ratios to estimate emissions of the non-CO2 tracegases. In the case of burning of cleared forests (and other land conversion if
appropriate), step 1 has been carried out in the previous section which included theestimation of carbon emissions from the portion of biomass from conversions which isburned on site in the Inventory Year. The total carbon release from this on site burning(not including any carbon released from decay or soils) provides the basis for theInventory Year release of non-CO2 trace gases. To complete the calculations, it isnecessary only to add step 2 of the calculation – the release of non-CO2 trace gases fromcurrent burning.
TABLE 5-7
EMISSION RATIOS FOR OPEN BURNING OF CLEARED FORESTS
CompoundRatios

CH4

0.012

CO

0.06

N2 O

0.007
0.121

NOx
Sources:(0.009 - 0.015) a(0.04 - 0.08) b(0.005 - 0.009) c(0.094 - 0.148) c

a Delmas, 1993b Lacaux et al., 1993
c Crutzen and Andreae, 1990

Note: Ratios for carbon compounds, i.e., CH4 and CO, are mass of carbon compound released (in unitsof C) relative to mass of total carbon released from burning. Those for the nitrogen compounds are
expressed as the ratios of emission (in units of N) relative to total nitrogen released from the fuel.

Once the total carbon released from on site burning of cleared biomass has been
estimated, the emissions of CH4, CO, N2O, and NOx can be calculated (Crutzen and
Andreae, 1990). The total carbon released due to burning is multiplied by the emission
ratios of CH4 and CO relative to emissions of total carbon to yield total emissions of
CH4 and CO (each expressed in units of C). The emissions of CH4 and CO are
multiplied by 16/12 and 28/12, respectively, to convert to full molecular weights.
To calculate emissions of N2O and NOx, first the total carbon released is multiplied by
the estimated N/C ratio of the fuel by weight (0.01 is a general default value for thiscategory of fuel (Crutzen and Andreae, 1990)) to yield the total amount of nitrogen (N)released. The total N released is then multiplied by the ratios of emissions of N2O andNOx relative to the total N released from the fuel to yield emissions of N2O and NOx(expressed in units of N). To convert to full molecular weights, the emissions of N2O
and NOx are multiplied by 44/28 and 46/14, respectively.26
The trace gas emissions from burning calculation are summarised as follows:•

CH4 Emissions = (carbon released) x (emission ratio) x 16/12

the level of confidence associated with various point estimates provided in the inventory.
Procedures for reporting this uncertainty or confidence information are discussed in theReporting Instructions.
26 The molecular weight ratios given above for the emitted gases are with respect tothe weight of nitrogen in the molecule. Thus for N2O the ratio is 44/28 and for NOx it is
46/14. NO2 has been used as the reference molecule for NOx.

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CO Emissions = (carbon released) x (emission ratio) x 28/12•

