Susan E Page

John O Rieley

Christopher J Banks

Global and regional importance of the tropical peatland carbon pool

Keywords: peat, carbon, tropical peatland, peat area, peat thickness, peat volume, carbon pool peat, carbon, tropical peatland, peat area, thickness, volume, carbon pool 16 17 18

Accurate inventory of tropical peatland is important in order to (a) determine the magnitude of the carbon pool; (b) estimate the scale of transfers of peat-derived greenhouse gases to the atmosphere resulting from land use change; and (c) support carbon emissions reduction policies. We review available information on tropical peatland area and peat thickness and calculate peat volume and carbon content in order to determine their best estimates and ranges of variation globally, regionally and nationally. Our best estimate of tropical peatland area is 439,238 km2 (~11% of global peatland area) of which 247,778 km2 (57%) is in Southeast Asia. We estimate the volume of tropical peat to be 1,756 Gm3 (~22-33% of global peat volume) with the highest share in Southeast Asia (77%). This new assessment reveals a larger tropical peatland carbon pool than previous estimates, with a best estimate of 88.5 Gt (range 81.5-91.8 Gt) equal to 17-19% of the global peat carbon pool. Of this, 68.5 Gt (77%) is in Southeast Asia. A single country, Indonesia, holds the largest share (57.4 Gt, 65%), followed by Malaysia (9.1 Gt, 10%). These data are used to provide revised estimates for Indonesian and Malaysian forest soil carbon pools of 77 Gt and 15 Gt, respectively, and total forest carbon pools (biomass plus soil) of 97 Gt and 19 Gt. Peat carbon comprises 60% of the total soil carbon pool in Malaysia and 74% in Indonesia. These results emphasise the prominent global and regional role played by the Southeast Asian peat carbon pool and the importance of including peat carbon in national and regional assessments of terrestrial carbon stocks. This information is essential given current interest in greenhouse gas emissions from

carbon emissions reduction policies. We review available information on tropical 5 peatland area and peat thickness and calculate peat volume and carbon content in 6 order to determine their best estimates and ranges of variation globally, regionally and 7 nationally. Our best estimate of tropical peatland area is 439,238 km 2 (~11% of global 8 peatland area) of which 247,778 km 2 (57%) is in Southeast Asia. We estimate the 9 volume of tropical peat to be 1,756 Gm 3 (~22-33% of global peat volume) with the 10 highest share in Southeast Asia (77%). This new assessment reveals a larger tropical 11 peatland carbon pool than previous estimates, with a best estimate of 88.5 Gt (range 12 81.5-91.8 Gt) equal to 17-19% of the global peat carbon pool. Of this, 68.5 Gt (77%) 13 is in Southeast Asia. A single country, Indonesia, holds the largest share (57. 4 Gt, 14 65%), followed by Malaysia (9.1 Gt, 10%). These data are used to provide revised 15 estimates for Indonesian and Malaysian forest soil carbon pools of 77 Gt and 15 Gt, 16 respectively, and total forest carbon pools (biomass plus soil) of 97 Gt and 19 Gt. Peat 17 carbon comprises 60% of the total soil carbon pool in Malaysia and 74% in Indonesia. 18 These results emphasise the prominent global and regional role played by the [START_REF] Jaenicke | Determination of the 20 amount of carbon stored in Indonesian peatlands[END_REF] Southeast Asian peat carbon pool and the importance of including peat carbon in 20 national and regional assessments of terrestrial carbon stocks. This information is 21 essential given current interest in greenhouse gas emissions from developed and 22 Peatlands are globally important terrestrial carbon pools and vital components of 2 carbon soil-atmosphere exchange processes [START_REF] Immirzi | The Global Status of Peatlands and their 8 Role in Carbon Cycling. A report for Friends of the Earth by the Wetlands 9 Ecosystems Research Group[END_REF]Strack, 2008). [START_REF] Koh | Conversion of Indonesia's peatlands[END_REF] area, peatlands have their greatest extent of 3,570,000 km 2 in boreal and temperate 4 zones [START_REF] Immirzi | The Global Status of Peatlands and their 8 Role in Carbon Cycling. A report for Friends of the Earth by the Wetlands 9 Ecosystems Research Group[END_REF] but tropical peatlands, which are located in Southeast 5 Asia, Africa, the Caribbean, Central and South America, are also an important 6 component of the global peatland resource, contributing to terrestrial carbon storage 7 in both their above-ground biomass and underlying thick deposits of peat (Rieley et 8 al., 1996;Page et al., 1999;Page et al., 2004). Most tropical peatlands are located at 9 low altitude, although a proportion occurs at high altitude in the mountains of Africa, 10 South America and Papua New Guinea. 11 12 There is growing recognition of the importance of carbon storage in, and carbon gas 13 emissions from, tropical peatlands and their role in global environmental change 14 processes. Degradation of tropical peatlands leads to release of carbon and a reduction 15 in the size of their carbon stores [START_REF] Weiss | The geochemistry of major and selected trace elements in a forested peat bog, 10 Kalimantan, SE-Asia, and its implications for past atmospheric dust deposition[END_REF][START_REF] Jauhiainen | 6 Carbon fluxes from a tropical peat swamp forest floor[END_REF][START_REF] Jauhiainen | Carbon dioxide and 9 methane fluxes in drained tropical peat before and after hydrological restoration[END_REF]16 Hooijer et al., 200616 Hooijer et al., , 2009;;[START_REF] Jaenicke | Determination of the 20 amount of carbon stored in Indonesian peatlands[END_REF]. The most rapid degradation of tropical 17 peatland is currently taking place in Southeast Asia where there are strong economic 18 and social pressures for timber, land for agriculture and plantations of oil palm and 19 pulp trees [START_REF] Koh | Conversion of Indonesia's peatlands[END_REF]. As a consequence, this region's peatlands have 20 undergone rapid deforestation in the last two decades (Langner et al., 2007, Langner 21 & Siegert, 2009), widespread drainage (Hooijer et al., 2006(Hooijer et al., , 2009)), and frequent and 22 intensive fires [START_REF] Weiss | The geochemistry of major and selected trace elements in a forested peat bog, 10 Kalimantan, SE-Asia, and its implications for past atmospheric dust deposition[END_REF](Page et al., , 2009a;;[START_REF] Langner | Land cover change 2002-2005 in Borneo 12 and the role of fire derived from MODIS imagery[END_REF]Langner & Siegert, 23 2009). These have caused high levels of carbon gas emissions to the atmosphere 24 [START_REF] Van Der Werf | Continental-scale 18 partitioning of fire emissions during the 1997 to 2001 El Niño/La Niña period[END_REF][START_REF] Van Der Werf | 22 Climate regulation of fire emissions and deforestation in equatorial Asia[END_REF]. Tropical peatlands are also 2 sensitive to changes in temperature and precipitation and evidence shows that 3 prolonged periods of drought can change them from carbon sinks to carbon sources 4 [START_REF] Suzuki | Influences of deforestation on 5 carbon balance in a natural tropical peat swamp forest in Thailand[END_REF]Hirano et al., 2007). 5 6 As a consequence of these impacts, tropical peatlands are areas of high carbon 7 density, which play an important role in carbon-gas land-atmosphere interactions 8 (Canadell et al., 2004;Gruber et al., 2004). This updated and improved inventory is 9 important in order to determine the global area of tropical peatland and the magnitude 10 of its carbon pool, estimate the likely scale of transfers of peat-derived greenhouse 11 gases to the atmosphere resulting from changes in tropical peatland use both now and 12 in coming decades, and predict what is likely to happen to the peatland carbon sink 13 and store under the influence of future climate change. These data are also necessary 14 for improving global climate-carbon models and supporting initiatives to improve 15 peatland management planning and policy for climate change mitigation and carbon 16 accounting. For example, new policy initiatives to reduce greenhouse gas emissions 17 through avoided deforestation in developing countries (REDD and the voluntary 18 carbon market) have emphasised the potential for conserving tropical peatlands by [START_REF] Jaenicke | Determination of the 20 amount of carbon stored in Indonesian peatlands[END_REF] negotiating carbon offset and trading agreements [START_REF] Van Der Werf | 22 Climate regulation of fire emissions and deforestation in equatorial Asia[END_REF]. 20 21 This paper augments other recent regional studies of carbon pools in peat 22 accumulating ecosystems (e.g. North American wetlands (Bridgham et al., 2006), [START_REF] Rayment | Hydraulic conductivity and bulk density changes in the 24 cultivated layer of a Newfoundland peat soil and initial effects of soil amendments[END_REF] west Siberian peatlands [START_REF] Yefremov | Present stocks of peat and organic carbon in bog 17 ecosystems of west Siberia[END_REF]) and the northern permafrost 24 region [START_REF] Tarnocai | Soil 8 organic carbon pools in the northern circumpolar permafrost region[END_REF]) by re-evaluating the status of tropical peatlands in 1 regional and global peatland and soil carbon pools. Several previous peatland 2 inventories (e.g. [START_REF] Immirzi | The Global Status of Peatlands and their 8 Role in Carbon Cycling. A report for Friends of the Earth by the Wetlands 9 Ecosystems Research Group[END_REF]Lappalainen, 1996) presented the area and 3 volume of tropical peatland on a country by country basis, but were hampered by a 4 lack of reliable data and knowledge of their accuracy and provenance. Our study 5 builds upon these inventories and incorporates new published and unpublished 6 information, assesses the reliability of the data and, where possible, provides an 7 evaluation of their uncertainty by giving ranges of estimates. 8 9

