Environmental Engineering Reference
In-Depth Information
bogs are common in Ireland and England. Plateau bogs,
large lowland bogs with a flat top, used to be common
in north-west Europe, but are now almost unknown.
Kettle-hole mires are relatively small but usually very
deep bogs, since they grow in Pleistocene areas
where, after the retreat of the glaciers, ice blocks have
melted, forming small lakes, which later terrestrialized
as bogs. Kettle-hole mires are common in eastern
Germany and Poland (Dierssen & Dierssen 2001,
Succow & Joosten 2001). In northern Germany and
in the Netherlands, with much precipitation during the
summer, even smaller bogs of
c
.1-2 m deep occur in
wet heathlands (Tüxen 1983, Grootjans
et al.
2003).
These bogs are usually found in former riverbeds, which
have been cut off from groundwater discharge.
9.2 Description of natural mire systems
Mire systems are by definition peat-forming systems.
They accumulate organic matter, but mineral sediments
- such as sand, clay or chalk - can also be deposited
in a mire system. Mire systems are basically natural
systems that can develop without human interference.
Present-day mire systems, however, are almost all
influenced by humans in one way or another, particu-
larly in densely populated areas. The description of
the following mire types is, therefore, based mainly
on stratigraphical and palynological studies, describ-
ing natural mire systems without human influences
(Kulczynski 1949, Brinson 1993, Grünig 1994, Dierssen
& Dierssen 2001). Succow (1988) integrated vegetation
composition of mires based on geomorphological and
hydrological features with palynological records and
nutrient status of the mire systems. He distinguished
c
.10 different mire systems. In the present study we
will only distinguish between the following five mire
types: rainwater-fed bogs, groundwater-fed fens, ter-
restrialization mires, spring mires and floodplain mires.
9.2.2 Groundwater-fed fens
Groundwater-fed fens are usually large peat-forming
systems almost without trees and dominated by
bryophytes (Plate 9.1b). Usually these fens are gently
sloping, in particular in mountain areas, but they have
also been known to occur in almost flat lowland river
valleys, where the groundwater was supplied by sur-
rounding hills. As in natural bogs, natural fens have
loosely structured topsoils that permit a rather fast
flow of water during wet periods. Such mires are, there-
fore, referred to as percolating mires (Succow 1988).
Large natural fen systems of hundreds of hectares
have been described from Canada (Glaser
et al.
1990),
Scandinavia (Dierssen 1996, Vasander 1996) and
Russia (Sirin & Minaeva 2001). Such large peat-
forming groundwater-fed mires were once wide-
spread in north-west Europe too, but they do not
occur any more under natural conditions in Europe
(Dierssen & Dierssen 2001, Succow & Joosten 2001).
In the best-preserved fen systems the typical fen
species co-occur with a large number of fen meadow
species. Locally some peat forming can occur, but many
of such fen systems do not accumulate peat any more
and in most systems that are regarded as fens the peat
is actually degrading (Kotowski 2002).
9.2.1 Rainwater-fed bogs
Bogs are fed by rainwater although some peripheral
parts of the bog system can be influenced by water
that has been in contact with the mineral soil
(Plate 9.1a). Normally they consist of a peat body, which
has grown above the immediate surroundings. This
rather firm and decayed peat layer (catotelm) is
covered by a loosely structured layer (acrotelm) of mainly
living mosses (
Sphagnum
species), intermingled with
phanerogams which have adapted to very wet and
nutrient-poor conditions. The acrotelm, consisting of
mainly living plants, maintains the required wet con-
ditions for the typical bog species. During very wet
periods the acrotelm expands and the resistance to
water flow becomes very low. Excess water can flow
through the acrotelm to the periphery of the bog. In
dry periods the acrotelm shrinks and becomes more
firm, thus increasing the resistance to water flow. This
feedback mechanism prevents a rapid loss of water from
the bog during dry periods (Succow & Joosten 2001).
Various types of bog have been distinguished on
the basis of their geomorphology (Vasander 1996,
Dierssen & Dierssen 2001). Shallow-layered blanket
9.2.3 Terrestrialization mires
Terrestrialization mires (see Plates 9.1c and 9.2a) are
peat-forming systems, which in western Europe are
