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 The Registry of Nature Habitats
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Wetland Basics | Ponds
| Streams
| Bogs & Fens
Marshes
| Pollution | Wetlands
Bogs & Fens
A bog is a wetland type
that accumulates acidic peat, a deposit of
dead plant material. The term peat bog in common usage is not
entirely redundant, although it would be proper to call these
sphagnum bogs if the peat is composed mostly of acidophilic moss
(peat moss or Sphagnum spp.).
Lichens are a principal component of peat in the far north. Moisture
is provided entirely by precipitation, and for this reason bog
waters are acidic and termed ombrotrophic (or cloud-fed), which
accounts for their low plant nutrient status. Excess rainfall
outflows, with dissolved tannins from the plant matter giving a
distinctive tan colour to bog waters.

Bogs are
recognized as a significant habitat type by a number of governmental
and conservation agencies. For example, the United Kingdom in its
Biodiversity Action Plan establishes bog habitats as a priority for
conservation. Bogs are challenging environments for plant life
because they are low in nutrients and very acidic. Carnivorous
plants have adapted to these conditions by using insects as a
nutrient source. The high acidity of bogs and the absorption of
water by sphagnum moss reduce the amount of water available for
plants. Some bog plants, such as Leatherleaf, have waxy leaves to
help retain moisture. Bogs also offer a unique environment for
animals.
Bogs
The
word bog often evokes a picture of a pond with a ring of
sphagnum moss, but the term bog actually describes the larger
area of wet organic soil in which these ponds occur. Bogs are
generally formed in depressions where the combination of cool
climates and abundant moisture retard the rate of
decomposition resulting in an accumulation of organic matter.
They are hydrologically open systems, but receive little or no
discharge of water from groundwater aquifers and are,
therefore, dependent on precipitation for moisture. Bogs
produce near normal amounts of surface runoff and may recharge
small amounts of water to regional groundwater systems. The
resulting chemistry produces nutrient poor acid conditions and
less than average productivity. However, low tree productivity
is largely offset by high moss productivity. This causes the
accumulation of peat further restricting water movement and
raising thewater table. This accumulation of water and peat is
self intensifying and can eventually result in expansion and
overlaying of adjacent areas with peat to create blanket
bogs.
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The dominant vegetation is adapted to the
cold, wet, nutrient poor, acidic environment and includes
black spruce, tamarack, Atlantic white cedar, Northern white
cedar, alder, sphagnum moss, sedges and heaths ubiquitous to
bogs such as highbush blueberry, cranberry and leatherleaf.
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Other plants adapted to this environment include
carnivorous plants such as pitcher plant and sundew and
managers should address the possibility of threatened and
endangered species in management plans for these
areas.
Wildlife using bogs include bog lemmings, four?toed
salamander, spruce grouse, massasauga rattlesnake, wood frog;
moose, spotted turtle, water shrew, ribbon snake and
neotropical birds such as the olivesided flycatcher, northern
parula warbler, baybreasted warbler and blackpole
warbler.
There is seldom reason to enter the floating
sphagnum mats surrounding open water portions of bogs and the
best advice is to go around them. Forested bog peatlands tend
to occur in areas such as Minnesota, Michigan and Maine where
the ground freezes during the winter months. Management
activities on the forested areas should be restricted to
periods when the surface is sufficiently frozen to support
harvesting and other equipment and should utilize the Best
Management Practices listed for frozen
conditions.
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Fens The feature that
distinguishes fens from bogs is the fact that fens receive
water from the surrounding watershed in inflowing streams and
groundwater, while bogs receive water primarily from
precipitation. Fens, therefore, reflect the chemistry of the
geological formations through which these waters flow. In
limestone areas the water is high in calcium carbonate
resulting in fens that are typically buffered to a near
neutral pH of 7. However, the level of calcium or magnesium
bicarbonate varies widely in fens. At low levels of
bicarbonate the pH may be closer to pH 4.6 resulting in an
acidfen. At very high levels of bicarbonate, the water may
reach a pH of 9. Thus, there is much variation among fens with
respect to acidity and they often do not have the extreme acid
conditions associated with bogs.
The common features of
both bogs and fens are that they accumulate peat and occur in
similar climatic and physiographic regions. Indeed, they often
occur side by side, one grading into the other. Under the
right conditions, peat can accumulate in low domes that
effectively separate rain water in the dome from calcium rich
groundwater in the underlying fen.
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The
plant community of the fen is more varied than that of a bog
and where heaths are more plentiful in bogs, sedges tend to be
more plentiful in fens. Typical cool, wet climate vegetation
is common with acid loving species occurring as scattered
inclusions on hummocks. The species composition of acid fens
is similar to bogs and includes black spruce, Virginia pine,
tamarack, willow, birch, orchids, leatherleaf and random
sphagnum mats.

As noted previously, many fens are
acidic. However, fens receiving water from limestone or
calcium carbonate geologic sources are much less acidic. These
calcareous fens support a groupof plants that differs somewhat
from the group of plants found in acid fens. The calcareous fens
tend to be dominated by grasses and sedges as well as calcium
loving trees such as northern-white cedar and Atlantic white
cedar instead of the sphagnum moss common to acid
fens.
Some species of wildlife such as bog turtles are
more common in calcareous fens where they use the shrub layer
for aestivation, or summer hibernation, particularly in the
northeast.
Wildlife species groups associated with
acidic fens are similar to those associated with bogs. They
include species such as the bog lemming, fourtoed salamander,
spruce grouse, wood frog, moose, spotted turtle, water shrew
and ribbon snake and neotropical birds such as the northern
waterthrush and palm warbler. Because surface outflows trigger
their damming instinct, beaver will occasionally occupy fens
when more desirable habitat is unavailable.

The
tea colored surface outflow from fens, while not trophy trout water, provides
important habitat for small, newly hatched brook trout. The trout survive because the organic
acids that impart color to the water also tend to congeal
soluble forms of aluminum which could otherwise be toxic to
trout, particularly young trout. Trout in these streams have
been observed to survive spring "acid shock" loading when
trout in nearby clear streams have not
survived.
Europeans refer to peatlands, both bogs and
fens, with the word "mire" which says a lot about operating in
these areas. Machinery can be "mired down" unless operations
are con ducted under frozen conditions. These areas, by virtue
of the conditions necessary for their existence, are rarely
dry, so operating in a dry sea son is all but impossible.
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Reference: U.S.D.A. Forest Service, Functions, Benefits and the Use of
Best Management Practices
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