N2O Emissions = (carbon released) x (N/C ratio) x (emission ratio) x 44/28•

NOx Emissions = (carbon released) x (N/C ratio) x (emission ratio) x 46/14

5.2.4 Abandonment of Managed Lands
If managed lands, e.g., croplands and pastures, are abandoned, carbon may re-accumulate
on the land and in the soil. In this section, only the carbon accumulation in biomass is
considered; accumulation in the soil, as organic carbon, is dealt with in Section 5.3. The
response of these converted systems to abandonment depends upon a complex suite ofissues including soil type, length of time in pasture or cultivation, and the type of original
ecosystem. It may be that some of the abandoned agricultural lands are too infertile,saline, or eroded for regrowth to occur. In this case, either the land remains in its
current state or it may further degrade and lose additional organic material (i.e., carbon in
the biomass and the soils). Therefore, to calculate changes in carbon flux from thisactivity, the area abandoned should first be split into parts: lands that re-accumulate
carbon naturally, and those that do not or perhaps even continue to degrade.
In the basic calculation, only those that begin to return to an approximation of theirprevious natural state are considered. Those that remain constant with respect tocarbon flux can be ignored. Likewise, the CO2 flux to the atmosphere for those lands
that continue to degrade is likely to be small on a global basis and hence is ignored in theinitial application of basic calculations. In some countries, abandoned lands which degrade
may be a significant problem and could be an important source of CO2 emissions.
Where lands continue to degrade, both aboveground biomass and soil carbon may decline
rapidly, e.g., due to erosion. However, carbon in eroded soil could be re-deposited inrivers, lakes, or other lands downstream. For countries which have significant areas ofsuch lands this issue should be considered in a more refined calculation.
Abandoned lands must be evaluated in the context of the various natural ecosystems
originally occupying them. In addition, the effect of previous patterns of abandonment
should be considered while recognising the desire for simplicity and practicality. The
process of recovery of aboveground biomass generally is slower than the human-induced
oxidation of biomass. With this in mind and in consideration of possible data sources it isrecommended that abandoned lands be evaluated in two time horizons. A twenty year
historical time horizon is suggested to capture the more rapid growth expected after
abandonment. A second time period – from 20 years after abandonment up to roughly
100 years – may be considered if data are available. 27
The calculation, by original ecosystem is straightforward. To estimate gains in biomass
carbon stocks the total area abandoned (total over the previous 20 years including theInventory Year) is multiplied by the average annual uptake of carbon in the abovegroundbiomass. If landuse data are available to support calculations over a longer time horizon,
national experts may want to consider adding a pool of forests and grasslands that are
27 It is clear that most forest systems will take longer than 100 years to return to thelevel of biomass contained in an undisturbed state. If data are available, it is possible tocalculate carbon sinks from regrowth on lands abandoned more than 100 years prior to
the Inventory Year. As a practical matter, however, it is unlikely that such data will beavailable in most countries, or that the magnitude of annual carbon accumulation would
be large. Therefore, it is not generally recommended to carry these calculations backmore than the specified period of 100 years.

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regrowing from abandonment that occurred more than 20 years ago. The growth rates
of aboveground biomass in these forests would be slower than those of forests
regrowing from abandonment that occurred less than 20 years ago. The same
calculations can be repeated for lands abandoned for more than 20 years and up to about
100 years prior to the Inventory Year.
Table 5-2 presents estimates of average annual aboveground biomass accumulation invegetation in various regrowing forest ecosystems following abandonment of cultivatedland or pasture28. These general growth rates, averaged over large regions and many
specific ecosystem types, should be considered only approximations as applied to theparticular lands regrowing in a given region or country. If more accurate data on thesegrowth rates are locally available, they should be used. If lands are regenerating tograssland, then the default assumption is that no significant changes in abovegroundbiomass occur. This can be varied based on locally available data. Accumulation ofaboveground dry biomass can be converted to carbon using a general default conversion
value for biomass of 0.5 t C/t dm.

5.3

CO2 Emissions and Uptake by Soils fromLand-Use Change and Management
5.3.1 Overview
The principal sources/sinks of CO2 in soils are associated with changes in the amount oforganic carbon stored in soils. Release of CO2 also occurs from inorganic sources, either
from naturally occurring carbonate minerals or from applied lime. CO2 flux fromweathering of native carbonate minerals is not a significant source in most agriculturalsoils, with the possible exception of irrigated arid and semi-arid soils (Schlesinger, 1986).
This section focuses on a methodology to estimate net fluxes of CO2 due to changes insoil organic carbon stocks. CO2 releases from liming applications are also dealt with.
Fundamentally, changes in organic carbon content are a function of the balance between
inputs to soil of photosynthetically-fixed carbon and losses of soil carbon via
decomposition. Soil erosion can also result in the loss (or gain) of carbon locally, but the
net effect of erosion on carbon losses as CO2 for large areas on a national scale isunclear and demands separate consideration.
For soils, both the quantity and quality of organic matter inputs and the rate ofdecomposition of soil organic carbon will be determined by the interaction of climate, soil
and landuse/management (including land-use history). In native ecosystems, climate andsoil conditions are the primary determinants of the carbon balance, because they control
both production and decomposition rates.
In agricultural systems, landuse andmanagement act to modify both the input of organic matter via residue production, crop
selection, fertilisation, harvest procedures, residue management and the rate ofdecomposition (by modifying microclimate and soil conditions through crop selection, soil
tillage, mulching, fertilisation, irrigation and liming).
28 Values given in Table 5-2 assume linear regrowth of aboveground biomass in eachof the two time periods (0-20 years and 21-100 years). In reality, as shown in Brown and
Lugo (1990), the regrowth in tropical forests is closer to a logistic function. The
calculation could be improved by breaking the 20 year period into finer segments,
assuming availability of data, to determine a weighted average.

