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Mire

A mire (or quagmire) is a wetland type, dominated by living, peat-forming plants. Mires arise because of incomplete decomposition of organic matter, usually litter from vegetation, due to waterlogging and subsequent anoxia. (add citation? Page & Baird 2016, p. 35, 41) Like coral reefs, mires are unusual landforms in that they derive mostly from biological rather than physical processes, and can take on characteristic shapes and surface patterning. All types of mires share the common characteristic of being saturated with water at least seasonally with actively forming peat, while having its own set of vegetation and organisms.^[9]

There are four types of mire: bog, fen, marsh and swamp.^[1] A bog is a mire that due to its location relative to the surrounding landscape obtains most of its water from rainfall (ombrotrophic), while a fen is located on a slope, flat, or depression and gets most of its water from soil- or 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.^[2] Although marshes are wetlands within which vegetation is rooted in mineral soil, some marshes form shallow peat deposits: these should be considered mires. Swamps are characterized by their forest canopy and, like fens, are typically of higher pH and nutrient availability than bogs. Some bogs and fens can support limited shrub or tree growth on hummocks.

Mires, and the peat that forms within them, provide a number of ecosystem services, most importantly thesequestration of carbon (C), protection and habitat for biodiversity, the mitigation of natural risk as well as providing food and fuel (Paige & Baird 2016, p. 36)

For botanists and ecologists, the term peatland is a more 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, but a mire by definition must be actively forming peat).

Mires are a kind of "...living relic... [A] living skin on an ancient body"^[3] in which successive layers of regular plant growth and decay are preserved stratigraphically with a quality of preservation unknown in other wetland environments.

The formation of mires today is primarily controlled by climatic conditions, however there is a growing anthropogenic influence in the accumulation of peat and peatlands around the world (Page & Baird 2016, p. 37).

Global distribution

Mires, although perhaps at their greatest extent at high latitudes in the Northern Hemisphere, are found around the globe. Mires occur wherever conditions are right for peat accumulation: largely where organic matter is constantly waterlogged. The distribution of mires therefore depends on topography, climate, parent material, biota and time.^[5] The type of mire - bog, fen or swamp - depends also on each of these factors.

The largest accumulations of mires, constituting around 64% of global peatlands, are found in the temperate, boreal and subarctic zones of the Northern Hemisphere (Page & Baird 2016, pp. 37-38). In polar regions, mires are usually shallow, because of the slow rate of accumulation of dead organic matter, and often contain permafrost. Very large swathes of Canada, northern Europe and northern Russia are covered by boreal mires. In temperate areas mires are typically more scattered due to historical drainage and peat extraction, but can cover large areas as blanket bog where precipitation is very high (e.g. in maritime climates inland near the coasts of the north-east and south Pacific, and the north-west and north-east Atlantic). In the sub-tropics, mires are rare and restricted to the wettest areas.

In the tropics, mires can again be extensive, typically underlying lowland tropical rainforests (for example, in Kalimantan, Indonesia), although tropical peat formation occurs in coastal mangroves, as well as in areas of high altitude. (Page & Baird 2016, p. 38). Tropical peatlands cover an area of over 0.4 million square kilometres, mostly in Indonesia, Papua New Guinea and Malaysia (Page & Baird 2016, pp. 38-39).

Mires are in rapid decline globally due to drainage for agriculture and forestry, and for peat harvesting. For example, more than 50% of original European mire area, more than 300000 km², has been lost.^[6]

Topogaphy

Mires elevate the ground surface above the original topography. Mires can reach considerable heights above the underlying mineral soil or bedrock: peat depths of above 10m have been recorded in temperate regions (many temperate and most boreal mires were removed by ice sheets in the last Ice Age), and above 25m in tropical regions.^[7] When the absolute decay rate in the catotelm (the lower, water-saturated zone of a mire) matches the rate of input of new peat into the catotelm, the mire will stop growing in height.^[8] A simplistic calculation, using typical values for a Sphagnum bog of 1mm new peat added per year and 0.0001 proportion of the catotelm decaying per year, gives a maximum height of 10m. More advanced analyses incorporate expectable nonlinear rates of catotelm decay.

