Environmental Engineering Reference
In-Depth Information
deeply drained adjacent agricultural fields. In eastern
Europe, however, most of the remaining fen meadows
declined in diversity due to abandonment (Ilnicki
2002, Kotowski 2002).
9.3 Consequences of land-use changes
The change from natural mires to semi-natural eco-
systems is depicted in Plate 9.2, showing typical
remnants of natural mire types after large changes in
land use during the past centuries. The time period in
which such land-use changes began may differ greatly
between various parts of Europe, but the outcome is
practically always the same. Important changes in land
use are excavations of mires for fuel, intensification
of agriculture and afforestation (see also Chapter 10).
All these changes affected hydrological conditions,
including the composition of groundwater and the
direction of water flows. Most fens and bogs have
been excavated for fuel and the peat structure of the
remaining peat layers has been irreversibly changed
due to deep drainage in adjacent agricultural fields.
Moreover, the natural fluctuation pattern has been
reversed in many European lowlands: low water
levels in winter and spring and high water levels in
summer. This is done to increase agricultural produc-
tion, but the effects are also noticeable in nature
reserves, causing direct or indirect eutrophication.
Before the introduction of artificial fertilizer,
farmers depended on the natural soil fertility within
a landscape and manure from livestock. Before the
Second World War, almost every bit of landscape was
used for food production in densely populated areas.
This led to large losses of forested areas and natural
mires. Fens were transformed into fen meadows on a
large scale, but they had gained in species richness due
to shallow drainage, resulting in spreading of meadow
species into the fen without a total replacement of the
original mire vegetation. In order to increase the soil
fertility of the fen meadows artificial flooding was
applied on a large scale in north-west Europe (Girel
1994, Hassler
et al.
1995). Extensive networks of
ditches and small dams guided base-rich surface
water to and from the meadows. Soil fertility became
less dependent on landscape processes after the intro-
duction of artificial fertilizer. Many peat areas were
drained deeply for food production. Towards the end
of the 20th century the semi-natural landscape had
almost disappeared in most of Europe. In western
Europe small remnants of mires and species-rich
meadows had been preserved as nature reserves, but
they were usually influenced in a negative way by
9.3.1 Rainwater-fed bogs
Large bog complexes (Plate 9.2a) that have been
excavated commercially are left with a totally bare peat
surface, consisting of remnants of the catotelm, the
firm and decayed peat layer underneath the flexible
and little-decomposed top layer. Hydrological and
ecological conditions are very extreme after strip
cutting. In particular in the summer water tables drop
deep and desiccation can be severe. The bog peat is
very acidic with pH values between 3 and 4. Under
such conditions
Sphagnum
species cannot form a new
vegetation layer. Grasses such as
Molinia caerulea
and
Deschampsia flexuosa
and trees such as
Betula pub-
escens
soon cover the whole area. In western Europe
most of the former bog areas have been reclaimed for
agriculture.
Kettle-hole mires and other small bog systems can
still be found in large numbers in afforested areas
in the Netherlands, northern Germany and Poland.
Remnants of such mires can also be found in agricul-
tural areas, but most often they have been severely
eutrophicated due to inflow of surface water with high
amounts of nutrients. In afforested areas peat cutting
may have occurred and drainage ditches are usually
present to drain the direct surroundings of the mire.
9.3.2 Fens and fen meadows
From a hydrological point of view fens and fen
meadows (Plate 9.2b) are very different systems
(Grootjans & van Diggelen 1995). The distinction
between the two systems, however, cannot easily be
made in the field and many ecologists do not make
a distinction between fens and fen meadows (Wheeler
& Proctor 2000). Fen vegetation can still be found in
numerous ecosystems ranging from small terrestrial-
izing peat pits to spring mires and fen meadows,
and even in calcareous dune slacks on almost bare
mineral soil. Well-preserved fen and fen meadow
complexes are still present in the mountain areas in
