Peatlands and carbon storage
Global distribution of peatlands
The extent of global peatland coverage has been estimated in the range of 3,880 x 103 to 4,080 x 103 km2 by Maltby and Immirzi (1993), although they acknowledge that this is likely an underestimate and as such an estimate by Bridgham et al. (1996) of 5,961 x 103 km2 is probably closer to the true extent of peatland area.
Of the global peatland area, the estimates of peatland in tropical regions vary. Maltby and Immrizi (1993) estimated tropical areas at 333,820 – 497,120 km2. This is slightly lower than more recent estimates such as that by Page et al. (2011) in which tropical peatland area was estimated between 387,201 – 657,430 km2 with a best estimate of 441,025 km-2, which fell within the range estimated by Maltby and Immirzi (1993). Tropical peatlands represent ca. 11 % of the global peatland area and contain ca. 89 Gt of carbon (Page et al. 2011).
The majority of peatlands are located in temperate and boreal regions with estimates in the range 3,460 x 103 to 3,589 x 103 km2 presented by Gorham, (1991) and Maltby and Immirzi, (1993). This represents ca. 89 % of global peatlands.
Depth of peat deposits
Peatlands fall into three broad categories determined by latitudinal position. They are boreal, temperate and tropical. The location of a peatland determines the environmental factors that will potentially affect the C storage capacity and what applies to one category will not necessarily apply to the others, for example, permafrost melting in boreal peatlands or monsoons in tropical peatlands. This means that whilst the fundamental mechanisms of ecosystem functioning will be similar across peatland types, the primary factors that influence them may differ.
Tropical peatlands form deep peat deposits in comparison to temperate and boreal peatlands. Asian peatlands are thought to have the deepest peat deposits of the tropical regions. Extensive studies in Kalimantan, Indonesia have generally found that peat deposits are in the range of 0.5 – 11 m (Page et al. 1999; Shimada et al., 2001; Hope et al., 2005). Some studies have found deposits of greater depths including 13.7 m in a coastal peatland of West Kalimantan (Shimada et al. 2001). In Eastern Kalimanton deposits of up to 16 m were found by Hope et al. (2005). In comparison, other tropical peatlands have so far been found to be shallower than those of SE Asia. For example, Phillips et al. (1997) found peat deposits in the range of 0.5 – 8 m in San San Pond Sak peatland, Panama. Lahteenoja et al. (2009) found deposits of up to 5.9 m in Peruvian Amazon peat deposits. When all tropical regions were considered Page et al. (2011) estimated the majority of peat deposits to be in the range of 0.5 – 9 m.
Temperate peatlands have deposits that are typically in the range of 0.5 – 4.0 m. For example, Buffam et al. (2010) found that the majority of peat deposits in Wisconsin and Michigan fell between 0.1 – 4.0 m. In an Irish blanket bog Laine et al. (2007) found peat deposits of 2 – 3 m. Plado et al. (2011) found peat of up to 4.0 m depths in East Estonia. Not all peat depths were found to be lower than in tropical peatlands, some exceed the general majority of depths seen in tropical peatlands, for example, peat deposits of up to 14.6 m were found in Wisconsin and Michigan, USA in a study by Buffam et al. (2010). Peat depths of this extent are the exception rather than the norm in temperate peatlands.
Boreal peatlands tend to have the shallowest peat deposits compared to temperate and tropical peatlands. Some boreal locations were found to be comparable to temperate peatland depths, for example, Sheng et al. (2004) found peat deposits of 1 – 4 m depth in West Siberia. This was comparable with depths of ca. 0.3 – 4.5 m found in Finland by Silvola et al. (1996). Other studies such as that by Plug (2003) found shallow deposits of < 2 m in Northwest Alaska. Slightly greater depth ranges were found in Northern Sweden of 0.5 – 3.0 m by Nilsson and Bohlin (1993). Deeper peat deposits seldom occur, though there are exceptions, for example, Silvola et al. (1996) found one location in Finland of 6 – 7 m depth. No peat deposits comparable to the deepest tropical deposits have been found in boreal peatlands.
