wise use of mires and peatlands - Peatland Ecology Research Group

MIRES AND PEATLANDS252.2 PEAT FORMATIONThe cycling of matter in most ecosystems isrelatively fast and complete. In contrast, miresare characterised by an incomplete cyclingresulting in a positive carbon balance.Because plant production exceeds decay, acarbon surplus is accumulated as peat. Peataccumulation generally takes place as a resultof limited decay (decomposition) of plantmaterial 17 . An important factor for peataccumulation is the chemical and structuralcomposition of the organic material,determining the “ability to decay”. The abilityto decay varies with species (e.g. Phragmitesversus Typha), plant parts (e.g. rhizomsversus flowers), and substances (e.g. waxesversus sugars) 18 . This means that some plantspecies, organs, and substances are moreinclined to accumulate peat than others. Alarge number of plant species that occur inmires can contribute to peat formation, suchas sedges, grasses, Sphagnum and othermosses, and woody plants. Consequently awide variety of “botanical” peat types 19 exist.Water is the most important external factorlimiting decay. Because of its large heatcapacity water induces lower than ambienttemperatures 20 . The limited diffusion rate ofgasses in water leads to a low availability ofoxygen 21 . Both factors inhibit the activitiesof decomposing and decompositionfacilitatingorganisms, leading to a decreasedrate of decay of dead organic material and,consequently, to the accumulation of peat 22 .Mires have been developing on Earth sincewetland plants first existed. Peat from thetropical mires of the Upper Carboniferous (320- 290 million years ago) and the sub-tropicalmires of the Tertiary (65 - 3 million years ago)is currently found as coal and lignite 23 . Thegreat majority of present-day peatlandsoriginated in the last 15,000 years. Sincedeglaciation, mires have developed intounique organic landforms with hydrological,biogeochemical, and biological links toupland and aquatic ecosystems. It isestimated that 4 million km 2 on Earth (some3% of the land area) is covered withpeatlands. The largest known concentrationsare found in Canada and Alaska, NorthernEurope and Western Siberia, Southeast Asia,and parts of the Amazon basin, where morethan 10% of the land area is covered withpeatlands 24 . Mires store about one third ofthe soil carbon in the world (see §2.5 below),and contain some 10% of the global liquidfresh water resources 25 .2.3 MIRE AND PEATLAND TYPESThere are many different ways of classifying 26wetlands, peatlands and mires that varyaccording to the purposes of theclassification 27 . It is not possible to describethem all in this document. The typologiesdescribed here are those appropriate to thediscussion in the rest of the document.Historically peatlands were distinguished onthe basis of their situation and the after-useof the remaining land, leading to theidentification of:● bogs, raised above the surroundinglandscape. After peat extraction, which wasnormally carried out under dry conditionsfollowing drainage, a mineral subsoil●suitable for agriculture often remained.fens, situated in depressions. After peatextraction, which was carried out bydredging, open water remained.These ‘pre-scientific’ terms were adopted andadapted (on different conceptual bases) byvarious scientific disciplines, which has ledto much confusion. More recently 28 , mireshave been classified into two mainhydrogenetic types: ombrogenous mires thatare fed only by precipitation, and geogenousmires 29 that are also fed by water which hasbeen in contact with the mineral bedrock orsubstrate 30 .