Bogs

Bogs are functionally ombrotrophic. At least in the Northern Hemisphere, they have ground layers dominated by the bryophyte genus Sphagnum (Figure 4). Sedges (Carex spp.) are absent or nearly so. The shrub layer is well developed and trees may or may not be present. Nearly, all of the vascular plants have associations with mycorrhizal fungi. Microrelief of raised mounds (hummocks) and depressions (hollows) is generally well developed. The peat column consists of a deep anaerobic layer (the catotelm), wherein decompositional processes are extremely slow and a surficial layer of

Figure 4 Mixed lawn of the peatmosses: Sphagnum angustifolium, mostly to the left, and S. magellanicum (red), mostly to right.

1-10 dm of the peat column that occupies an aerobic zone (the acrotelm). The acrotelm extends upward from the anaerobic catotelm and is mostly made up of living and dead components of Sphagnum plants, wherein vascular plant roots and fallen vascular plant aboveground litter occur. Well-developed acrotelms are unique to ombro-trophic bogs and provide opportunities to study atmospheric deposition and ecosystem response to such deposition.

Bogs are acidic ecosystems that have pH's of around 3.5-4.5. Base cations are limited owing to the ombrogen-ous source of water and to the cation exchange abilities of Sphagnum (see below). Bicarbonate is lacking in bogs and carbon is dissolved in the water column only as CO2. The lack of geogenous waters limits nutrient inputs to those derived only from atmospheric deposition, and thus nitrogen and phosphorus are in short supply.

Bogs appear to be limited in distribution to areas where precipitation exceeds potential evapotranspiration. In many oceanic regions of the Northern Hemisphere (especially Britain, Ireland, Fennoscandia, and coastal eastern Canada), bogs form large treeless expanses. In Europe, the Ericaceous shrub, Calluna vulgaris, forms a characteristic component of these treeless landscapes. Many of these oceanic bogs are patterned, with a series of pools of waters separated by raised linear ridges. This sometimes spectacular pool/ridge topography forms either concentric or eccentric patterns (Figure 5), with water flowing from the highest raised center of the bog to the lower surrounding edges. Runoff from the surrounding upland (and from the raised bog itself) is concentrated at the margins of these raised bogs and due to increased nutrients, decomposition processes are greater and peat accumulation somewhat less. Thus, the central, open, raised 'mire expanse' part of a bog is surrounded by a wetter, often shaded lagg, or moat, and this 'mire margin' zone may be dominated by plants indicative of fens. Some

Figure 5 An oceanic eccentric bog. Maine, USA. Highest elevation of bog is to center left, with elongate axis sloping to distant right. Photo is courtesy of Ronald B. Davis.
Figure 6 A continental ombrotrophic bog from western Canada. Tree species is Picea mariana (black spruce).

oceanic bogs have a rather flat mire expanse, with occasional pools of water. Whereas the mire expanse surface of these raised bogs is flat, the dome of water contained within the bog peat is convex and thus the driest part of the bog is at the edges just before contact with the fen lagg. This marginal, relatively dry upslope to the mire expanse is usually treed and is termed the 'rand'.

In continental areas, bogs have a very different appearance (Figure 6). These continental bogs have a conspicuous tree layer and abundant shrubs (mostly Ledum spp. or Chamaedaphne calyculata) while pools of water are not present. In North America, the endemic tree species, Picea mariana, dominates these continental bogs, while in Russia bogs have scattered individuals of Pinus sylvestris. Farther north in the subarctic and northern boreal zones, peat soils contain permafrost. When entire bog landforms are frozen, the bog becomes drier and dominated by lichens (especially species of the reindeer lichen, Cladina). Unfrozen or melted areas contained within these peat plateaus are easily recognized features termed collapse scars (Figure 7). Peat plateaus form

Figure 7 Extensive peat plateaus with permafrost (whitish areas dominated by the reindeer lichens in the genus Cladina), with isolated collapse scars (without permafrost - greenish circular to oblong areas), and with lush growth of Sphagnum species and sedges.
Figure 8 Bog dominated by Picea mariana in background, with dead snags in foreground, indicating recent permafrost collapse and the formation of an internal lawn and dominated by carpet and lawn species of Sphagnum.

extensive landscapes across the subarctic zone of both North America and Siberia. Farther south in the boreal zone, bog landforms may contain only scattered pockets of permafrost (frost mounds), that over the past several decades have been actively melting. Recent melting of the raised frost mounds results in collapse of the mound and active revegetation by fen vegetation to form wet, internal lawns with associated dead and leaning trees (Figure 8).

Fens

Fens are peatlands that are minerotrophic that when compared to bogs have higher amounts of base cations and associated anions. All fens have an abundance of Carex and Eriophorum spp. and water levels at or near the surface of the peat (thus acrotelms are poorly developed). Unlike bogs that are characterized by high microrelief of

Figure 9 A carpet of the brown moss, Scorpidium scorpioides, a characteristic species of rich fens.

hummocks and hollows, fens feature a more level topography of extensive carpets and lawns dominated by species of mosses (Figure 9). Depending on the characteristics of the surrounding water, fens can by divided into three types.

Poor fens

These Sphagnum-dominated peatlands are associated with acidic waters (pH 3.5-5.5) that contain the least amount of base cations and little or no bicarbonate alkalinity.

Rich fens

True mosses dominate the ground layer of rich fens, especially a series of species that are red-brown in color and often termed 'brown mosses'. Examples of important species would be Drepanocladus, Hamatocaulis, Warnstorfia, Meesia, Campylium, Calliergon, and Scorpidium. Waters have pH varying from 5.5 to more than 8.0 and base cations are relatively abundant, especially calcium. Alkalinity varies from very little to extremely high amounts ofbicarbonate. Rich fens occur as two types centered on the chemistry of

Figure 10 A patterned fen in western Canada characterized by elongate pools (flarks) separated by raised ridges (strings), oriented perpendicular to water flow.

the pore waters. 'Moderate-rich fens' have pH values between 5.5 and 7.0, with little alkalinity. Both brown mosses and some mesotrophic species of Sphagnum (e.g., S. subsecundum, S. teres, and S. warnstorfii) dominate the ground layer. 'Extreme-rich fens' are bicarbonate-rich peatlands, often with deposits of marl (precipitated CaCO3) and pH ranging from around neutral to over 8.0. Species of Scorpidium, Campylium, and Hamatocaulis dominate the ground layer.

Whereas water quality (= chemistry) is the main factor controlling fen type and flora, water quantity (= flow) controls vegetation structure and surface topography. Fens, whether poor or rich, are vegetationally extremely variable, ranging from sites having abundant trees (dominated by Larix laricina in North America), to sites dominated by shrubs (mostly Betula, Alnus, and Salix), to sites having only sedges and mosses. Topographically, fens may be homogeneous and dominated by lawns and carpets. However, as water flowing through the fen increases, the surface vegetation develops a reticulation of wet pools and carpets separated by slightly raised ridges. Further increase in flow of water directs the patterns into linear pools (some filled with floating vegetation = carpets), sometimes termed flarks, alternating with linear ridges (termed strings; Figure 10). These pool/string complexes are oriented perpendicular to water flow, with smaller pools always upstream from the larger ones. Especially prevalent in Scandinavia and Russia, these patterned fens and associated bog islands form extensive peatlands termed aapamires.

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