Therefore although tropical peatlands cover a smaller land area the potential for extensive carbon stores is evident when considering the relatively greater depths of tropical peat in comparison to temperate and boreal peatlands.
Rate of carbon accumulation in peatlands
Peatlands are formed when environmental conditions promote slow rates of decomposition; this is often combined with a high rate of Net Primary Production (NPP) and results in the accumulation of partially decomposed plant material in the form of peat deposits (Moore et al. 2007).
Historical carbon accumulation in peatlands
Historically peat accumulation/degradation rates are likely to have varied with the climate, so for periods of favourable conditions there were greater carbon accumulation rates. However, when dating peat it is important to remember that absence of peat of a certain age is not necessarily an indication of a slow accumulation rate, but could instead be indicative of peat degradation after formation. For example, Page et al. (2004) found in Indonesia, that the carbon accumulation rates alternated from periods of rapid accumulation (2 – 2.55 mm peat yr-1) to slow or standstill periods (0 – 0.6 mm peat yr-1). The highest carbon accumulation rate was found to be 74 mg C m-2 yr-1 at ≈ 24, 000 yr before present (BP) and the lowest rates at ≈ 12,000 – 13,000 yr BP at 1.3 g C m-2 yr-1. From this data two time periods were identified as being conductive to peat formation in Indonesia, they were the lead up to the last glacial maximum (LGM) 36,000 – 21,000 yr BP and the last glacial-interglacial transition period 13,000 – 10,000 yr BP. This was supported in part in a study by Anshari et al. (2010) in which they found peat accumulations rates in West Kalimantan decreased in the time periods closer to the LGM, though it is speculated that this low accumulation may be due to oxidation in subsequent years as a result of climatic shift in favours of degradation processes.
Ancour et al. (1999) found in the Rusaka swamp in Africa a similar decrease in carbon accumulation rates during ≈ 13,600 – 12,000 yr BP as well as during 5,500 – 1,600 yr BP. These time periods are thought to represent drier climatic conditions. Carbon accumulation rates found by Ancour et al. (1999) ranged between 20 – 200 mg C m-2 yr-1, which were generally greater than those found in Indonesian peatlands (Page et al. 2004).
From an analysis of available data, Dommain et al. (2011) found that during the Early Holocene, rapid sea-level rise lead to the initiation of many coastal peat domes in Kalimantan, however, inland peat formations in Indonesia were typically initiated in the period 29,000 – 21,000 yr BP and are some of the oldest tropical peat deposits.
In contrast, the peat deposits of Central and Southern America are typically younger than those of Asia, for example, Phillips et al. (1997) used radiocarbon dating to suggest that the Changuinola peat deposit in Panama was initiated ≈ 4,000 yr BP. Garcia et al. (2004) found that peat deposit initiation in the Jacarei deposits in Brazil was ≈ 9,700 yr BP and that in Venezuela peat formation in the central and northwestern delta plain of the Rio Grande was found to have initiated <3,000 yr BP in response to prior sea-level changes during the Holocene.
Generally it can be seen that peat deposits present in the tropical Americas are younger in age than those of Africa or Asia. However the absence of older peat deposits does not preclude the suggestion that there may have been peat deposits formed during similar time periods as the oldest Asian peats, but that instead shifting climates may have caused older peat deposits to degrade.
Current carbon accumulation in peatlands
The rate of carbon accumulation in peat deposits is greatest in the tropics, which is likely due to high NPP. Though there is a wide range of accumulation rates that have been measured in different peatlands. For example, Chimner and Ewel (2005) estimated carbon accumulation on the Island of Kosrae in Micronesia at 300 g C m-2 yr-1, which is at the higher end of carbon accumulation rates in the tropics. The majority of measurements fall below 100 g C m-2 yr-1. In central Kalimantan for example, carbon accumulation rates were determined to be 31.3 g C m-2 yr-1 on average by Dommain et al. (2011). However, Page et al. (2004) suggest a current carbon accumulation rate of 84.8 g C m-2 yr-1 which decreases to 56.2 g C m-2 yr-1 when the past 500 years are considered. Coastal peatlands in Kalimantan were also considered by Dommain et al. (2011) with a reported carbon accumulation rate of 77 g C m-2 yr-1 on average. Comparable C accumulation rates were reported in Peruvian Amazon peatlands by Lahteenoja et al. (2009), with accumulation rates ranging from 39 – 85 g C m-2 yr-1.