The specific objectives of this paper are to (1) present best estimates of the important 10 attributes of tropical peatland (area, thickness, volume, bulk density and carbon 11 content) by country and region in order to provide improved knowledge and certainty 12 of the amount of peat in tropical countries and the magnitude of the carbon pools, and 13

(2) assess the contribution of tropical peatland to national, regional and global 14 peatland inventories and soil carbon stocks. 15 16 17

18

MATERIALS AND METHODS

19

In carrying out this assessment, we constrained the data in several ways: firstly, by 20 including only those countries that lie between the Tropics of Cancer and Capricorn 21 (23.5°N and 23.5°S, respectively), which excludes some peatland in the sub-tropics 22 (e.g. Florida Everglades) but includes high altitude peatlands, some of which bear 23 greater resemblance to temperate than lowland tropical peatland and, secondly, by 24

F o r R e v i e w O n l y 6
defining peat as the surface layer of soil, consisting mostly of partially decomposed 1 vegetation, with an organic content of at least 65% in a minimum thickness of 30 cm 2 [START_REF] Andriesse | Nature and Management of Tropical Peat Soils[END_REF]Rieley & Page, 2005). Not all tropical peatland inventories follow 3 this definition, or define the resource, and there are inconsistencies in the data 4 available for assessment. 5 6 We encountered anomalies and mistakes in some estimates of tropical country 7 peatland areas that had been incorporated into subsequent reports and publications. 8

For example, the highest published estimate of peatland area in Indonesia (270,000 9 km 2 ) provided by [START_REF] Jansen | Peat Development for Power 3 Generation in West Kalimantan -An Ecological Appraisal[END_REF] is cited by several other authors. This area is 10 based on a definition of peat as a cumulative layer of 40 cm or more containing 11 greater than 30% organic matter and therefore is for the total area of Histosols that 12 includes both non-peat organic soil and true peat according to our definition. The 13 lowest reported value for Indonesia of 160,000 km 2 is also given by Jansen et al. 14 (1985), although its origin is probably [START_REF] Polak | Veen en veenontginning in Indonesia[END_REF], who considered peat as soils with 15 more than 65% organic matter in a cumulative layer of at least 1 metre. The 16 definitions of peat used to obtain these area values are, therefore, different and 17 illustrate the importance of using a standardised approach. In addition, we believe that 18 this lower value of 160,000 km 2 , which is cited frequently, applies only to Kalimantan 19 and Sumatra since Indonesia did not acquire Irian Jaya (now West Papua) until 1963 20 and its peatland area was not included in earlier inventories. 21 The principal sources of information consulted were global (Kivinen & Pakarinen, 2 1980[START_REF] Kivinen | Geographical distribution of peat resources and major 22 peatland complex types in the world[END_REF]Bord na Mona, 1984;[START_REF] Immirzi | The Global Status of Peatlands and their 8 Role in Carbon Cycling. A report for Friends of the Earth by the Wetlands 9 Ecosystems Research Group[END_REF]Lappalainen, 1996;Joosten & 3 Clarke, 2002;GPD, 2004;World Energy Council, 2004) and tropical peatland 4 inventories [START_REF] Shier | Tropical peat resources -an overview[END_REF][START_REF] Andriesse | Nature and Management of Tropical Peat Soils[END_REF]Rieley et al., 1996;Rieley & Page, 2005). 5

Where possible, primary reports and published papers, from which these inventories 6 were derived, were consulted. Later inventories tend to quote data from earlier ones 7 and the trail leads mostly to the same origins, namely Bord na Mona (1984) and 8 [START_REF] Kivinen | Peatland areas and the proportion of virgin peatlands in 17 different countries[END_REF][START_REF] Kivinen | Geographical distribution of peat resources and major 22 peatland complex types in the world[END_REF]; the former obtained information from official 9 government sources and both derived data from various proceedings of and surveys 10 undertaken by members of the International Peat Society (e.g. [START_REF] Ips | Tropical Peat Resources -Prospects and Potential[END_REF]. A few 11 sources of information for peatland area in Indonesia could not be checked owing to 12 their unavailability in a consultancy report [START_REF] Jansen | Peat Development for Power 3 Generation in West Kalimantan -An Ecological Appraisal[END_REF], symposium 13 proceedings (Soepraptohardjo & Driessen, 1976) and an old issue of a Dutch 14 scientific journal [START_REF] Polak | Veen en veenontginning in Indonesia[END_REF]. 15 16 These inventory data were collected and collated for different purposes using a variety 17 of criteria and methods and mostly provide ranges of values, the upper and lower 18 limits of which might have been obtained from different sources. For example, the 19 detailed report prepared by Bord na Mona (Bord na Mona, 1984) for the World Bank 20 focuses on the potential of peat in developing countries for energy and focussed on 21 peat with a minimum thickness of one metre. Detailed inventories and primary reports 22 were available for Southeast Asia, especially Indonesia, where a major proportion of 23 tropical peatland is located and whose peat resources are relatively well documented 24 (e.g. RePPProt, 1988RePPProt, -1990)). For most other countries, however, only single inventory 25

F o r R e v i e w O n l y
values were available giving the impression of precision of survey and accuracy of the 1 data when, in fact, the opposite was the case. We also found that many published 2 estimates had been rounded to tens, hundreds and even thousands of hectares, whilst 3 some inventories focused on the highest available estimates and ignored much lower 4 minimum values (e.g. [START_REF] Joosten | Wise Use of Mires and Peatlands -Background and 14 Principles, including a Framework for Decision-making[END_REF]. 5 67

Data components 8

Peatland area 9

The area of tropical peatland is usually included in national soil inventories but those 10 for different countries may not be strictly comparable owing to different definitions of 11 peat and inclusion of non-peat organic soils in the statistics. If, for example, the 12 minimum thickness of the surface organic layer adopted is 50 cm this will provide a 13 smaller estimate of peatland area than if it were 20 cm or less. Many country 14 inventories provide the area of Histosols and organic soils, of which peat is one type, 15 but which is not always separated, and therefore the area of non-peat Histosols (<30 16 cm) may exceed that of true peat. As far as was possible, non-peat Histosols are 17 included in the maximum estimates and dealt with separately. 18 19 The data that we acquired on peatland area revealed large variation, not only between 20 countries and regions but also within countries (minimum compared to maximum). In 21 determining best estimates it is important to treat the maximum estimates with caution 22 since they may include large areas of shallow peat and non-peat Histosols and organic 23 soils. In a few instances they also include other wetlands. In arriving at best estimates, 24 however, it is inappropriate to simply determine the means of the minimum and 25 maximum values, the approach adopted by [START_REF] Immirzi | The Global Status of Peatlands and their 8 Role in Carbon Cycling. A report for Friends of the Earth by the Wetlands 9 Ecosystems Research Group[END_REF] and others. Instead 1 we used a discriminating approach in which we assessed every piece of data to 2 determine its likely accuracy. In doing this we used our expert judgment to accept 3 data (maximum or minimum) as best estimates if the accounts were recent (post 1990) 4 and originated from official sources. This was possible for only a few countries, for 5 example, Indonesia and Malaysia. In those cases for which maximum values exceeded 6 minimum values greatly we subtracted them, calculated 25% of the difference and 7 added this amount to the minimum value. This was done to prevent distortion of best 8 estimates as a result of inflated maximum areas and to provide a conservative 9 evaluation. 10 11 12

Peat thickness 13

Knowledge of peat thickness, country by country and best estimates for all tropical 14 countries are essential in order to determine as accurately as possible the total volume 15 of tropical peat and hence its carbon content. Data on peat thickness are much fewer 16 than for area because acquisition requires time-consuming direct measurement in the 17 field and the difficulty of sampling peat with a thickness of up to 10 metres or more 18 containing a large proportion of very hard tree remains. Even when values are 19 available, there is usually no information on the sampling methods or means of data 20 evaluation and interpolation. Peat thickness cannot be treated in the same way as 21 peatland area because (a) the minimum is often the lowest value that can qualify as 22 peat (0. give a mean. Default values of 0.3 m and 2 m were applied as minimum and 2 maximum thicknesses, respectively, to those countries for which peat thickness data 3 could not be found. We used mean thicknesses as best estimates where these were 4 available, otherwise we derived them conservatively from 25% of the maximum 5 values in order to derive conservative estimates. We used only best estimate peat 6 thickness values to calculate minimum, maximum and best estimates of peat volume 7 and carbon pools. 8 9