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Estimates of CO2 fluxes can be made in two ways: (1) by direct measurement of CO2
flux, and (2) indirectly, through balance estimates of the net change in carbon stocks ofthe soil.
Direct estimates of CO2 influx, through photosynthesis, and CO2 efflux, throughdecomposition and respiration, are difficult to obtain for several reasons: both exhibit
large diurnal, short-term and seasonal variations requiring near continuous measurements
to derive annual estimates; different components of the overall fluxes operate on very
different time scales (e.g., plant respiration vs. mineralisation of recalcitrant organic
matter); and the magnitudes of influxes and effluxes of CO2 are large relative to theirdifference. The most practical means of directly measuring net CO2 fluxes for wholeecosystems is by using micrometeorological techniques, which are costly and timeconsuming. Such measurements for agricultural ecosystems are scarce.
Balance estimates of CO2 emissions from soils due to land use are based primarily onfield studies of changes in soil carbon occurring over several years, from repeatedmeasures of soil carbon in experimental plots or in sites representing a chronosequence,
e.g., sites that had experienced a change in land use or management at different times
prior to sampling. Such information, coupled with laboratory experiments, has been used
to develop statistical and simulation models of soil carbon change as a function of climate,soil and land-use and management factors. At the present time, we deem a carbon
balance-based approach as the most feasible alternative.

5.3.2

Description
Methodology

ofaCarbonBalance

The most rigorous approach to estimating net changes in soil carbon would be to
calculate inputs and outputs of carbon for a particular ecosystem and from the difference
calculate the annual net change in carbon storage. To integrate the various factors
determining the quantity and quality of carbon entering the soil as well as thedecomposition rates of soil organic carbon, a simulation model can be used. This kind of
an approach has recently been used at large spatial scales (i.e., regional and country-level)
to estimate changes in carbon stocks in agricultural soils in the United States (Donigian etal., 1994, Lee et al., 1993, Paustian et al., submitted) and Canada (Smith et al., in press,
Dumanski et al., submitted). Similar applications have been made at regional and global
scales to evaluate potential responses of soil carbon to global climate change (Jenkinson
et al., 1991, King et al., 1996, Potter et al., 1993, VEMAP, 1996). However, theseapplications have used relatively complex research models, requiring large numbers ofsimulation runs and sophisticated computer resources such as geographic information
systems to manage large amounts of input data. Moreover, the models have not been
extensively tested for many agricultural systems, particularly in the tropics. Thus, it is ouropinion that a "model-based" approach would not be feasible at the present time.
However, we feel this kind of an approach is a priority for further development and could
be incorporated in future inventory methodologies.
The current method employs an accounting approach based on estimating soil carbon
stocks and areas for major categories of agricultural land-use/management systems. A
similar approach has been used in an assessment of the carbon balance for the agricultural
sector in the former Soviet Union (Kolchugina and Vinson, 1996). The fundamental
principle in this approach is that after many years under a particular form of land use, soil
carbon levels tend towards an equilibrium state where carbon inputs from plant residues
and losses of carbon through decomposition roughly balance. A new steady state balanceis more rapidly reached in hot humid climates but the time to equilibrium can be muchslower in cool and/or dry climates. Hence net fluxes of CO2 can be associated with

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and management to another.
Since climate and soil type are major determinants of the kinds of management systems
employed, as well as the potential stocks of soil carbon and their responses tomanagement, the inventory approach is based on a classification according to eight major
climate regions and six major soil types.

