Characteristic peat forming plants in different parts of the Earth (Prager et al. 2006).

Climatic zones and sections	Dominant peat formers (physiognomy)	Dominant peat formers (taxonomy)	Dominant peat forming plant parts
Arctic / Boreal/ Oceanic	Mosses	Sphagnaceae, Hypnales	Stems, branches, leaves
Temperate / Subtropic	Reeds	Poaceae, Cyperaceae, Equisetaceae	Rhizomes, rootlets
Tropic	Trees	Angiospermae/Dicotyledoneae	Roots

The accumulation of peat implies an imbalance in the production and the decay of dead organic (plant) material. Such imbalance may be caused by both the production side and the decay side of the process. A high production rate is stimulated by ample availability of plant nutrients (CO2, P, K, N), water and warmth. A high CO2 concentration in the atmosphere has probably been responsible for the enormous accumulation of peat in the Carboniferous and Tertiary periods that has been passed on to us as coal and lignite (Lyons & Alpern 1989, Cobb & Cecil 1993, Demchuk et al. 1995). While NPK-fertilization and higher temperatures may lead to higher production, decay rates generally are even higher. This therefore frustrates peat accumulation (Clymo 1983). Differences in the chemical and structural composition of the plant material mean that some plant species and plant parts may produce peat, whereas others do not (Koppisch 2001). The most important reason for peat accumulation, however, is retarded decay due to the abundance of water (Clymo 1983, Koppisch 2001). Water is the single most important factor enabling peat accumulation. Water-logging is a prerequisite for the creation and preservation of peat. The large heat capacity of water and the large energy demand for vaporization induce lower than ambient temperatures, whereas the limited diffusion rate of gases in water leads to a low availability of oxygen (Ball 2000, Denny 1993). The resulting relatively cold and anaerobic conditions inhibit the activities of decomposing organisms (Moore 1993, Freeman et al. 2001). Peat accumulation only takes place when the water level is just under, at, or just over the surface over the long-term. When water levels are too low, plant remains decay too rapidly to allow accumulation. Water levels that are too high obstruct the production of plant material because the submersed plant parts are suffocated through lack of oxygen and carbon dioxide (Ivanov 1981, Ingram and Bragg 1984, Alexandrov 1988, Sjörs 1990, Lamers et al. 1999). Peat accumulation therefore only takes place in the range of water “availability” (both in space, with regard to water levels, and time, with regard to seasons), in which the decay of organic material is inhibited more than its production. In areas with deeper and fluctuating water levels a larger part of the organic material decays. This leads to less peat accumulation and more strongly humified peat. Activities that substantially lower or raise the water level in peatlands negatively affect their peat accumulation capacity and their associated functions (Ivanov 1981). In different parts of the world, different plant groups and plant parts are the main peat formers. Mosses (Bryophytes) determine peat growth in cold (e.g. boreal and subarctic) and wet-and-cool (e.g. oceanic) places (table 2.1). A lack of water-conducting organs enables peat formation by mosses only where water loss by evapotranspiration is restricted. In these areas, where most peatlands are concentrated, peatland science came into being. Therefore moss growth is the central model of peatland development, to the extent that the same words refer to tiny Bryophyte plants and to the extensive peatlands of e.g. Flanders Moss, Lille Vild Mose, and Katin Moch (Prager et al. 2006). This north/west European bias has hampered the recognition of peatlands that are not dominated by mosses. In more temperate and continental parts of the world, the drier climate forces peat formation to “go underground”. There, peat is formed from the downward growing rhizomes and rootlets of grasses (Poaceae) and sedges (Cyperaceae). Peat accumulates in the first 10–20 cm below the surface, as new root material is injected into the older peat soil matrix. In tropical lowlands peat is forms even further under the surface by the roots of tall forest trees (Prager et al. 2006). In natural peatlands peat typically accumulates with a long-term rate of 0.5-1 mm and 10 – 40 tonnes C per km2 per year, with locally strong variation. These general rates may be slower under less favourable climatic or hydrological conditions such as in the Arctic tundra, or faster, particularly in the tropics (Lavoie et al. 2005, Prager et al. 2006). The peatlands existing today largely originated from the end of the Late-Glacial and in the first part of the Holocene (Halsey et al. 1998, Campbell et al. 2000, MacDonald et al. 2006).