Mire

A mire, peatland 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 water-logging and subsequent anoxia. 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. 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.

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. Ombrotrophic types of quagmire may be called quaking bog (quivering bog). Minerotrophic types can be named with the term quagfen.

There are four types of mire: bog, fen, marsh and swamp. 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. 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 characterised 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.

The formation of mires today is primarily controlled by climatic conditions, such as precipitation and temperature, although terrain relief is a major factor, as water-logging occurs more easily on flatter ground. However, there is a growing anthropogenic influence in the accumulation of peat and peatlands around the world.

A valley mire creates a level ground surface in otherwise dramatic topography. Upper Bigo Bog, Rwenzori Mountains, Uganda.

Topographically, 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 10 m have been commonly recorded in temperate regions (many temperate and most boreal mires were removed by ice sheets in the last Ice Age), and above 25 m in tropical regions. 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. A simplistic calculation, using typical values for a Sphagnum bog of 1 mm new peat added per year and 0.0001 proportion of the catotelm decaying per year, gives a maximum height of 10 m. More advanced analyses incorporate expectable nonlinear rates of catotelm decay.

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).