Peat volume 10

This is the product of peatland area and peat thickness. 11

V p = A p x T p (1) 12 13
Where Vp = peat volume (m 3 ); Ap = area of tropical peatland (m 2 ); Tp = mean peat 14 thickness (m) 15 16 The minimum mean peat volume in a country is obtained by multiplying the 17 minimum area by the minimum thickness. The best estimate mean volumes are 18 derived in a similar manner. In the determination of maximum peat volumes the areas 19 in excess of the best estimate area values were considered to have thin peat only, with 20 a mean thickness 0.3 m, and these volumes were computed separately and added to 21 the best estimates to provide maximum peat volume values. This was felt necessary in 22 order to maintain the conservative approach used throughout this assessment. [START_REF] Rayment | Hydraulic conductivity and bulk density changes in the 24 cultivated layer of a Newfoundland peat soil and initial effects of soil amendments[END_REF] 24 plus pore space and water) that has been dried to constant weight at 80 o C and is 2 usually expressed in grammes per cubic centimetre (g cm -3 ) or kilogrammes per cubic 3 metre (kg m 3 ). Bulk density depends on the degree of peat compaction, water content, 4 plants from which the peat has formed, degree of peat decomposition, mineral content 5 of the peat and land use. The method used to measure bulk density and the way in 6 which it is expressed (e.g. dry or wet BD) are important considerations when 7 comparing data from different authorities and between countries. 8 9 Carbon content of peat is usually expressed as fraction of the dry peat dry weight 10 (50% = 0.5). Carbon contents published in the literature have been determined by 11 different methods that have changed over time and have become more automated and 12 sophisticated. Consequently, differences between carbon contents may be partly a 13 result of these differences in analytical procedures, although the methods used are 14 seldom specified. 15 16 Unfortunately, there are few published data on either bulk density or percentage 17 carbon for tropical peatlands and these vary spatially over the surface of tropical 18 peatland and at different depths within peat profiles. Most bulk density values in the 19 literature are for surface or subsurface tropical peats to a maximum depth of 100 cm, 20 but mostly in the upper 50 cm or less. Bulk density is often higher at the surface 21 compared to the rest of the peat profile although the highest values are usually 22 obtained for bottom peat samples, close to the underlying mineral ground, in which 23 the organic matter content is lowest and mineral content highest [START_REF] Weiss | The geochemistry of major and selected trace elements in a forested peat bog, 10 Kalimantan, SE-Asia, and its implications for past atmospheric dust deposition[END_REF]24 Page et al., 2004). It is probably true to say that low bulk densities are associated with 25 profiles than bulk densities. Lowest values are for samples taken near to the 5 underlying mineral substrate or for very shallow peats in which there is a larger 6 proportion of inorganic material and these do not fit with our definition of peat. The 7 carbon content of surface tropical peat varies depending on vegetation cover and land 8 use and, as with bulk density, may not provide a true representation of entire peat 9 deposits. 10 11 As with peat thickness, the most detailed information on bulk density and carbon 12 content was available for Indonesian and Malaysian peat deposits and very few data 13 were found for peat in other tropical countries. In the absence of data, we applied best 14 estimate values for bulk density and carbon content that were derived from our 15 assessment of the literature on peats in these two countries and taking into account 16 values from other countries where available. 17

Bulk density and carbon content of tropical peat 25

Carbon pool 19

The magnitude of the tropical peatland carbon pool is obtained by multiplying peat 20 volume by bulk density and percentage carbon content (Equation 2). 21 22

C p = Vp x BD be x C c / 10 9 (2) 23 F o r R e v i e w O n l y
Where C p is the peatland carbon pool in Gt 1 ; V p is the volume of tropical peat in 1 Gm 32 ; BD be is best estimate mean dry bulk density determined as explained in the text 2 and expressed in g m -3 ; C c is percentage carbon content expressed as a fraction. 3 4 5

Results

6

Peatland area 7

Data on the area of peatland in tropical countries and geographical regions together 8 with their maximum, minimum, best estimates and areas of shallow Histosols and 9 organic soils are presented in Table 1. 10

<<INSERT TABLE 1 NEAR HERE>> 11 12

The total area of tropical peatland lies within the range 384,776-656,430 km 2 , with a 13 best estimate of 439,238 km 2 . The Southeast Asia region contains the largest share of 14 this resource (247,778 km 2 , 57% of the best estimate value), followed by South 15 America (106,363 km 2 ; 24%), Africa (55,616 km 2 ; 13%), Central America and the 16 Caribbean (22,956 km 2 ; 5%), Asia (other) (6,335 km 2 ; 1%) and the Pacific region 17 (190 km 2 ; <1%) (Table 1). Within Southeast Asia, Indonesia has the largest area 18 (206,950 km 2 , 47% of the total best estimate), followed by Malaysia (25,889 km 2 ; 19 6%) and Papua New Guinea (10,986 km 2 ; 3%) with other countries in this region 20 containing a much smaller amount (1% collectively in Brunei, Myanmar, the 21 Philippines, Thailand and Vietnam). In South America the largest peatland area is in Peru (50,000 km 2 ; 11%) followed by 1 Brazil (23,875 km 2 ; 5%), Venezuela (10,000 km 2 ; 2%) and Guyana (8,139 km 2 ; 2%). 2

In Africa, Zambia contains 12,201 km 2 (3%) of the tropical peatland area, followed by 3 Sudan with 9,068 km 2 (2%) and Uganda 7,300 km 2 (2%). Panama in Central America 4 has a peatland area of 7,870 km 2 (2%). All other tropical countries have smaller areas 5 of peatland equal to 1% or less of the total. 6 7

Peat thickness 8

Those tropical countries for which peat thickness values were available are listed in 9

Table 2 and the best estimates of the means are used in subsequent calculations of 10 peat volumes and carbon pools. For countries without information on peat thickness, 11 default values were applied (see above). 12 13

The thickest peat deposits in this assessment are in Africa with best estimates of mean 14 peat thickness of 11 m in Rwanda, 8 m in Burundi, 7.5 m in Congo, 5 m in Nigeria 15 and 4 m in both Democratic Republic of Congo and Uganda. In Southeast Asia the 16 thickest peat is in Malaysia (7 m) followed by Indonesia (5.5 m). In Central America 17 and the Caribbean peat is thickest in Panama with a best estimate of 6 m, followed by 18 The maximum, minimum and best estimates of peatland volume by country and by 2 region are given in Table 3. The total volume of tropical peat is in the range 1,618 to 3 1,822 Gm 3 with a best estimate of 1,756 Gm 3 . Southeast Asia has the largest share of 4 the tropical peatland resource by volume (1,359 Gm 3 ; 77% of the best estimate), 5 followed by South America (190 Gm 3 ; 11%), Africa (136 x 10 9 m 3 ; 8%), Central 6 America and the Caribbean (62 x Gm 3 ; 4%), with Asia (other) and the Pacific region 7 together containing only 9 Gm 3 (1%) (Table 3). 8

<<INSERT TABLE 3 NEAR HERE>> 10 11

Within Southeast Asia, as with area, Indonesia has the largest share of the tropical 12 peatland resource by volume (1,138 Gm 3 ; equivalent to 65% of the best estimate 13 global total), followed by Malaysia (181 Gm 3 ; 10%) and Papua New Guinea (27 Gm 3 ; 14 2%) (Table 3). All other countries in this region have less than 1% and contribute very 15 little to the overall global tropical peat resource. In South America, Peru has 88 G m 3 16 (5%), followed by Brazil (48 G m 3 ; 3%) and Venezuela (40 x 10 9 Gm 3 ; 2%). Congo 17 has the largest volume in Africa with 47 Gm 3 (3%), followed by Uganda (29 Gm 3 ; 18 2%). In Central America and the Caribbean, Panama has a peat volume of 47 Gm 3 19 (3%). All other countries contain 1% or less of the tropical peat volume. [START_REF] Van Der Werf | 22 Climate regulation of fire emissions and deforestation in equatorial Asia[END_REF] 21

Peat bulk density 22

Virtually all of the bulk density data available in the literature are for Southeast Asia, 23 especially Sarawak, Malaysia and Kalimantan and Sumatra in Indonesia, although a 24 few values were found for some other countries (Table 4). It is difficult to obtain 25