5.3.3 Climate Categories
Eight broad climatic regions are defined for inventory purposes. Climate will have an
overriding effect on land use, management and cropping practices and a significant effecton soil carbon levels. Major soil taxonomic groups will to some degree co-vary withclimate regions. The eight climate regions chosen are: 1) cold temperate, dry, 2) cold
temperate, moist, 3) warm temperate, dry, 4) warm temperate, moist, 5) tropical, dry 6)
tropical moist (with long, dry season), 7) tropical moist (with short, dry season), and 8)
tropical, wet. These regions were chosen largely on the basis of major constraints on
crops and cropping practices.
The cold thermal regime in the temperate zone represents areas with short growing
seasons dominated by spring-sown cereals (e.g., wheat, barley, rye), root crops andperennial forages. The warm temperate zone is characterised by warmer summers andmilder winters, which are favourable for over-wintering cereals (e.g., winter wheat) andfor heat-demanding crops such as maize and soybeans. In the tropical zone, lowtemperatures are not generally a constraint on crop growth.
The moisture regimes are intended to coincide with major differences in management as
a function of moisture availability. In the dry temperate zone, moisture limits thepotential for continuous cropping, thus summer fallowing is widespread anddrought-tolerant crops are required. Irrigation, where available, is an importantmanagement practice. In the temperate, moist zone, precipitation is sufficient for
cropping every year, summer fallowing is not generally practised and irrigation is mainly
used for speciality crops. Moisture regimes in the tropics can be roughly classified byamounts of precipitation and length of the dry season. The dry tropics receive less than
1000 mm of precipitation, with a highly seasonal distribution and prolonged dry seasons.
Agricultural systems are mainly extensive and subsistence-based, with grazing being animportant component. Diverse and highly productive systems exist where irrigation ispossible. The moist tropics are divided into two classes, both having annual precipitationin the range of 1000-2000 mm, but with classes having a prolonged dry season (> 5months) versus short dry seasons (<5 months).
Management systems become
increasingly intensive as precipitation increases and is more evenly distributed. The wet
tropics are classified as areas receiving >2000 mm per year. In this zone, production
systems are mainly limited by fertility factors due to the preponderance of highly
weathered soils.

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5.3.4 Soil Categories
A stratification of up to six major soil groups is proposed, based on major differences intheir inherent carbon stocks and their response to management. The soil groups chosen
are:

TABLE 5-8
SOIL TYPE CLASSIFICATIONS
Soil Type

Examples for major FAO soil
taxonomic groups

Examples for major
USDA soil taxonomic
groupsHigh clay activity mineral soils

Vertisols, Chernozems,
Phaeozems, Luvisols

Vertisols, Mollisols,
high-base status Alfisols

Low clay activity mineral soils

Acrisols, Nitosols, Ferralsols

Ultisols, Oxisols, acidic
Alfisols

Sandy soils

Arenosols, sandy Regosols

Psamments

Volcanic soils

Andosols

Andisols

Aquic soils (wet soils)

