Environmental Engineering Reference
In-Depth Information
gled with phanerogams adapted to very wet and
nutrient-poor conditions. There is a distinct gradient in
hydraulic conductivity in this situation. The underly-
ing, more fi rm and decayed peat layer called
catotelm
can store a large amount of water, thus maintaining
the required wet conditions for the typical bog species.
Excess water fl ows to the periphery of the bog through
the acrotelm. In dry periods, the water level in the
acrotelm drops, and reaches peat layers with much
lower hydraulic conductivities. Water fl ow in these
layers then becomes very low. This regulating feed-
back mechanism prevents a rapid loss of water from
the bog during dry periods (Joosten 1993; Succow &
Joosten 2001 ).
Various types of bog have been distinguished on the
basis of their morphology (Glaser 1992; Glaser
et al
.
1997 ). ' Blanket bogs ' , which cover the undulating
landscape like a blanket, are common in Ireland and
Scotland. 'Plateau bogs' are large bogs with a fl at top
that used to be common in north-western Europe, but
are now - as living mire systems - very rare. Still larger
bog systems occur in Canada (Warner 2005), Russia
(Yurkovskaya 2005), Scandinavia, the Baltic states
and Belarus, as well as in Tierra del Fuego, in south-
ernmost South America (Coronato
et al
. 2006 ).
forming, groundwater-fed mires were also widespread
in north-western Europe, some centuries ago, but at
present only remnants of percolating fens occur there,
which no longer form peat (Succow & Joosten 2001).
In the best-preserved fen systems, such as those in the
Biebrza Valley, Poland, typical fen species co-occur with
a large number of fen meadow species that mark the
transition towards grasslands. Locally peat formation
can occur, but many such fen systems no longer accu-
mulate peat, and in most systems regarded as fens the
peat is actually degrading. One of the largest fen
systems in the world, the Everglades in Florida, United
States, is a large calcareous fen that originally occupied
more than one million hectares (Kadlec 2009). At
present more than half of it has been reclaimed for
agriculture (Richardson 2008). In section 16.4.2 we
will discuss attempts to restore damaged parts of the
Everglades.
16.2.3
Terrestrialization fens
Terrestrialization fens occur almost everywhere where
peat formation can occur, and they are abundant
under natural conditions. In human-dominated envi-
ronments, however, most terrestrialization mires are
artifi cial. In the lowlands of north-western Europe, for
instance, the natural mires (fens and bogs) are long
gone (Plate 16.2a and 16.2c) and secondary terrestri-
alization has taken place in former peat extraction
sites. Most of these newly formed terrestrializing mires
( ' fl oating mats') are small, ranging in size from less
than 1 ha to a few hundred hectares, such as the
Norfolk Broads in England and the Dutch Weerribben
or Wieden (van Wirdum
et al
. 1992 ; van Diggelen
et al
. 1996). In the Mississippi River delta plain in Loui-
siana, United States, there is a very large, fl oating
freshwater system of circa 3000 ha (Sasser & Gos-
selink 1984 ).
Terrestrialization mires are ecologically very diverse.
Eutrophic plant communities, such as reeds, form
fl oating rafts on which in later stages mesotrophic or
even oligotrophic plant communities can develop when
the peat layer is suffi ciently thick to allow the formation
of rainwater lenses (van Wirdum
et al
. 1992 ). When
groundwater supply is insuffi cient or the surface water
contains too many nutrients and sulphates, these ter-
restrializing fens are rapidly overgrown by shrubs and
acidify rather quickly (van Diggelen
et al
. 1996 ).
16.2.2
Percolation fens
These groundwater-fed fens are large peat-forming
gently sloping systems, sometimes semiforested, but
often almost treeless, in which case they are dominated
by sedges and bryophytes (Plate 16.2b). These fens
occur in mountain areas and lowland river valleys, in
situations where the groundwater comes from the sur-
rounding catchment area. As in natural bogs, the
uppermost peat of natural fens is loosely structured
and permits a rather rapid fl ow of water during wet
periods of the year. This type is referred to as percola-
tion mire since, in contrast to other peatland types, a
substantial part of the groundwater fl ow in this case
occurs through the rather permeable peat in the upper
layers of the mire (Succow 1988). Large natural fen
systems of thousands of hectares occur in Canada
(Warner 2005 ), Scandinavia (Lindholm & Heikkil รค
2005), the Baltic States (Pakalne & Kalnina 2005),
Russia (Yurkovskaya 2005), Tierra del Fuego (Coro-
nato
et al
. 2006 ), Tibet (Tsuyuzaki 2006 ) and South
Africa (Grundling & Grobler 2005). Such large peat-