Bio-chemical processes

Mires have unusual chemistry, which influences inter alia their biota and the chemistry of the water outflow. Peat has very high cation-exchange capacity due to its high organic matter content: cations such as Ca²⁺ are preferentially adsorbed onto the peat in exchange for H⁺ ions. Water passing through peat declines in nutrients and in pH. Therefore mires are typically nutrient-poor and acidic unless the inflow of groundwater (bringing in supplementary cations) is high.^[7]

Mires generally form whenever inputs of carbon exceed carbon outputs. This occurs due to the anoxic state of water-logged peat, and the process of photosynthesis by which peat grows (Page & Baird 2016, p. 41). Due to this, mires are collectively a major carbon store, containing between 500 and 700 billion tonnes of carbon, despite accounting for just 3% of Earth's land surfaces (Page & Baird 2016, p. 36-37). Carbon stored within mires equates to over half the amount of carbon found in the atmosphere (Page & Baird 2016, p. 36).

Mires influence carbon dioxide levels in the atmosphere such that when the water table rises, such as during a rainstorm, the peat and its microbes are submerged under water and inhibits the access to oxygen, giving opportunity for anaerobic microorganisms to flourish. Carbon dioxide is released when the water table shrinks, such as during a drought, as this supplies the aerobic microbes with oxygen to decompose the peat, subsequently releasing carbon dioxide.^[10] Levels of methane also vary with the water table position and somewhat with temperature. Methanogens are responsible for producing methane via decomposition of the peat which consequently increases as the water table rises and oxygen levels are depleted. Increased temperatures in the soil also contributes to increased seasonal methane flux, though at a lower intensity. It is shown that the methane increased by as much as 300% seasonal from increased precipitation and temperature of the soil.^[11]

Mires are important reservoirs of climatic information to the past because they are sensitive to changes in the environment and can reveal levels of isotopes, pollutants, macrofossils, metals from the atmosphere, and pollen.^[12] For example, carbon-14 dating can reveal the age of the peat. The dredging and destruction of a mire will release the carbon dioxide that could reveal irreplaceable information about the past climatic conditions. It is widely known that a plethora of microorganisms inhabit mires due to the regular supply of water and abundance of peat forming vegetation. These microorganisms include but are not limited to methanogens, algae, bacteria, zoobenthos, of which Sphagnum species are most abundant.^[13] The peat in mires contain a substantial amount of organic matter, where humic acid dominates. Humic materials are able to store very large amounts of water, making them an essential component in the peat environment, contributing to an increased amount of carbon storage due to the resulting anaerobic condition. If the peatland is dried from long-term cultivation and agricultural use, it will lower the water table and the increased aeration will subsequently release carbon content.^[14] Upon extreme drying, the ecosystem can undergo a state shift, turning the mire into a barren land with lower biodiversity and richness. The formation of humic acid occurs during the biogeochemical degradation of vegetation debris, animal residue, and degraded segments.^[15]The loads of organic matter in the form of humic acid is a source of precursors of coal. Prematurely exposing the organic matter to the atmosphere promotes the conversion of organics to carbon dioxide to be released in the atmosphere.

Anthropogenic uses

Mires are used by humans for a range of purposes, most significantly being agriculture and forestry, which accounts for around a quarter of global peatland area. (Page & Baird 2016, p. 46)

The commercial extraction of peat from mires for energy production is widely practiced in Northern European countries, such as Russia, Sweden, Finland and the Baltic states. (Page & Baird 2016, p. 46)

In addition, records of past human behaviour and environments can be contained within mires. These may take the form of human artifacts, or paleoecological and geochemical records. (Page & Baird 2016, p. 45)

Tropical peatlands comprise 0.25% of Earth’s terrestrial land surface but store 3% of all soil and forest carbon stocks and are mostly located in developing countries. The exploitation of these ecosystems, such as the draining and harvesting of tropical peat forests, releases a large amount of carbon dioxide. In addition, fires caused by dry peat due to the draining of peat bogs releases even more carbon dioxide. The economic value of a tropical peatland used to be derived from raw materials, such as wood, bark, resin, and latex; the extraction of which did not release carbon emissions. Today, many of these ecosystems are drained for conversion to palm oil plantations, releasing the stored carbon dioxide and preventing the system from sequestering carbon again. The planned Carbopeat Project will attempt to assign economic value to the carbon sequestration performed by peat bogs to stop the exploitation of these ecosystems.^[20]