Gleysols

Aquic suborders

Organic soils

Histosols

Histosols

The strong effect of texture and clay mineralogy on organic matter contents has been
well demonstrated in both temperate and tropical regions. Thus we distinguish four
major soil groups according to these criteria. Soils with high clay activity are defined ashaving appreciable contents of high activity clays (e.g., 2:1 expandable clays such asmontmorillonite) which are implicated in the long-term stabilisation of soil organic
matter, particularly in many carbon-rich temperate soils (Martel and Paul, 1974). Soils
with low clay activity are defined as soils with low activity clays (e.g., 1:1 non-expandable
clays such as kaolinite and gibbsite and hydrous oxide clays of iron and aluminium) whichhave a much lower ability to stabilise organic matter (Trumbore, 1993) and faster
responses to changes in the soil's carbon balance (Tiessen et al., 1994). Among these areincluded highly-weathered acid soils of subtropical and tropical regions. Sandy soils aredefined as soils having less than 8 per cent clay and greater than 70 per cent sand, which
generally have poor structural stability and a low capacity to stabilise carbon. These
coarse-textured soils can occur in many of the major taxonomic soil classes. The
Andosol soil group includes soils derived from volcanic materials, with allophane as theprimary colloidal mineral. These soils are generally rich in carbon and highly fertile.
The other two soil groups are distinguished on the basis of drainage and soil water status.
Aquic soils are defined as mineral soils which have developed in poorly-drained, wetenvironments resulting in reduced decomposition rates and high organic matter contents.
If artificially drained for agriculture they are subject to large losses of carbon. Histosols
or peat soils are organic soils which form under water-saturated conditions where
decomposition is greatly reduced. They can lose massive amounts of carbon over asustained period upon drainage and cultivation.

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5.3.5

Soil Carbon Responses to AgriculturallyRelated Practices

Land-use practices affect soil carbon stocks by modifying carbon inputs to soil as well as
the decomposition rate of soil organic matter. The most important kinds of management
practices, from the standpoint of their effects on carbon levels, are briefly described.

Land clearing from native vegetation
The clearing of native vegetation (e.g., forests, savanna, grassland, wetlands) to agriculture
almost invariably leads to a reduction in soil carbon as a result of decreased carbon inputs
and enhanced decomposition from the disturbed soil. This has been well documented fortemperate (Haas et al., 1957; Kononova, 1966; Dalal and Mayer, 1986; Paul and van Veen,
1978) as well as tropical (Nye and Greenland, 1960; Detwiler, 1986; Brown and Lugo,
1990) environments. Generally, the majority of the losses occur within the first few (<10) years, particularly in the tropics (Detwiler, 1986), although slower declines in soil
carbon can continue for many decades in organic matter rich soils, particularly intemperate regions (Tiessen et al., 1982; Rasmussen and Rohde, 1988). Erosion (by wind
and water) can be a significant factor in carbon losses (and redistribution) locally, but thepresent inventory methodology does not account for effects of erosion on net CO2
fluxes. Currently conversion of native vegetation to agricultural uses is occurring almost
exclusively in the tropics.
The magnitude of soil carbon decline can vary according to native vegetation, climate, soil
type, land clearing method, and subsequent management. Several reviews of theliterature (Schlesinger, 1986; Mann 1986; Detwiler 1986) show losses of 20-40 per centor more of the original soil carbon stock following cultivation. Mann (1985) reported anaverage of 40 per cent lower carbon contents in cultivated vs. uncultivated soils (0-15cm), from a sample of more than 300 temperate forest- and grassland-derived soils.
Davidson and Ackerman (1993) analysed soil carbon inventories, from paired
comparisons including both concentration and bulk density changes. They reported
average reductions in soil carbon stocks of about 40 per cent for the surface (A) horizon
and about 30 per cent for the top 30 cm.
Clearing of native vegetation and conversion directly to pasture can result in lower
carbon losses compared to clearing for cultivation. After initial decreases following forest
clearance, soil carbon may even increase under pasture to levels comparable to preclearance levels within 10 years (Cerri et al., 1991; Eden et al., 1991; Lugo and Brown,1993). With improved management (fertiliser and moderate grazing), high soil carbon
levels may be sustainable over longer periods. However, unmanaged and overgrazed
pastures are likely to decline in productivity and be subject to erosion and soil
degradation, and subsequent decreases in soil carbon (Eden et al., 1991).