Some carbon accumulation rates from temperate peatlands are comparable with the majority of measured tropical accumulation rates. For example, Craft and Richardson (1993) found accumulation rates in the range 54 – 161 g C m-2 yr-1 in the North and Central Florida Everglades. Though the C accumulation rate can vary dependent on the types of peatland, as shown by Craft et al. (2008), when investigating peatlands between South Florida and Minnesota (26 – 47O N). Bog peatlands were found to have carbon accumulation rates that were greater than typical tropical rates, at 132 – 198 g C m-2 yr-1. However fen peatlands had accumulation rates in the range 19 – 46 g C m-2 yr-1, lower than typical tropical accumulation rates. Anderson (2002) also found carbon accumulation rates that were lower than those of tropical peatlands in a bog in North West Scotland, with a long-term average accumulation of 21.3 g C m-2 yr-1. Roulet et al. (2007) found a similar carbon accumulation rate of 21.5 g C m-2 yr-1 on average in a peatland located in Ottawa Canada.
Carbon accumulation rates in boreal peatlands are generally lower than those found in temperate and tropical peatlands. Robinson and Moore (1999) found, in a range of boreal Canadian peatlands, carbon accumulation rates that fell between 13.3 – 21.8 g C m-2 yr-1. Slightly lower rates were found in West – Central Canadian peatlands at 12.5 – 12.7 g C m-2 yr-1 by Sannel and Kuhry (2009). Substantially lower rates at 6.3 g C m-2 yr-1 were found by Turunen and Turunen (2003) in a Canadian peat bog. In comparison, carbon accumulation rates found in Finland had a greater degree of overlay with the range of rates found in temperate and tropical peatlands. For example, Turunen et al. (2002) found accumulation rates in the range 15.4 – 35.3 g C m-2 yr-1. Whilst Ukonmaanaho et al. (2006) found in an ombrotrophic bog in Hietajarvi, Finland, substantially greater carbon accumulation rates at 32.8 g C m-2 yr-1 on average. This is a greater accumulation rate than some areas of the tropics, so it can be seen that whilst accumulation rates are typically greatest in the tropical regions and decrease with latitude, this is not always the case.
Factors other than climate affecting carbon accumulation in peatlands
Carbon accumulation can be affected by processes other than decomposition, the most prominent of which are land-use change and fire.
Land-use change is an important factor relating to the carbon stores of tropical peatlands. One of the most important factors influencing peatlands is the soil moisture status, often inferred via the water table (WT) depth below or above the peat surface and typically when peatland is converted to other land uses the peatland is either drained or WTs are artificially controlled, with a subsequent impact on surface gas fluxes. For example, Hooijer et al. (2010) estimated that in 2006, 632 Mt CO2 was emitted from drained peatlands in SE Asia (with a range of 355 – 855 Mt CO2). Peatlands that are drained or WT controlled are often used as farmland. Sago palm plantations are particularly common in SE Asia. Sago palm plantations need high WTs, so land that was previously peat is ideal. When used to produce commercial products fertilisers would commonly be applied, this can also affected gas fluxes, for example, Watanabe et al. (2009) found in the Riau Province of Indonesia, that fertiliser application on Sago palm plantation during the rainy season increased CH4 flux from ca. 0.05 mg C m-2 h-1 (no fertiliser) to ca. 3.10 mg C m-2 h-1 with fertiliser application. Sago palm plantations as a CH4 source was also found by Melling et al. (2005b), in Sarawak, Malaysia, with an annual carbon loss in the form of CH4 of ca. 180 mg C m-2 yr-1 compared to ca. 18 mg C m-2 yr-1 in a mixed peat swamp forest in the same region. This was thought to be due to the high WTs required for Sago Palm plantations increasing the susceptibility of these areas to flooding and favouring anaerobic respiration. Conversely the conversion to Oil palm plantation resulted in a CH4 sink site, with an uptake rate of 15 mg C m-2 yr-1, again thought to be due to WT depth (in this case increased depth below the peat surface) together with the peat temperature (conversion to plantation lead to greater exposure of the peat surface and therefore peat temperature increased).
