Peat-forming wetlands are in general ecosystems that have accumulated sufficient organic matter over time to have a well-developed layer of peat. In many soil classifications, this is defined as soils having greater than 30% organic matter that forms deposits greater than 30-40 cm in depth. Non-peat-forming wetlands such as marshes (wetlands without trees) and swamps (wetlands dominated by a tree layer) mostly have less than 30-40 cm of accumulated organic material and over time have not been able to sustain continued accumulation of a carbon-rich peat deposit. Numerous classifications have been proposed that distinguish between various peatland types. For example, peatlands have been classified based on the source of water that has the primary influence on the peatland. Thus, peatlands that are influenced by water that has been in contact with soil or lake waters are termed geogenous and are divided into three types. Peatlands may be topogenous (influenced by stagnant water, mostly soil water, but also nonflowing water bodies as well), limnogenous (influenced by flood water from water courses resulting in lateral flow away from the direction of stream flow), or soligenous (influenced by flowing water, especially sheet flow on gentle slopes, including seepages and springs). Contrasted to these geo-genous types of peatlands, others may be ombrogenous (influenced only by rain water and snow).
Peatlands are extremely variable in vegetation structure; they may be forested (closed canopy), wooded (open canopy), shrub dominated, or sedge dominated. Ground layers may be moss dominated, lichen dominated, or bare. Finally, peatlands vary as to where they occur on the landscape: in association with streams, lakes, springs, and seeps or isolated at higher elevations in the watershed. Peatlands often occur on the landscape as 'complex peatlands', wherein several distinctive peatland types occur together (Figure 3). Finally, and perhaps most universally utilized, is a classification that combines aspects of hydrology, vegetation, and chemistry into a functional classification of peat-forming wetlands. In general, this view of peatlands would consider hydrology as
fundamental to peatland function and recognize two peat-land types - fens and bogs.
Fens are peatlands that develop under the influence of geogenous waters (or waters that influence the peatland after being in contact with surrounding mineral, or upland, substrates). Waters contacting individual peat-lands have variable amounts of dissolved minerals (especially base cations (Na+, K+, Ca2+, Mg2+) and associated anions (HCO-, SO2-, Cl-)), and may also vary in the amount of nutrients (N and P) as well as the number of hydrogen ions. Further complicating this minerotrophy is variation in the flow of water, including amount of flow and as well as source of the water (surface, ground, lake, or stream). Peatlands receiving water only from the atmosphere via precipitation are hydrologically isolated from the surrounding landscape. These ombrogenous peat-lands, or bogs, are ombrotrophic ecosystems receiving nutrients and minerals only from atmospherically deposited sources.
In summary and from a hydrological perspective, in fens water flows into and through the peatland after it has been in contact with surrounding materials, whereas in bogs water is deposited directly on the peatland surface and then flows through and out of the bog directly onto the surrounding landscape. Thus, fens are always lower in elevation than the surrounding landscape, while bogs are slightly raised about the connecting upland areas.
The recognition that hydrology is the prime factor for dividing peatlands into fens and bogs dates back to the 1800s. However, in the 1940s, Einar DuReitz recognized that vegetation composition and floristic indicators could be used to further characterize bogs and fens. Somewhat later, Hugo Sjors associated these floristic indicators with variation in pH and electrical conductivity (as a surrogate for total ionic content of the water). The results of these early field studies in Sweden provided an overarching view of how hydrology, water chemistry, and flora are associated, and more recent studies delineate how these combined attributes together form a functional classification of northern peatlands that provides an ecosystem perspective.
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