F o r R e v i e w O n l y
primary data for bulk density and most published information provides ranges only; 1 there are few individual values, means or standard deviations. For surface peats the 2 bulk density values represent the spatial variation while, for peat cores, the ranges 3 indicate upper and lower extremes only and do not provide detail of the variation in 4 bulk density throughout the peat profile. If surface values only are used the bulk 5 density will be higher than if the lower values in the rest of the profile are taken into 6 account. Similarly, in peatland converted to agriculture the high bulk densities in the 7 upper one metre or so of the peat soil following compaction do not indicate the much 8 lower values in the permanently waterlogged peat beneath. In the absence of sufficient 9 information on peat bulk density across the tropics we used a single best estimate of 10 0.09 g cm -3 that is a combination of the weighted means of 0.08 g cm -3 (Page et al., 11 2004) for peatland in Central Kalimantan, Indonesia and 0.13 g cm -3 for Central 12 Kalimantan, 0.08-0.13 g cm -3 for Sumatra and 0.09-0.13 g cm -3 for West Kalimantan 13 (Neuzil, 1997) 4). The much lower values that appear in some studies, e.g. 23.8% (Jaya, 22 2007) and 26.0% (Sajarwan et al., 2002), are for samples taken near to the underlying 23 mineral substrate or for non-peat organic soils in which there is a large proportion of 24 inorganic material and these do not fit with our definition of peat. For the purposes of 25

F o r R e v i e w O n l y
this assessment, the mean carbon content in peat from the Sabangau catchment, 1

Central Kalimantan (Page et al., 2004) of 56±3% was adopted for calculation of 2 tropical peat carbon content. This value is virtually the same as the value of 57±3% 3 obtained by Neuzil (1997) for several cores from Central Kalimantan, West 4 Kalimantan and Sumatra and is similar to the peat carbon content values that could be 5 found for a few other countries. [START_REF] Page | Ecological restoration of tropical 7 peatlands in Southeast Asia[END_REF] 7

Peat carbon pool 8

The values for global, regional and national tropical peatland carbon pools follow a 9 similar pattern to peat volume. The total tropical peatland carbon pool is in the range 10 82-92 Gt with a best estimate of 89 Gt. The largest pool is in Southeast Asia (69 Gt, 11 77% of the best estimate total), followed by South America (10 Gt; 11%), Africa (7 12 Gt; 8%), Central America and the Caribbean (3 Gt; 4%) and Asia (other) and the 13 Pacific region (<1 Gt; 1% combined) (Table 5). Within Southeast Asia, Indonesia has 14 by far the largest share of the tropical peatland carbon pool (57 Gt, 65%), followed by 15 Malaysia (9 Gt, 10%), with Brunei, Myanmar, Papua New Guinea, the Philippines, 16

Thailand and Vietnam, collectively, containing a smaller proportion of the total (2%). 17 There have been several previous evaluations of the global area of peatlands and 1 estimates of their carbon store but these differ widely owing to a lack of detailed 2 information from many countries and differences in definitions of peat and estimates 3 of peat thickness (e.g. Moore & Bellamy, 1974; Bord na Mona, 1984; Armentano & 4 Menges, 1986;[START_REF] Andriesse | Nature and Management of Tropical Peat Soils[END_REF]Gorham, 1991;[START_REF] Immirzi | The Global Status of Peatlands and their 8 Role in Carbon Cycling. A report for Friends of the Earth by the Wetlands 9 Ecosystems Research Group[END_REF]. We used the 5 evaluation by [START_REF] Immirzi | The Global Status of Peatlands and their 8 Role in Carbon Cycling. A report for Friends of the Earth by the Wetlands 9 Ecosystems Research Group[END_REF] as the most comprehensive estimate of the area of 6 boreal and temperate peatlands and added to this our updated tropical peatland area to 7 arrive at an improved global total ( 2 , make up 10 to 16% of the global peatland extent. The 13 peatlands of Southeast Asia, with a best estimate area of 247,778 km 2 , represent 14 between 6% and 8% of the global peatland area. South America has a peatland area of 15 106,363 (3% of the global peatland area), Africa has 55,616 km 2 (1%) and Central 16 America and the Caribbean have 22,956 km 2 (<1%). 17 maximum and mean estimates of non-tropical peatland area from Immirzi et al. 13 (1992), multiplying these by their peat carbon density (1099.5 t C ha -1 ) in a mean 14 global peat thickness of 1.5 m, and adding the new tropical peat carbon store values 15 obtained in this assessment, provides a new overall global estimate of 469 -486 Gt 16 with a best estimate of 480 Gt (Table 8). Using the greater mean thickness value of 17 2.3 m for boreal and sub-arctic peat (Gorham, 1991) and thus a higher carbon density 18 value of 1466 t C ha -1 , results in a larger estimate of the global peat carbon store of 19 between 598 to 618 Gt with a best estimate of 610 Gt (Table 8). On this basis of these 20 assessments, the tropical peat carbon pool is between 11 and 14% of the global peat 21 carbon pool. 22 23 The Southeast Asian region contains the largest proportion of the tropical peat carbon 24 store with between 66.4-69.8 Gt C (best estimate 68.5 Gt C). We found that 25 Asia plus Asia (other)) increases the value of this region's forest soil carbon pool to 3 ~130 Gt, 68% of which is in peat. This new data assessment draws attention to the 4 large contribution of Southeast Asian peatlands to both national and regional forest 5 soil carbon and emphasises the importance of considering peat carbon stores in 6 assessments of emissions from tropical land use change and fire. 7 8 Our assessment highlights countries for which there is a lack of primary data on 9 peatland area, thickness, bulk density and carbon content, leading to uncertainty in the 10 calculation of peat volumes and carbon pools. Some inventories include organic soils 11 and shallow non-peat Histosols in peatland inventories. This applies to peatland area 12 but it is a much greater problem for peat thickness, bulk density and carbon content 13 which have been inadequately determined in many countries. Further detailed field 14 surveys would undoubtedly contribute to more precise and better constrained 15 estimates of tropical peatland carbon pools, particularly in Africa, Central and South 16 America where there is still relatively little spatial information on peat thickness. 17 Even in Southeast Asia, where the peatlands of Indonesia and Malaysia have been [START_REF] Jaenicke | Determination of the 20 amount of carbon stored in Indonesian peatlands[END_REF] relatively well studied, there is still a lack of knowledge of the aerial extent and 20 volume of peatlands in some locations. In West Papua (Irian Jaya), for example, there 21 are at least 70,000 km 2 of thick peat deposits that have received very little study. This 22 is also the case in Papua New Guinea, where the difference between non-peat and 23 peat-forming wetlands is ill defined. In this assessment we have used a best estimate 24 peatland area of 16,971 km 2 1992) indicates that this is the extent of Histosols (i.e. organic soils associated with a 2 wide range of wetlands, not all of which conform to our definition of peat), thus the 3 best estimate value we have used could underestimate the true extent of the resource. 4

This uncertainty regarding classification of wetlands and wetland soils is not confined 5 to Southeast Asia. For example, our best estimates of peatland areas in Sudan and 6

Zambia are 9,068 and 11,060 km 2 , whilst the extent of Histosols and non-peat organic 7 soils (according to our definition of peat) is 33,270 and 15,645 km 2 , respectively 8 (GPD, 2004); again, we may have underestimated the true extent of peatland in these 9 and several other countries. 10 11 In other cases, the lack of precise information is because of limited field survey in 12 remote locations. Ruokalainen et al. (2001) have suggested that Amazonian peatlands 13 could have a total area of 150,000 km 2 . They do not provide verifiable evidence for 14 their assertions and these data should be treated with care until they are confirmed. 15 Most of these are small, topogenous (as opposed to ombrogenous) wetlands 16 associated with Mauritia flexuosa (aguaje palm) swamps that are predominantly 17 riverine or flood plain wetlands interspersed amongst dryland forest types (Lähteenoja 18 et al., 2009;[START_REF] Phillips | Vegetation Zones and Diagnostic Pollen 19 Profiles of a Coastal Peat Swamp, Bocas del Toro, Panamá[END_REF]. There is no general agreement that these are peat 19 forming in all situations, although they may accumulate plant litter to a thickness in 20 excess of 0.5 m that could be classified as peat. Undoubtedly, Amazonian peatlands 21 warrant further more detailed investigation and assessment, although, owing to their 22 Tropical mountain peatlands also warrant further 1 investigation. These are mostly small in area, occurring primarily in basins and on 2 slopes, but they can be numerous and, collectively, could make a substantial 3 contribution to regional peat resources, particularly in Andean countries (Chimner & 4 Karberg, 2008). 5 6 African peatlands are also under investigated. Some peatlands have thick peat 7 deposits with maximum recorded thickness in excess of 30 m (Table 2) in Burundi, 8

Congo and Democratic Republic of Congo. At present it is difficult to carry out field 9 investigations in these countries owing to their political situation, but if it were, our 10 best estimates of peatland areas might be increased with implications for the size of 11 their carbon pools. 12 13 Improved knowledge and understanding of the tropical peatland resource is vital 14 given the current rapid rate of peatland development occurring across the tropics and 15 specifically in Southeast Asia where the vast majority of the resource is located. 16 Consideration of the scale of carbon flux from deforestation or degradation of tropical 17 peatland should take into account the high below-ground carbon storage, which will 18 typically be an order of magnitude greater than that in the above-ground biomass. For 19 example, our best estimate of carbon density in Indonesian peatland is 2,772 t C ha -1 20 (based on a best estimate peat thickness of 5.5 m), which is much higher than typical 21 values for above ground peat swamp forest biomass of 100 to 150 t C ha -1 (Page et al., 22 2006). Deforestation of Southeast Asian peatlands is proceeding at rates as high as 23 106,000 km 2 (43%) of the tropical peatland resource across Southeast Asia had been 7

deforested, drained and converted to some other form of land use. Based on our best 8 estimate of the regional peatland carbon pool, this renders ~29 Gt C vulnerable to 9 release to the atmosphere as a result of peat oxidation and fire over coming decades. 10

Even with improved land management, the magnitude of emissions from tropical 11 peatland is unlikely to be reduced, since climate modelling studies have shown that 12 peatland areas of equatorial Southeast Asia and Amazonia will experience reduced 13 Table 1: Area of tropical peatland on a country and regional basis expressed as best 3 estimate, maximum and minimum area values (km 2 ) (see notes for details). 4 5 Table 2: Thickness of tropical peat on a country and regional basis expressed as 6 maximum, range, mean and best estimate (m) (see notes for details). 7 8 Table 3: Volume of tropical peatland on a country and regional basis expressed as 9 best estimate, maximum and minimum values (m 3 x 10 6 ) (calculated from area in 10

  peatlands and the need to predict future trends under the influence of land use and climate change.

