Peatland

A peatland is a type of wetland whose soils consist of organic matter from decaying plants, forming layers of peat. Peatlands arise because of incomplete decomposition of organic matter, usually litter from vegetation, due to water-logging and subsequent anoxia. Peatlands are unusual landforms that derive mostly from biological rather than physical processes, and can take on characteristic shapes and surface patterning.

The formation of peatlands is primarily controlled by climatic conditions such as precipitation and temperature, although terrain relief is a major factor as waterlogging occurs more easily on flatter ground and in basins. Peat formation typically initiates as a paludification of a mineral soil forest, terrestrialisation of lakes, or primary peat formation on bare soils on previously glaciated areas. A peatland that is actively forming peat is called a mire. All types of mires share the common characteristic of being saturated with water, at least seasonally with actively forming peat, while having their own ecosystem.

Peatlands are the largest natural carbon store on land. Covering around 3 million km² globally, they sequester 0.37 gigatons (Gt) of carbon dioxide (CO₂) a year. Peat soils store over 600 Gt of carbon, more than the carbon stored in all other vegetation types, including forests. In their natural state, peatlands provide a range of ecosystem services, including minimising flood risk and erosion, purifying water and regulating climate.

Peatlands are under threat by commercial peat harvesting, drainage and conversion for agriculture (notably palm oil in the tropics) and fires, which are predicted to become more frequent with climate change. The destruction of peatlands results in release of stored greenhouse gases into the atmosphere, further exacerbating climate change.

For botanists and ecologists, the term peatland is a general term for any terrain dominated by peat to a depth of at least 30 cm (12 in), even if it has been completely drained (i.e. a peatland can be dry). A peatland that is still capable of forming new peat is called a mire, while drained and converted peatlands might still have a peat layer but are not considered mires as the formation of new peat has ceased.

There are two types of mire: bog and fen. A bog is a mire that, due to its raised location relative to the surrounding landscape, obtains all its water solely from precipitation (ombrotrophic). A fen is located on a slope, flat, or in a depression and gets most of its water from the surrounding mineral soil or from groundwater (minerotrophic). Thus, while a bog is always acidic and nutrient-poor, a fen may be slightly acidic, neutral, or alkaline, and either nutrient-poor or nutrient-rich. All mires are initially fens when the peat starts to form, and may turn into bogs once the height of the peat layer reaches above the surrounding land. A quagmire is a floating (quaking) mire, bog, or any peatland being in a stage of hydrosere or hydrarch (hydroseral) succession, resulting in pond-filling yields underfoot (floating mats). Ombrotrophic types of quagmire may be called quaking bog (quivering bog). Minerotrophic types can be named with the term quagfen.

Some swamps can also be peatlands (e.g. peat swamp forest), while marshes are generally not considered to be peatlands. Swamps are characterized by their forest canopy or the presence of other tall and dense vegetation like papyrus. Like fens, swamps are typically of higher pH level and nutrient availability than bogs. Some bogs and fens can support limited shrub or tree growth on hummocks. A marsh is a type of wetland within which vegetation is rooted in mineral soil.

Peatlands are found around the globe, although are at their greatest extent at high latitudes in the Northern Hemisphere. Peatlands are estimated to cover around 3% of the globe's surface, although estimating the extent of their cover worldwide is difficult due to the varying accuracy and methodologies of land surveys from many countries. Mires occur wherever conditions are right for peat accumulation: largely where organic matter is constantly waterlogged. Hence the distribution of mires is dependent on topography, climate, parent material, biota and time. The type of mire—bog, fen, marsh or swamp—depends also on each of these factors.