Conversion of cultivated land to perennial vegetation and shifting
cultivation
In most cases, land that has been cultivated for many years is depleted in organic matter
relative to its original state. If converted to perennial vegetation, either through land
abandonment and natural succession or as an active management decision (e.g.,conversion to pasture, land set-asides for conservation practices), soil carbon levels
generally increase. An exception is where the land has been degraded to the extent that
productivity is permanently impaired in which case soil carbon levels may decline further.
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Europe. Agricultural set-aside programmes have also been implemented in North
America and Europe during recent years. The rate of increase and the level at which soil
carbon is eventually stabilised on these lands will depend on productivity levels and soil
conditions. Low rates of carbon accrual, 0.05-0.15 tonnes C/ha/yr over 25-50 year
periods, have been reported for abandoned fields in semi-arid regions of the UnitedStates and Canada (Dormaar and Smoliak, 1985; Burke et al., 1995). For land set aside
planted to perennial grasses and then left unmanaged, Paustian et al. (submitted)
estimated rates of carbon increase ranging from 0.05-0.30 tonnes C/ha/yr, for semi-arid
to sub-humid regions in the central United States. Higher rates of 0.25-0.50 tonnes
C/ha/yr, averaged over an 80 year period, have been reported for abandoned fields undermore mesic conditions, in the UK (Jenkinson , 1971a). For managed conversions toimproved pastures, much higher rates of 0.75-1.0 tonnes C/ha/yr, for 15-20 years, havebeen achieved (Tyson et al., 1990; Haynes et al., 1991). Improved management ofsubtropical and tropical grasslands, with the incorporation of legumes, fertilisation andcontrol of burning have been shown to increase soil carbon levels (Greenland, 1995).
Recent data also suggest the potential for substantial C sequestration in tropicalgrasslands with the use of deep-rooting exotic grasses (Fisher et al., 1994).
In the tropics, abandonment of land occurs as an integral phase of shifting cultivation.
Shifting cultivation systems are characterised by a cycle of forest or bush clearing,followed by a few years of cropping and then abandonment to natural revegetation(fallow). Soil carbon is rapidly lost during the cropping phase and re-accumulates during
fallow. In a study of six slash-and-burn chronosequences in Southern Cameroon,
Woomer et al. (submitted) report the loss of 8 tonnes C/ha from soils (0-40 cm) within
two years of forest conversion. Soil organic matter losses continued into the early fallow
but the initial levels of 77 tonnes C/ha were re-established within the secondary forest
after approximately 18 years. The mean soil carbon stocks maintained under shifting
cultivation depend on the carbon lost during the cropping phase, the rate of accumulation
under fallow and the length of fallow (Nye and Greenland, 1960). The length of the
fallow period varies depending on climate, soil, vegetation type, land scarcity, and humanpopulation pressure.
Recovery of soil carbon during the fallow phase can be rapid (Greenland and Nye, 1959).
In North-east Brazil, Tiessen et al. (1992) reported recoveries to native soil carbon levels
after about 10 years of bush fallow (following a 5-6 year cultivation cycle), by which time
vegetation had not nearly attained native size or composition. Global estimates of carbon
fluxes assume that 75 per cent (Houghton et al., 1987) to 90 per cent (Palm et al., 1986)of the original carbon stocks from deforested soils are recovered with forest succession.
Abandonment which leads to the establishment of degraded or unimproved grasslands,
such as the extensive areas of Imperata in South-east Asia, has been assumed to result inincomplete recovery (50 per cent) of original carbon stocks (Palm et al., 1986).
However, other data suggest little difference between soil carbon inventories under
Imperata vs. secondary forest (van Noordwijk et al., submitted). The discrepancies indata on the carbon content of Imperata grasslands may be partly due to differences in fire
frequency. Imperata grasslands without fire build up reasonable stocks of soil carbon, but
frequent burning may lead to serious soil degradation. The rapid invasion of Imperata
into crop fields after forest conversion in the humid parts of Asia probably slows down
further carbon losses, as compared to other areas where cultivation is continued. Lugoet al. (1986) found carbon stocks for pastures derived from abandoned cultivated lands in
Puerto Rico to have similar or higher levels than in secondary forests. Differences in soil
fertility are also important in that infertile soils are less prone to a build up of soil C upon
abandonment. Large spatial variability in soils, varying sample depths, patchiness ofvegetation regrowth and whether or not litter mats are included in soil C inventories
make it difficult to compare field studies.

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