Different countries classify their peatlands differently, but commonly peatlands are referred to by various names such as bogs, fens, and mires.
According to Joosten & Clarke (2002), a mire refers to a peatland where peat is actively being formed. Bogs, also known as ombrogenous mires, are higher than their surrounding landscapes and receive water only from precipitation. Fens, or geogenous mires, are located in depressions and receive water that has been in contact with mineral bedrock or soil. Geogenous mires may be further subdivided into those that are soligenous, receiving water from precipitation and surface runoff, and lithogenous, receiving mostly deep groundwater.
Peatlands are commonly classified on the basis of its water source, which may change in time during succession, and also governs water and nutrient chemistry. Geogenous peatlands, i.e. fens, are nutrient-rich (minerotrophic) and strong connections to groundwater. Fens commonly have neutral pH all year, and are characterized by abundance of base cations, Ca and Mg. Vegetation is dominated by grasses, sedges, and rushes. With peat layer thickness increasing in time, it becomes more and more isolated from groundwater, relying more on precipitation for water and nutrients. Grasses and sedges are replaced by Sphagnum sp. mosses and woody vegetation. These so-called mesotrophic peatlands are characterized by a thicker peat layer, high organic content and acidic conditions. These acidic and nutrient-poor, now ombrogenous peatlands receive water from precipitation only and represent the successional zenith along the continuum from minerotrophic to oligotrophic. In these peatlands, production of organic material like Sphagnum sp. and other vegetation is effectively preserved through a combination of anaerobic soils, acidic conditions and low nutrient availability.
Because bogs and fens represent a continuum of environmental conditions, it was suggested to describe them on the basis of both water source (ombrogenous, geogenous) and pH (circumneutral, moderately acid, strongly acid) rather than on nutrient availability. This way of precise classification, however, is very challenging due to the variation and global distribution of peat properties and vegetation types. Joosten & Clarke (2002) and many authorities therefore suggest using the generic term peatland, rather than bog or fen, when describing them.
Common terms used to describe peatlands (Joosten & Clarke 2002):
Mire: A peatland where peat is actively being formed
Bog or ombrogenous mire: A peatland that is raised above the surrounding landscape and that receives water only from precipitation
Fen or geogenous mire: A peatland that is situated in a depression and receives water that has been in contact with mineral bedrock or soil
Soligenous mire: A fen that receives water from precipitation and surface water
Lithogenous fen: A fen that receives water from precipitation and deep groundwater
Histosol: A peatland whose soil contains at least 12–18% organic C and whose thickness is at least 40 cm
Craft, C., 2016. Creating and Restoring Wetlands: From Theory to Practice. Elsevier.
Joosten, H. & Clarke, D. 2002: Wise Use of Mires and Peatlands: Background and principles including a framework for decision making. IMCG/IPS