25 F

 25 degraded peatlands and the need to predict future trends under the influence of land 23 use and climate change.24

  biomass, peat oxidation and combustion (Hooijer et al., 2006, 2009; 1 Page et al., 2002; van der

25 F 10 A

 2510 3 m), (b) the maximum may relate to only one or a few high values in the 23 literature, and (c) mean peat thicknesses and ranges are seldom available. 24 few sources provide maximum peat thickness, others indicate ranges while some 1

2 3

 2 high mineral and low carbon contents.Carbon contents of tropical peats are less variable across their surface and down 4

18

 18

22

 22

23 1

 23 Gt = Gigatonnes = Billion tonnes = t x 10 9 = g x 1015 = Petagrammes 2 Gm -Giga cubic metres = m3

Cuba ( 1 .

 1 8 m) and Trinidad and Tobago (1.3 m). There is no information on peat 19 thickness for most other countries in this region and the default best estimate of 0.5 m 20 has been applied to them. South American peats are shallower with a mean thickness 21 of 4 m in Venezuela, 2 m in Brazil and 1.75 m in Peru. 22 23 << INSERT TABLE 2 NEAR HERE >> 24 25

18 <<PUT

 18

18

  According to[START_REF] Immirzi | The Global Status of Peatlands and their 8 Role in Carbon Cycling. A report for Friends of the Earth by the Wetlands 9 Ecosystems Research Group[END_REF], the total global peat volume (mean=best estimate) 19 is 5,958 Gm 3 , based on an average peat thickness of 1.5 m. This underestimates the 20 greater thickness of tropical peat compared to boreal and temperate peat. By 21 combining the non-tropical peat volume of 5,335 Gm 3[START_REF] Immirzi | The Global Status of Peatlands and their 8 Role in Carbon Cycling. A report for Friends of the Earth by the Wetlands 9 Ecosystems Research Group[END_REF] with our 22 best estimate of 1,756 Gm 3 for the volume of tropical peat provides a larger global 23 estimate of 7,091 Gm 3 showing that tropical peatlands contain 25% of the global 24 peatland volume (

5 6

 5 store 57.4 Gt compared to 42 Gt used by Hooijer et al.1 (2006, 2009) in their assessment of CO 2 emissions from drained peatlands in 2 Southeast Asia but close to the value of 55 Gt C calculated for the Indonesian 3 peatland carbon pool by[START_REF] Jaenicke | Determination of the 20 amount of carbon stored in Indonesian peatlands[END_REF] using a combination of 3D modelling 4 and satellite imagery.The soil is the largest terrestrial pool of organic carbon, with global estimates ranging 7 between 1395 Gt[START_REF] Adams | Increases 2 in terrestrial carbon storage from the Last Glacial Maximum to the present[END_REF], 1462-1548 Gt (Batjes, 1996) and 1600-1800 8 Gt (Bouwman, 1990). Compared to a median value of 1500 Gt C in soils, tropical 9 peatland represents between 5-6% of the global soil carbon pool. The size of the 10 carbon pool in tropical soils generally is poorly known (Batjes, 1996), but in countries 11where peatland occupies a significant proportion of the land area, e.g. Indonesia 12 (10.8%) and Malaysia (7.9%), this ecosystem holds a major proportion of the national 13 soil carbon stock. Brown et al.(1993) estimated that the carbon pool in Indonesia's 14 forest soils (to 100 cm depth) and vegetation was 40 Gt, of which soil carbon 15 accounted for 50% (i.e. 20 Gt); values for Malaysia were 10 Gt C (6 Gt in soil plus4 16 Gt in biomass). Including our values for peat carbon pools could increase the 17 Indonesian forest soil carbon pool to about 77 Gt (even allowing for the fact that 18 Brown et al.(1993) included 100 cm thickness of peat swamp forest soil carbon in 19 their estimate), producing a total forest carbon pool value of 97 Gt, of which the soil 20 component is 79% and the peat 58%. Likewise the Malaysian forest soil carbon store 21 increases to 15 Gt, with a total forest store of 19 Gt. Of this total value, soil carbon 22 makes up 79% and the peat carbon proportion is 47%. Thus in these two countries, 23 peat carbon comprises 60% of the total soil carbon pool in Malaysia and 74% in 24 Indonesia. Forests across the entire tropical Asian region are estimated to have a total 25 of 43 Gt with an additional 42 Gt in biomass(Brown et al., 1993); 1 including our best estimate of the tropical Asian peatland carbon pool (i.e. Southeast 2

18

 18

  for Papua New Guinea, whilst Joosten and Clarke (2002) 25 value of 28,900 km 2 . Their original data source (Wayi and Freyne, 1

  shallow nature (average depth 1.75 m, according to data presented for 12 peat cores in 23 Peru (Lähteenoja et al. (2009)) they will likely make only a small additional 24 contribution to the tropical peatland carbon pool unless extensive thick, ombrogenous 25

2 .

 2 2% yr -1 across Borneo (2002 to 2005; Langner et al., 2007) and 9.0% yr -1 for some 24 specific locations (former Mega Rice Project, Central Kalimantan, 1997 to 2005; 25 al., 2004, 2008; Hooijer et al., 2006, 2009; Fargione et al., 2008) have explicitly 2 recognised the scale of carbon emissions arising from disturbance of tropical 3 peatlands where the flux from the below-ground carbon pool can be several orders of 4 magnitude greater than that from the above-ground pool and extend over a much 5 longer time period. For the year 2000, Hooijer et al. (2006) estimated that some 6

  rainfall and greater seasonality(IPCC, 2007; Li et al., 2007; Mahli et al., 2008), 14 which will lead to lower peatland water tables, enhanced peat decomposition and an 15 increased likelihood of fire.16 17 New policy initiatives to reduce greenhouse gas emissions through avoided 18 deforestation in developing countries (REDD and the voluntary market) are likely to 19 become a dominant component of land-based carbon mitigation in the future(Agus, 20 2008;[START_REF] Van Der Werf | 22 Climate regulation of fire emissions and deforestation in equatorial Asia[END_REF]. This study emphasises that tropical peatlands have 21 one of the highest carbon densities of all terrestrial ecosystems. Tropical peatlands, 22 particularly in Southeast Asia, combine a large carbon forest sink with an even larger 23 peat carbon store, thus policies that promote avoided deforestation and degradation as 24