The effect of land-use change to plantation on CO2 flux at first appears to be negligible. Some studies have found no significant difference in CO2 flux between undisturbed peatlands and peatlands that have been converted to plantation. For example, Page et al. (2011) in the lowlands of Central Kalimantan, Indonesia estimated that CO2 fluxes from a non-drained peat forest were 3713 ± 520 g m-2 yr-1 compared to flux from a drained peat forest site of 3719 ± 383 g m-2 yr-1 (however under dry conditions the drained forest site had greater CO2 fluxes than the non-drained site due to WT drawdown). Other studies have found that undisturbed peat sites produced higher CO2 fluxes than those peat sites that had been converted to plantations, for example, Melling et al. (2005a) found in Sarawak, Malyasia, that a mixed peat swamp forest had a CO2 flux of ca. 100 – 533 mg C m-2 h-1 compared to an oil palm plantation ca. 63 – 245 mg C m-2 h-1 and a Sago palm plantation ca. 46 – 335 mg C m-2 h-1. It was suggested that these differences in CO2 flux rates were driven by the decreased biomass and productivity of the plantations (compared to peat forest) and the subsequent decreased root biomass, and therefore root derived CO2 from root respiration. The extent of land-use change can also affect the potential CO2 flux. Jauhiainen et al. (2008) compared a drainage affected, selectively logged peat forest with a drained, deforested and burnt peatland in Central Kalimantan. They found that the selectively logged forest had higher CO2 fluxes (400 – 1600 mg m-2 h-1) compared to the deforested and burnt peatland (0 -700 mg m-2 h-1; although this does not include the CO2 emissions from the peat fires).
Whilst plantations may appear to have similar or lower CO2 fluxes, the CO2 produced from former peatlands is derived from the degradation of peat material (excluding the portion of CO2 flux fro root respiration) that is not being replaced by the formation of new peat material as in carbon accumulating peatlands, thereby reducing the carbon store of peatands converted to alternate land-use.
Degraded peatlands, particularly those that have been drained, have been found to be more susceptible to fires (Miettinen and Liew, 2010) and hence large point emissions of carbon from the peat stores, for example, Page et al. (2002) found in Central Kalimantan, Borneo, that the 1997 fires burned 51.3 % of the mega-rice project (MRP) area (a degraded area, heavily influenced by anthropogenic clearing) in comparison to only 19.3 % of the peatlands outside the MRP. The present an overall estimate of 2.18 – 2.57 Gt C released from Indonesian peat fires in 1997.
Using satellite data, van der Werf et al. (2008) investigated the areas of Indonesia, Malaysia and Papua New Guinea and found strong links between the intensity of drought and carbon emissions from fires. They found that fire occurrences and carbon emissions were exponentially linked, with an increase in carbon releases with an increase in drought severity. For example, in Borneo in 2000, the dry season was short due to la Niña conditions, with 7 ± 3 Tg C yr-1 estimated losses due to fires. Compared to 2006, when there were El Niño conditions and an extended dry season, with an estimate of 236 ± 106 Tg C yr-1 losses from fires.
These studies show how land-use change and other anthropogenic influences can lead to enhanced degradation of peat material. These changes also link in together, as land-use change tends to result in the peatland becoming more susceptible to drought, which increases the risk of fires, leading to increased greenhouse gas emissions, which further enhance climatic change leading to drier conditions, which then further enhance drought and so on in a feedback loop.
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