Peatlands initiate in one of four ways. The first, the most common, appears to through paludification (or swamping), wherein peat forms on previously drier, vegetated habitats on inorganic soils and in the absence of a body of water, generally due to regional water table rise and associated climatic moderation. Additionally, local site factors also have strong influences on paludification. Second, peat may form directly on fresh, moist, nonvegetated mineral soils. This primary peat formation occurs directly after glacial retreat or on former inundated land that has risen due to isostatic rebound. Third, shallow bodies of water may gradually be filled in by vegetation that develops floating and grounded mats - thus terrestrializing the former aquatic habitat. Both lake chemistry and morphometry as well as species of plants in the local area influence the rates and vegetative succession. Fourth, peat may form and be deposited on shallow basins once occupied by extinct Early Holocene lakes. These former lake basins, lined with vegetated impervious lake clays, provide hydrologically suitable sites for subsequent peat development.
Across the boreal zone, peatland initiation appears to be extremely sensitive to climatic controls. For example, in oceanic areas, peatlands often initiated soon after glacial retreat some 10 000-12 000 years ago. Many of these oceanic peatlands began as bogs and have maintained bog vegetation throughout their entire development. In more continental conditions, most peatlands were largely initiated through paludification. In areas where the bedrock is acidic, most of these early peatlands were poor fens, whereas in areas where soils are base rich and alkaline, rich fens dominated the early stages. Like oceanic peatlands, subcontinental peatlands initiated soon after glacial retreat; however, throughout most of the large expanses of boreal Canada and Siberia, peatland initiation was delayed until after the Early Holocene dry period, initiating 6000-7000 years ago. Many of these peatlands initiated as rich or poor fens and have remained as fens for their entire existence, whereas others have undergone succession and today are truly ombrotrophic bogs. A recent study in western Canada correlated peak times of peatland initiation to Holocene climatic events that are evident in US Midwest lakes, North Atlantic cold cycles, and differing rates of peat accumulation in the one rich fen studied in western Canada.
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