1 36 2 Finland. 3 RePPProt

 123 rehabilitation(Page et al., 2009b) would yield high benefits per 1 hectare if applied to tropical peat swamp forest. funded by the European Commission through theCARBOPEAT 2 Project (INCO-CT-2006-43743) and (for SEP) formed part of the Peat-Water-Carbon 3 research programme of the Singapore-Delft Water Alliance at the National University 4 of Singapore. The authors are grateful to members of CARBOPEAT Working Group 51 for supplying data and information, and to Dr Pep Canadell and Dr JyrkiJauhiainen 6 for commenting on a draft of the manuscript. wetlands of the temperate zone. Journal of Ecology, 74,755-774. 1 Batjes NH (1996) Total carbon and nitrogen in the soils of the world. European 2 Journal of Soil Science, 47, 151-163. 3 Bord na Mona (1984) Fuel Peat in Developing Countries. Dublin, Ireland. 4 Bouwman AF (1990) Global distribution of the major soils and land cover types. In: 5 Soils and the greenhouse effect, Proceedings of the International Conference on Soils 6 and the Greenhouse Effect (Bouwman AF ed.), p. 31-59. John Wiley & Sons, New 7 York. 8 Bridgham SD, Megonigal JP, Keller JK, Bliss NB, Trettin C (2006) The carbon 9 balance of North American wetlands. Wetlands, 26, 889-916. 10 Brown S, Iverson LR, Prasad A, Liu D (1993) Geographical distributions of carbon in 11 biomass and soils of tropical Asian forests. Geocarto International, 4, 45-59. 12 Canadell J, Ciais P, Cox P, Heimann P (2004) Quantifying, understanding and 13 managing the carbon cycle in the next decade. Climatic Change, 67, 147-160. 14 Chimner RA, Karberg JM (2008) Long-term carbon accumulation in two tropical 15 mountain peatlands, Andes Mountains, Ecuador. Mires and Peat, 3, 1-10. 16 Cohen AD, Raymond RJr, Mora S, Alverado A, Malavassi L (1985) Economic 17 characteristics of the peat deposits of Costa Rica: preliminary study. Proceedings of 18 the Symposium: Tropical Peat Resources -Prospects and Potential. 25 February -1 19 March 1985, Kingston, Jamaica. International Peat Society, Helsinki, Finland. Pp. 20 146-169. 21 Egglesmann R (1976) Peat consumption under influence of climate, soil condition and 22 utilization. Proceedings of the 5 th International Peat Congress, Poznan, Poland, 21-25 23 September, 1976. Volume 1, pp. 233-247. International Peat Society, Jyväskylä, Van den Berg E, Reich P (1993) Organic carbon in soils of the world. Soil 1 Science Society of America Journal, 57, FAO-ISRIC. 2 Fargione J, Hill J, Tilman D, Polasky S, Hawthorne P (2008) Land clearing and the 3 biofuel carbon debt. Science, 319, 1235-1238. 4 Franzén IG (2006) Increased decomposition of subsurface peat in Swedish raised 5 bogs: are temperate peatlands still net sinks of carbon? Mires and Peat, Volume1, 6 Article 03, pp. 8-9. http://www.mires-and-peat.net 7 Garcia MJ, de Oliveira PM, de Siqueira E, Fernandes RS (2004) A Holocene 8 vegetational and climati8c record from the Atlantic rainforest belt of coastal State of 9 São Paulo, SE Brazil. Review of Palaeobotany and Palynology, 131, 181-199. 10 GPD (2004) Global Peatland Database. International Mire Conservation Group. 11 Available at: http://www.imcg.net/gpd/gpd.htm (last updated 5th February 2004). 12Gorham, E. (1991) Northern peatlands: Role in the carbon cycle and probable 13 responses to climatic warming. Ecological Applications, 1, 182-195. 14 Gruber N, Friedlingstein P, Field CB, Valentini R, Heimann M, Richey JF, Romero P, 15 Schulze E-D, Chen A (2004) The vulnerability of the carbon cycle in the 21st 16 Century: An assessment of carbon-climate-human interactions. In: Global Carbon 17 Cycle, integrating human, climate, and the natural world, Field CB, Raupach M (eds). 18 Island Press, Washington, DC., pp 45-76 19 Hirano T, Segah H, Harada T, Limin S, June T, Hirata R, Osaki M (2007) Carbon 20 dioxide balance of a tropical peat swamp forest in Kalimantan, Indonesia. Global 21 Change Biology, 13, 412-425. 22 Hooijer A, Silvius M, Wösten H, Page S (2006) PEAT-CO2, Assessment of CO 2 23 Emissions from Drained Peatlands in SE Asia. Delft Hydraulics report Q3943, Delft, evolution. Journal of African Earth Sciences, 12, 171-2 179.Li W, Dickinson RE, Fu R, Niu G-Y, Yang Z-L, Canadell JG (2007) Future 3 precipitation changes and their implications for tropical peatlands. Geophysical 4 Research Letters, 34, L01403, doi:10.1029/2006GL028364. 5 Ma X, Wang R (1992) Peat in south coastal area of China. Proceedings of the 6 International Symposium on Tropical Peatland. Kuching, Sarawak, Malaysia, 1991, 7 pp. 96-100. 8 Mahli Y, Roberts JT, Betts RA, Killeen, TJ, Li W, Noble CA (2008) Climate change, 9 deforestation and the Fate of the Amazon. Science, 319. 169-172. 10 Markov VD, Olunin AS, Ospennikova LA, Skobeeva EI, Khoroshev PI (1988) World 11 Peat Resources. Moscow Nedra. 383 pp. (in Russian). 12 Moore PJ, Bellamy DJ (1974) Peatlands. Paul Elek Scientific Books Ltd, London. 13 Murdiyarso D, Suryadiputra N, Dewi S, Agus F (2008) How can REDD scheme 14 support the management of vulnerable carbon pools of Indonesian peatlands. In: 15 Future of Tropical Peatlands in Southeast Asia as Carbon Pools and Sinks; papers 16 presented at the special session on tropical peatlands at the 13 th International Peat 17 Congress, Tullamore, Ireland, 10 th June 2008 (eds Rieley, JO, Banks CJ, Page SE), 18 pp. 57-60. Department of Geography, University of Leicester, Leicester, UK. 19 (http://www.geog.le.ac.uk/carbopeat/media/pdf/tullamorepapers/ipc_tropical_peat_sp 20 ecial_session.pdf) 21 Mutalib AA, Lim JS, Wong MH, Koonvai L (1992) Characterization, distribution and 22 utilization of peat in Malaysia. Proceedings of the International Symposium on 23 Tropical Peatland. Kuching, Sarawak, Malaysia, 1991, pp. 7-16. 24 Swampland in Sumatra, Kalimantan and Irian Jaya. Volume 1, Main Report. 2 Neuzil SG (1997) Onset and rate of peat and carbon accumulation in four domed 3 ombrogenous peat deposits, Indonesia. In: Biodiversity and Sustainable Management 4 of Tropical Peatlands (eds Rieley JO, Page SE), pp. 55-72. Samara, Cardigan, UK. 5 Oravainen H, Sänkiaho K, Klemetti, V, Crisologo S, Abando R, Herrera N (1989) Pre-6 feasibility Study of the Philippine Peat Resources. Final Report. Technical Research 7 Centre of Finland, Vapo Oy Finaland and Office of Energy Affairs of the Philippines. 8 Jyväskylä, Finland. 9 Oraveinen H, Crisologo EJ, Abando R, Herrera N, Sankiaho K, Klemetti V (1992) Pre-10 feasibility study of the Philippines peat resources. Proceedings of the International 11 Symposium on Tropical Peatland. Kuching, Sarawak, Malaysia, 1991, pp. 49-56. 12 Page SE, Rieley, JO, Shotyk W, Weiss D (1999) Interdependence of peat and 13 vegetation in a tropical peat swamp forest. Philosophical Transactions of the Royal 14 Society, Series B, 354, 1885-1897. 15 Page SE, Siegert F., Rieley JO, Boehm H-DV, Jaya A, Limin SH (2002) The amount 16 of carbon released from peat and forest fires in Indonesia during 1997. Nature, 420, 17 61-65. 18 Page SE, Wüst RAJ, Weiss D, Rieley JO, Shotyk W, Limin SH (2004) A record of 19 Late Pleistocene and Holocene carbon accumulation and climate change from an 20 equatorial peat bog (Kalimantan, Indonesia): implications for past, present and future 21 carbon dynamics. Journal of Quaternary Science, 19, 625-635. 22 Page SE, Rieley JO, Wüst RAJ (2006) Lowland tropical peatlands of Southeast Asia. 23 In: Peatlands: Evolution and Records of Environmental and Climate Changes (eds. 24 Proceedings of the 5 th International Peat Congress, Poznan, Poland, 21-25 1 September, 1976. Volume 1, pp. 282-292. International Peat Society, Jyväskylä, (1988-1990) A National Overview from the Regional Physical Planning 4 Programme for Transmigration. UK Overseas Development Administration and 5 Directorate Bina Program. Ministry of Transmigration, Jakarta. 6 Rieley JO, Page SE (eds) (2005) Wise Use of Tropical Peatlands -Focus on 7 Southeast Asia. Alterra, Wageningen, The Netherlands. 8 (http://www.restorpeat.alterra.wur.nl/p_download.htm ) 9 Rieley JO, Ahmad-Shah AA, Brady MA (1996) The extent and nature of tropical peat 10 swamps. In: E Maltby, CP Immirzi, RJ Safford (Eds.) Tropical Lowland Peatlands of 11 Southeast Asia, Proceedings of a Workshop on Integrated Planning and Management 12 of Tropical Lowlands Peatlands, Cisarua, Indonesia, 3-8 July 1992, pp. 17-53. IUCN, 13 Gland, Switzerland. 14 Rieley JO, Wüst RAJ, Jauhiainen J et al. (2008) Tropical peatlands: carbon stores, 15 carbon gas emissions and contribution to climate change processes. In: M. Strack 16 (Ed.) Peatlands and Climate Change pp. 148-181, International Peat Society, 17 Jyväskylä, Finland. 18 Ruokalainen, K, Schulman L, Tuomisto H (2001) On Amazonian peatlands. 19 International Mire Conservation Group Newsletter. Issue 2001/4, 8-10. 20 Rydin H, Jeglum J (2006) The Biology of Peatlands. Oxford University Press, United 21 Kingdom. p. 232. 22 Sajarwan A, Notohadiprawiro T, Radjagukguk B, Hastuti S (2002) Diversity of 23 tropical peat characteristics in intact peatland forest, under the influence of forest 24 type, peat thickness, and position of peat deposit. In: Peatlands for People: Natural 25

  . The bulk densities obtained for a few other countries are in 14

	15	accordance with these.
	16
	17	<<INSERT TABLE 4 ABOUT HERE>>
	18
	19	Peat carbon content
	20	Published values of carbon content of Southeast Asian peats range from 41.6%
	21	(Sajarwan et al., 2002) to 62.0% in Central Kalimantan (Neuzil, 1997; Page et al.,
		2004) (Table

Table 6

 6

	).