Impacts on global climate

Wetlands provide an environment where organic carbon is stored in living plants, dead plants and peat, as well as converted to carbon dioxide and methane. Microbial activity is promoted by the large amounts of dissolved organic matter in wetlands, containing 45-50% carbon. Mineralization through bacterial oxidation converts this carbon to inorganic substances, allowing carbon storage to occur. Three main factors giving wetlands the ability to sequester and store carbon are the high biological productivity, high water table and low decomposition rates. Suitable meteorological and hydrological conditions are necessary to provide an abundant water source for the wetland. Fully water-saturated wetland soils allow anaerobic conditions to manifest, storing carbon but releasing methane.^[16] Wetlands make up about 5-8% of Earth’s terrestrial land surface but contain about 20-30% of the planet’s 2500 Gt soil carbon stores.^[17] Mires, as well as bogs, fens and marshes are the wetland types that contain the highest amounts of soil organic carbon, and can thus be considered peatlands.^[18] Wetlands can become sources of carbon, rather than sinks, as the decomposition occurring within the ecosystem emits methane.^[16] Natural peatlands do not have a measurable cooling effect on the climate in a short time span as the cooling effects of sequestering carbon are offset by the emission of methane, which induces warming. Despite this, peatlands do result in cooling of the Earth's climate over a longer time period as methane is oxidized quickly and removed from the atmosphere whereas atmospheric carbon dioxide is continuously absorbed.^[19] However, in the paper ‘Wetlands, carbon and climate change’ by Mitsch et al., it was determined “that methane emissions become unimportant within 300 years compared to carbon sequestration in wetlands. Within that time frame or less, most wetlands become both net carbon and radiative sinks.”^[17] Peatlands insulate the permafrost in subarctic regions, thus delaying thawing during summer, as well as inducing the formation of permafrost.^[19] As the global climate continues to warm, wetlands could become major carbon sources as higher temperatures cause higher carbon dioxide emissions. Compared with untilled cropland, wetlands can sequester around two times the carbon, and planted wetlands may be able to store 2-15 times more carbon than what they release. Carbon sequestration can occur in constructed wetlands, as well as natural ones. Estimates of greenhouse gas fluxes from wetlands indicate that natural wetlands have lower fluxes, but man-made wetlands have a greater carbon sequestration capacity. The carbon sequestration abilities of wetlands can be improved through restoration and protection strategies, but it takes several decades for these restored ecosystems to become comparable in carbon storage to peatlands and other forms of natural wetlands.^[16]

The drainage of peatlands for agriculture and forestry has resulted in the emission of extensive greenhouse gasses into the atmosphere, most notably carbon dioxide and methane. By allowing oxygen to enter the peat column within a mire, drainage disrupts the balance between peat accumulation and decomposition, and the subsequent oxidative degradation results in the release of carbon into the atmosphere. (Page & Baird 2016, p. 46)

Mire drainage or drying due to climactic factors may also increase the risk of fires, presenting further risk of carbon and methane release into the atmosphere. (Page & Baird 2016, p. 35) Due to their naturally high moisture content, pristine mires have a generally low risk of fire ignition. The drying of this waterlogged state means that the carbon-dense vegetation becomes vulnerable to fire. In addition, the oxygen poor nature of the vegetation causes peat fires to smoulder beneath the surface, causing incomplete combustion of the organic matter and resulting in extreme emissions events. (Page & Baird 2016, pp. 47-48)

When undertaken in such a way that preserves the hydrological state of a mire, the anthropogenic use of mires' resources can avoid significant greenhouse gas emissions. However, continued drainage will result in increased release of carbon and methane, contributing to global warming. As of 2016, it was estimated that drained peatlands account for around ten percent of all greenhouse gas emissions from agriculture and forestry. (Page & Baird 2016, p. 37)

Management and rehabilitation

Rehabilitation projects undertaken in North America and Europe usually focus around the rewetting of peatlands and revegetation with native species. This acts to mitigate carbon release in the short term, before providing organic litter to fuel the peat formation process in the long term. (Page & Baird 2016, p. 49)

The United Nations Convention of Biological Diversity targets highlights peatlands as key ecosystems to be conserved and protected. The conventions requires governments at all levels to present action plans for the conservation and management of wetland environments. Wetlands are also protected under the 1971 Ramsar Convention. (Page & Baird 2016, p. 46)