Peat swamp forests are tropical moist forests where waterlogged soil prevents dead leaves and wood from fully decomposing. Over time, this creates a thick layer of acidic peat. Peat swamp forests are typically surrounded by lowland rain forests on better-drained soils, and by brackish or salt-water mangrove forests near the coast. They are a kind of wetland which store and accumulate vast amounts of carbon as soil organic matter—much more than forests on mineral soil contain. Peat formation is a natural carbon sink; because the decomposition of the organic matter is slower than its production rate, the surplus accumulates as peat. The stability of these forests has important implications for climate change; they are among the largest near-surface reserves of terrestrial organic carbon. Tropical peat swamp forests, which have ecological importance, are one of the most threatened yet least studied and most poorly understood biotypes. Large areas of these forests are being logged at high rates. Since the 1970s, tropical peat swamp forest deforestation and drainage have greatly increased in South East Asia. In addition, El Niño–Southern Oscillation drought and large-scale fires are accelerating peatland devastation. Peat fires, drainage and deforestation enhance the decomposition of soil organic matter, increasing the release of stored carbon into the atmosphere as carbon dioxide. Tropical peat swamp forests are home to thousands of animals and plants, including many rare and critically endangered species such as the orangutan and Sumatran tiger, whose habitats are threatened by peatland deforestation.
Distribution Tropical peat ecosystem are found in three regions: Central America, Africa and South East Asia. About 62% of the world's tropical peat lands occur in the Indomalayan realm (80% in Indonesia, 11% in Malaysia, 6% in Papua New Guinea, and pockets in Brunei, Vietnam, the Philippines, and Thailand). Peat in Indonesia is distributed over three islands, Sumatra (8.3 million ha), Kalimantan (6.3 million ha) and Papua (4.6 million ha). 36% of the world's tropical peat occurs in Africa's central Congo Basin.
Formation Tropical peat forms on low-lying areas, such as river deltas, floodplains or shallow oxbow lakes. The formation process usually follows hydrosere successional steps, where the ponds or flooded area eutrophicate by water plants, then transform into swamp with grasses or shrubs, and eventually form a forest that continues to grow and accumulate. Peat located on the fringing areas might form through lateral expansion. This peat accumulation often forms a convex shape called a dome, which could rise up to 4 m (13 ft) on coastal peat and up to 18 m (59 ft) on inland peat. At the beginning of its formation, peat is largely topogenous or minerotrophic, receiving high nutrient input from rivers or groundwater. As the peat thickens and the dome becomes elevated, the top of the peat is no longer affected by the river or groundwater input, instead soil becomes ombrotrophic, exclusively obtaining water from the precipitation Input only from the rain causes a low nutrient and mineral content, especially calcium. The peat thus becomes highly acidic and only able to support low biodiversity and stunted forest.
Inland and coastal peat differ greatly in their age, where coastal peat formed during the mid Holocene, about 8,000 years before present (YBP). Inland peat formed much earlier during the Late Pleistocene, more than 26,000 YBP. Coastal peat formation is highly affected by the sea level rise with strong accumulation around 8,000-4,000 YBP when El Niño is less intense. Because the Sunda Shelf is tectonically stable, the sea level change in this area is only affected by the eustatic sea level, and during the glacial period the Karimata Strait dried, causing Asian Peninsula, Sumatra, Borneo and Java to become connected. After the Last Glacial Maximum, this coastline moved inland as the ice sheet melted and finally reached the level of modern coastline around 8,500 YBP. Inland peat formation is highly affected by climate with little or no effect of sea level rise because it located around 15–20 m (49–66 ft) above sea level, where the most recent record of higher sea level was 125,000 YBP when the sea level was 6 m above the modern level. Peat cores from Sebangau, South Kalimantan show a slow growth of 0.04 mm/y around 13,000 YBP when the climate was colder, then accelerated to 2.55mm/y around 9,900 YBP in warmer Early Holocene, then slower again to 0.23–0.15 mm/y during intense El Niño. A similar pattern is observed in cores from Sentarum, West Kalimantan, where the peat shows slower growth, which was caused by a drier climate during Heinrich Event I and the emergence of El Niño. It has long been assumed that the peat underlying tropical peat swamp forests accumulates because the extreme conditions (waterlogged, nutrient poor, anaerobic and acidic) impede microbial activity. Studies in a tropical Malaysian peat swamp (North Selangor peat swamp forest) show that although the sclerophyllous, toxic leaves of endemic peat-forest plants (Macaranga pruinosa, Campnosperma coriaceum, Pandanus atrocarpus, Stenochlaena palustris) were barely decomposed by bacteria and fungi, the leaves of M. tanarius, another plant species, were almost completely decomposed after one year. Thus it is intrinsic properties of the leaves (that are adaptations to deter herbivory in the nutrient poor environment) that impede microbial breakdown.
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