Table 7 )

 7 , making a much larger contribution than their area 25[START_REF] Immirzi | The Global Status of Peatlands and their 8 Role in Carbon Cycling. A report for Friends of the Earth by the Wetlands 9 Ecosystems Research Group[END_REF] conclude that the amount of carbon stored within peatlands 9 globally is in the range 329 to 525 Gt with a mean value of 462 Gt, although other 10 published values range from 234 to 679 Gt(Gorham, 1991; Eswaran et al., 1993; 11 Batjes, 1996; Lappalainen, 1996;[START_REF] Joosten | Wise Use of Mires and Peatlands -Background and 14 Principles, including a Framework for Decision-making[END_REF]. Using the minimum, 12

	1	suggests. By using a higher mean peat thickness value of 2.3 m for boreal and
	2	temperate peat, as suggested by Gorham (1991), produces a peat volume of 8,180
	3	Gm 3 which, when combined by our assessment of the tropical peat volume, produces
	4	a global estimate of 9,936 Gm 3 , of which tropical peat comprises 17-18% and
	5	Southeast Asian peat 14%.
	6
	7	<<INSERT TABLE 7 AROUND HERE>>
	8	r o F
		R
		e v
		i e
		w
		O n l
		y

Table 1

 1 and peat thickness in Table2). 1112

Table 4 :

 4 Bulk density and carbon concentration of tropical peat obtained from the 13 literature (psf: peat swamp forest; BD: bulk density) with some values for temperate 14

	15	peat for comparison.
	16

Table 5 :

 5 Carbon store in tropical peatland on a country and regional basis expressed 17

	18	as best estimate, maximum and minimum values (Gigatonne) (calculated from volume
	19	in Table 1, bulk density of 0.09 g cm -3 and carbon concentration of 56% (0.56))
	20

Table 6 :

 6 Updated estimates of global and tropical peatland areas derived fromImmirzi 21

	22	et al. (1992) and this assessment.
	23

Table 7 :

 7 Updated estimates of global and tropical peat volumes derived fromImmirzi 24

	25	et al. (1992) and this assessment.
	26

Table 8 :

 8 Updated estimates of global and tropical peatland carbon pools derived from 27

	28	Immirzi et al. (1992) and this assessment.
	29
	30

Table 1 : Area of tropical peatland on a country and regional basis expressed as best 5 estimate, maximum and minimum area values (km 2 ) (see notes for details).

 1

	6
	Region	Country	Minimum area (km 2 )	%	Best estimate of area (km 2 )	soils and organic Histosols % Shallow	%	(km 2 ) area Maximum	%	Notes
							(km 2)
	AFRICA	Angola	100 0	2640 1	7621 4	gives 10,261 km 2 as max for Histosols. 10,261 2 Min from Andriesse (1988). WSM 3 in GDB (2004)
		Botswana	2500	1	2625 1	375 0	3000 0 Max and Min from Sliva (pers. comm.) mentioned in GPD (2004).
		Burundi	140 0	323 0	555 0	878 0 Min from Bord na Mona (1984). WSM in GPD (2004) gives 878 km 2 as max for
										Histosols.
		Cameroon	100 0	1077 0	2930 1	4007 1 Min from Joosten & Clarke (2002) but source not
										given; Max from WSM in GPD (2004).
		Congo	2900 1	6220 1	9957 5	16177 2 Min from Bord na Mona (1984); WSM in GPD (2004) gives 16,177 km 2 as max for
										Histosols.
		Democratic	400 0	2800 1	7,200 3	10000 2 Most inventories give the 400 km 2 cited in Shier
		Republic								(1985); Andriesse (1988) and GPD (2004) give
		of Congo								higher value.
		Gabon	80 0	548 0	1,403 1	1951 0 Min from Joosten & Clarke (2002) but source not
										given; Max from WSM in GPD (2004).
		Ghana	49 0	59 0	41 0	100 0 Min from WSM in GPD (2004); Max from Joosten
										& Clarke (2002)
		Guinea	853 0	1952 0	3,298 2	5250 1 Min from WSM in GPD (2004); Max from Bord na
										Mona (1984) but includes mangroves.
		Ivory Coast	300 0	725 0	1,275 1	2000 0 Min from Bord na Mona (1984); Max from Markov
										(1988) in GPD (2004).
		Kenya	1600 0	2440 1	2,519 1	4959 1 Min from Lappaleinen & Zurek (1996), source credited to Markov (1988) in GPD (2004); Max
										from WSM in GPD (2004).
		Liberia	26 0	120 0	280 0	400 0 Min from WSM in GPD (2004); Max from Bord na
										Mona (1984).
		Madagascar	1903 0	1920 0	50 0	1970 0 Min from WSM in GPD (2004); Max and Best
										Estimates from Bord na Mona (1984).
		Malawi	353 0	492 0	418 0	910 0 Min from WSM in GPD (2004);
										Max from Bord na Mona (1984).
		Mauritania	60 0	60 0	0 0	60 0 Joosten & Clarke (2002); source not given.
		Mauritius	1 0	1 0	0 0	1 0 Joosten & Clarke (2002); source not given.
		Mozambique	100 0	575 0	1,425 1	2000 0 Min from Andriesse (1988); Max fom Grunding (pers. comm.) in GBD (2004).
		Nigeria	120 0	1600 0	5,400 2	7000 1 Min from Joosten & Clarke (2002) but cannot be
										verified. Max from Lappaleinen & Zurek (1996) but
										cannot be verified, mostly mangrove and other
										wetland.
		Reunion	1 0	1 0	0 0	1 0 Joosten & Clarke (2002); source not given.
		Rwanda	800 0	830 0	90 0	920 0 Min from Bord na Mona (1984); Max from WSM in
										GPD (2004).

Table 2 : Thickness of tropical peat on a country and regional basis expressed as 1 maximum, range, mean and best estimate (m) (see notes for details).

 2

	2
	Region	Country	Maximum peat thickness (m)	Peat thickness range (m)	Peat thickness (m) mean	(m) Best estimate Peat thickness	Notes
	AFRICA	Angola	N.I.	N.I.	N.I.	0.5 No information available.
		Botswana	4	N.I.	N.I.	1 Sliva (pers. comm.) in GPD (2004).
		Burundi	32.7	N.I.	N.I.	8 Pajunen (1985).
		Cameroon	N.I.	N.I.	N.I.	0.5 No information available.
		Congo	30	N.I.	N.I.	7.5 Markov (1988) in GPD (2004).
		Democratic Republic	30-60	1-15	N.I.	4 Shier (1985); Bord na Mona (1985).
		of Congo
		Gabon	N.I.	N.I.	N.I.	0.5 No information available.
		Ghana	N.I.	N.I.	N.I.	0.5 No information available.
		Guinea	N.I.	N.I.	N.I.	0.5 According to Bord na Mona (1984) all
						peats are shallow.
		Ivory Coast	5-7	N.I.	N.I.	1.5 Markov (1988) in GPD (2004).
		Kenya	N.I.	N.I.	N.I.	0.5 No information available.
		Liberia	0.5	N.I.	N.I.	0.5 Bord na Mona (1984); Shier (1985).
		Madagascar	10.5	2-10.5	N.I.	2.5 Markov (1988) in GPD (2004); Straka
						(1960) in GPD (2004).
		Malawi	3.2	N.I.	N.I.	1 GPD (2004).
		Mauritania	N.I.	N.I.	N.I.	0.5 No information available.
		Mauritius	1.9	N.I.	N.I.	0.5 Markov (1988) in GPD (2004).
		Mozambique	5	0.4-3	N.I.	1.5 Markov (1988) in GPD (2004).
		Nigeria	1.2-20	1.2-20	N.I.	5
		Reunion	3.6	N.I.	N.I.	1 Markov (1988) in GPD (2004).
		Rwanda	20	3-20	11	11 Bord na Mona (1984); Shier (1985).
		Senegal	10	7.7-10	3.5	3.5 Bord na Mona (1984); Shier (1985);
						Korpijaakko (1985); Lézine & Chateauneuf
						(1991).
		Sierra Leone	N.I.	N.I.	N.I.	0.5 No information available.
		Sudan	3	N.I.	N.I.	1 GPD (2004).
		Uganda	16	1-10-16	N.I.	4 Bord na Mona (1984).
		Zambia	N.I.	N.I.	N.I.	0.5 No information available.
	ASIA	Brunei	10	10-15	N.I.	3 James (1984); Jali (2002); Stoneman
	(SOUTHEAST) *					(1997).
		Indonesia	20	4-6	N.I.	5.5 Derived from values from Neuzil (1997);
						Page et al. (1999); Jaya (2007); Jaenicke et
						al. (2008).
		Malaysia	20	4-10	N.I.	7 Based on peat profiles in Anderson (1961)
						and Sayok et al. (2008).
		Myanmar (Burma)	2	1-2	N.I.	1.5 From Markov et al. (1988) quoted in GPD
						(2004).
		Papua New Guinea	10	1-10	N.I.	2.5 Wayi & Freyne (1992).
		Philippines	12	0.5-12	5.3	5.3 Bord na Mona (1984).
		Thailand	3	1-3	0.6-1	1 Urapeepatanapong & Pitayakajornwute
						(1996).
		Vietnam	2	N.I.	N.I.	0.5 GPD (2004).
	ASIA (OTHER) *	Bangladesh	0.3-4	0.3-4	1.5	1.5 Max mean value from GPD (2004); mean
			Av. 1.5			from Bord na Mona (1984).
		China	30	1-2-30	N.I.	1 Ma & Wang (1992); mostly buried by
						marine sediments; peat layers are generally
						less than 1 m.
		India	9	N.I.	4	4 Markov (1985) in GPD (2004).
		Sri Lanka	6.5	N.I.	4	4 Bord na Mona (1984).

