Bogs will form where precipitation exceeds
evaporation and drainage is very slow.
Establishment of Sphagnum further impedes drainage. Sphagnum lacks internal
water conducting tissues, so draws water by external capillary action through
spaces formed as its branches hang against its stem. Sphagnum grows in sponge-
like aggregations which hold large volumes of water, such that up to 98% of a living
Sphagnum carpet can be water. It has no roots and grows from the top, new growth
continually shading and killing older parts. The dead tissues collapse and become
compacted into a water-saturated, poorly permeable mass, the catotelm, covered
by the living layer, the acrotelm, up to 40cm thick and very permeable.
As the moss continues to grow, the bog
adds more organic matter to the catotelm,
allowing it to rise above the surrounding land and, in turn, raising the water table
with it. Such raised bogs typically develop over the top of fens, a rheotrophic mire
being replaced, through the action of Sphagnum, by an ombrotrophic mire.
The sequence starts with encroachment
of fen vegetation in a rapidly infilling basin.
Eventually the fen completely dominates the basin. The water table is high, allowing
patches of Sphagnum to establish. These patches expand and coalesce and
Sphagnum expands across the fen. As the Sphagnum continues to grow, it
develops a catotelm, which remains saturated, raising the water table. This allows
the Sphagnum to expand upwards, creating a raised bog of Sphagnum peat on top
of fen peat.
Quaking bog
Succession in kettle hole lakes may proceed
directly to Sphagnum dominated bog.
Floating macrophytes which grow outwards from the bank, such as leatherleaf
(Chamaedaphne calyculata) in northeast North America, are important in this
succession as they provide a substrate upon which Sphagnum and various sedges
can become established. The peat produced from the partial decomposition of
these species is slightly buoyant, creating a mat of peat which floats, albeit mainly
below the surface. Organic debris which erodes from the mat, along with planktonic
and inorganic debris from the pelagic zone, form a distinct layer of debris peat.
Kettle hole bogs contain two horizontal
layers, the debris peat and the mat peat
layer, with remains of leatherleaf in between. The mat peat can be further
subdivided into three vertical zones. The outer edge is a zone of thickening ol the
floating mat, as more organic detritus is deposited, pushing the mat deeper into the
water When it becomes so deep that it comes into contact with the debris peat,
further thickening cannot occur, so addition of new peat at the surface results in
compaction. Closest to the shore is peat at its maximum density; no further
compaction can occur and a zone of equilibrium is produced, in which organic
matter addition is matched by decomposition at the surface.
Peat encroachment will eventually cover
the open water and may overtop the lake
centre, leaving a lens of water trapped within the centre of the fen, creating a
quaking bog (so-called because the lens of water allow: it to move underfoot) or
schwingmoor. When the open water is completely covered, the water table is able
to rise and Sphagnum growth extend upwards, creating a raised bog.
Blanket bogs
Cool, constantly wet climates, in combinatior
with impermeable rocks, allow the
developmen of blanket bogs. Again, Sphagnum is the control ling influence, but
these are not confined tc previously flooded areas, but develop on anc spread over
flat or gently sloping ground. Blanket bogs are features of temperate zones
subjected to an oceanic climate, particularly in northwest Europe, and are typical of
much of the north and west of the British Isles, where they can extend over large
areas. The Flow Country of northern Scotland, for example, is a more or less
continuous expanse covering nearly 4000 km2 (Ratcliffe & Oswald, 1988).
Cyclical succession in Sphagnum bogs
A well established theory associated with
ombrotrophic Sphagnum mires is the
regeneration cycle, a cyclical succession which occurs in the hummock-hollow
complex of these environments. The regeneration cycle hypothesis can be
summarised as follows: as hummocks develop, they rise above the water table until
they are dry enough to support terrestrial species (such as Calluna) which produce
very little peat, so the hummock ceases to rise and may begin to erode. Within
hollows, meanwhile, constant waterlogging ensures deposition of large amounts of
peat, so that the hollows infill. Eventually, the hollows will reach the level of former
hummocks, which become waterlogged and themselves are converted into hollows;
the former hollows, which continue to grow, become hummocks. In due course,
these new hummocks will dry out and cease to grow, while the new hollows will infill
with peat. By constant repetition of this cycle, a single point will, alternately, be
occupied by hummock and hollow. This hypothesis is intuitively very appealing, as it
describes an apparently logical progression of bog development, and has been
widely presented as an established truth. Unfortunately, as Backeus (1990) points
out, there is no evidence for its occurrence and, in all probability, the regeneration
cycle does not occur.
poundment, Hog Lake, raised water levels
since embankments were constructed in
1973 have led to domination by Typha X glauca, which forms floating mats
consisting almost exclusively of this single species. Where these mats form, they
insulate the water beneath from summer warming, reducing decomposition rates
and nutrient release even further. This, is turn, leads to lower Typha vigour and
invasion of mats by Sphagnum. It is likely that, in due course, Sphagnum, which
acidifies the water and further suppresses Typha, will dominate (Krusi & Wein,
1988).
The raised bog illustrated in Figure 7.10
may be an end point in itself, but, if
precipitation is very high and evaporation very low, then expansion into a blanket
bog extending over the surrounding land is a possible outcome. Blanket bogs
expanded over much of the British Isles and Norway around 5000 Bl>, at a time of
climatic deterioration into cooler, wetter conditions. This, in itself, may have been
enough to trigger their development, but the role of human activity in this bog
expansion is also implicated (Moore, 1993). Areas that are climatically susceptible
to blanket bog expansion may remain dry if there is adequate tree cover; removing
trees reduces transpiration losses and interception of rainfall, leading to increased
groundwater levels and waterlogging, which favours development of Sphagnum.
The expansion of blanket bogs in northwest Europe, most of which cover formerly
forested areas, was probably enhanced by human activity, cutting and burning tree
cover, particularly in areas marginal for blanket mire formation but where trees were
under climatic stress.
Attempts at conservation of wetlands can
activate successional processes. At the
head of the Bay of Fundy in New Brunswick is a series of wetlands which have
been contained within embankments, stabilising water levels, in order to create
open water habitats for waterfowl. Whereas these wetlands would formerly have
dried out for short periods during the summer, allowing decomposition of organic
matter and remineralisation of nutrients, this can no longer occur where
embankments impede drainage.