Table 4 : Bulk density and carbon concentration of tropical peat obtained from 3 the literature (psf: peat swamp forest; BD: bulk density) with some values for

 4

	4
	5	temperate peat for comparison.
	6

Table 5 : Carbon store in tropical peatland on a country and regional basis expressed as 1 best estimate, maximum and minimum values (Gigatonne) (calculated from volume in 2Table 1 , bulk density of 0.09 g cm -3 and carbon concentration of 56% (0.56)) 3

 51

			Minimum		Best estimate		Carbon store in		Maximum peat
			carbon store		carbon store		shallow		carbon store
	Region	Country	(Gt)	%	(Gt)	%	Histosols and	%	(Gt)	%
							organic soils
							(Gt)
	AFRICA	Angola	0.003	0	0.067	0	0.114	4	0.181	0
		Botswana	0.126	0	0.132	0	0.006	0	0.138	0
		Burundi	0.056	0	0.13	0	0.008	0	0.138	0
		Cameroon	0.003	0	0.027	0	0.044	1	0.071	0
		Congo	1.096	1	2.351	3	0.149	5	2.5	3
		Democratic Republic	0.081	0	0.564	1	0.108	3	0.672	1
		of Congo
		Gabon	0.002	0	0.014	0	0.021	1	0.035	0
		Ghana	0.001	0	0.002	0	0.001	0	0.003	0
		Guinea	0.022	0	0.049	0	0.049	2	0.098	0
		Ivory Coast	0.023	0	0.055	0	0.019	1	0.074	0
		Kenya	0.04	0	0.061	0	0.038	1	0.099	0
		Liberia	0.001	0	0.003	0	0.004	0	0.007	0
		Madagascar	0.24	0	0.242	0	0.001	0	0.243	0
		Malawi	0.018	0	0.025	0	0.006	0	0.031	0
		Mauritania	0.002	0	0.002	0	0	0	0.002	0
		Mauritius	0	0	0	0	0	0	0	0
		Mozambique	0.008	0	0.043	0	0.021	1	0.064	0
		Nigeria	0.03	0	0.403	0	0.081	2	0.484	1
		Reunion	0	0	0	0	0	0	0	0
		Rwanda	0.444	1	0.46	1	0.001	0	0.461	1
		Senegal	0.003	0	0.006	0	0.001	0	0.007	0
		Sierra Leone	0	0	0	0	0	0	0	0
		Sudan	0.05	0	0.457	1	0.363	11	0.82	1
		Uganda	1.008	1	1.472	2	0.104	3	1.576	2
		Zambia	0.279	0	0.307	0	0.051	2	0.358	0
	SUB-TOTAL		3.536	4	6.872	8	1.19	37	8.062	9

Table 6 : Updated estimates of global and tropical peatland areas derived from 1 Immirzi et al. (1992) and this assessment. 2

 6 (1992) calculated the mean of the maximum and minimum values without assessing their degree of certainly (i.e. 3 likelihood of being correct); we considered the provenance of the data available and assessed, using criteria described in the text, 4 whether or not data were likely to be correct or not (hence best estimate) 52 Immirzi et al. (1992) estimates of global, boreal/temperate and tropical peatland areas

		Peatland Area	Minimum	Best Estimate 1	Maximum
			(km 2 )	(km 2 )	(km 2 )
		Global 2	3,858,374	3,971,895	4,085,416
		Tropical 2	333,820	415,485	497,119
		Boreal and temperate 2	3,524,554	3,556,410	3,588,297
		Revised tropical 3	384,776	439,238	656,430
		Revised global 3	3,909,330	3,995,648	4,244,727
		Tropical (% of revised	9.8	11.0	15.5
		global area)
		Southeast Asia 3	236,722	247,778	336,115
		Southeast Asia (% of	6.1	6.2	7.9
		revised global area)
	6 7	3 Area of tropical peatland from this assessment
	8
	9

1

Immirzi et al.

Table 7 : Updated estimates of global and tropical peat volumes derived from 1 Immirzi et al. (1992) and this assessment. 2

 7 Based on Boreal/Temperate peat mean thickness of 1.5 m (This is the mean thickness of global peat applied by Immirzi et al. Based on Boreal/Temperate peat mean thickness of 2.3 m (This is the mean thickness of global peat suggested by Gorham et al.

		Minimum	Best Estimate	Maximum
	Area of boreal and	3,524,554	3,556,410	3,588,297
	temperate peatland
	(km 2 ) 1
	Boreal/temperate peat	5,286	5,335	5,383
	volume 1 (Gm 3 ) 2
	Tropical peat volume	1,618	1,756	1,822
	(Gm 3 ) 3
	New global peat volume	6,904	7,091	7,205
	1 (Gm 3 )
	Tropical peat volume (%	23%	25%	25%
	of global 1)
	Southeast Asian peat	1,317	1,359	1,386
	volume (Gm 3 ) 3
	Southeast Asian peat	19%	19%	19%
	volume (% of global 1) 2
	Boreal/temperate peat	8,107	8,180	8,253
	volume 2 (Gm 3 ) 4
	Tropical peat carbon	1,618	1,756	1,822
	pool (Gm 3 ) 3
	New global peat volume	9,725	9,936	10,075
	2 (Gm 3 )
	Tropical peat volume (%	17%	18%	18%
	of global 2)
	Southeast Asian peat	1,317	1,359	1,386
	volume (Gm 3 )
	Southeast Asian peat	14%	14%	14%
	volume (% of global 2) 1 Immirzi et al. (1992) 2 4 3 (1992)). 5 3 This assessment 6 4 7 (1991) for boreal and sub-arctic peat). 8 9
	10

Table 8 : Updated estimates of global and tropical peatland carbon pools derived 1 from Immirzi et al. (1992) and this assessment. 2

 8

		Minimum	Best Estimate	Maximum
	Area of boreal and	3,524,554	3,556,410	3,588,297
	temperate peatland
	(km 2 ) 1
	Carbon density for peat	1099.5	1099.5	1099.5
	thickness 1.5 m (t ha -1 ) 1
	Boreal/temperate peat	387.5	391.1	394.5
	carbon pool (Gt 2 ) 1
	Tropical peat carbon	81.5	88.5	91.8
	pool (Gt) 3
	Global peat carbon	469.0	479.6	486.3
	pool (Gt)
	Tropical peat carbon	17%	19%	19%
	pool (% of global)
	Southeast Asian peat	66.4	68.5	69.8
	carbon pool (Gt)
	Southeast Asian peat	14%	14%	14%
	carbon pool (%)
	Carbon density of	1466	1466	1466
	boreal and temperate
	peatland for peat
	thickness of 2.3 m (t
	ha -1 ) 1
	Boreal/temperate peat	516.7	521.4	526.1
	carbon pool (Gt 2 ) 1
	Tropical peat carbon	81.5	88.5	91.8
	pool (Gt) 3
	Global peat carbon	598.2	609.9	617.9
	pool (Gt)
	Tropical peat carbon	14%	15%	15%
	pool (% of global)
	Southeast Asian peat	66.4	68.5	69.8
	carbon pool (Gt)
	Southeast Asian peat	11%	11%	11%
	carbon pool (%)

1

Immirzi et al.

(1992)

Acknowledgements