Environmental Engineering Reference
In-Depth Information
rely on the soil seed bank for restoration (Figure 14.3)
of grasslands and heathlands (Bossuyt & Hermy 2003)
and calcareous grasslands (Fagan
et al
. 2010 ).
Plant species composition may not change during the
fi rst years of woody plant encroachment (abandon-
ment) or of establisment of dominant grasses (eutrophi-
cation). As a result of decreasing light availability at soil
level, fewer species will fl ower, set seed and germinate.
This implies a depletion of the soil seed bank, especially
of species with a transient (<1 year) or short-term per-
sistent (2-5 years) seed bank (Thompson
et al
. 1997 ).
After encroachment had taken place over several
decades on the alvar of Öland, Sweden, both the species
richness of the soil seed bank and the established vegeta-
tion decreased dramatically (Bakker
et al
. 1996a ).
ered by restoration practitioners. It is necessary to fi rst
reduce the amount of nutrients in the soil to levels
appropriate for the re-establishment of communities
on nutrient-poor soils. The next step for restoration is
the re-establishment of species (Bakker
et al
. 1996b ).
Once species have disappeared, the manager or practi-
tioner may hope that target species are still present in
the soil seed bank, and if not, they must be encouraged
to disperse from nearby source sites and meet the
soil organisms to build the target community. Active
reintroduction of species is a relatively new element in
the restoration of dry grasslands and heathlands, and
will be discussed in the last section.
14.4 RESTORATION AFTER
INTENSIFICATION OR ABANDONMENT
14.3.4
Interaction between organisms
An important issue in the restoration of communities
on mesotrophic or oligotrophic soils in old fi elds that
were formerly exploited very intensively is the removal
of surplus nutrients, especially nitrogen and phospho-
rus, or the reduction in their availability in the soil.
Several techniques such as growing crops, mowing,
burning, grazing, sod cutting (< 10 cm) and topsoil
removal (>10 cm) can be considered. The target com-
munities of dry grasslands, dune grasslands and
heathlands need management to prevent succession
towards a community in which just a few herbaceous
species dominate that then become shrublands, wood-
lands or forests. This also applies to sites where sod
cutting and topsoil removal have occurred.
Interactions between plants and soil organisms is an
important area for intervention when seeking to
restoring plant communities on nutrient-poor soil.
Even when restoration has resulted in nutrient-poor
soil conditions and seeds of nutrient-poor soil condi-
tions are available, re-establishment of certain species
can be hampered (van der Heijden
et al
. 1998 ). The
problem may be found in the soil biotic community.
However, soil organisms disperse very slowly, and
hence their absence can be a constraint for restoration
(Frouz
et al
. 2009) in a similar way to seeds. In a micro-
cosm experiment (de Deyn
et al
. 2003 ), soil herbivores
such as nematodes foraged preferentially on grass
species such as those that are dominant in early
eutrophic successional stages of restoration projects.
Once the nematodes have reduced the early succes-
sional species, later successional target species in res-
toration of species-rich mesotrophic grasslands can
more easily establish.
Removal of nutrients by taking away soil does reduce
biomass, and also removes numbers of bacteria, fungal
biomass and nematode abundance. On initially bare
soil after, for example, topsoil removal, however, the
experimental introduction of soil organisms of the late
stage after topsoil removal did not facilitate the estab-
lishment of later successional plant species. Appar-
ently, each successional stage of the plant community
is related to a characteristic community of soil organ-
isms (Kardol
et al
. 2008 ).
All these constraints and complexities have different
consequences and ramifi cations that should be consid-
14.4.1
Growing crops
When cultivating barley in pots with soil from an old
fi eld that was formerly intensively used, or from oligo-
trophic soil, the nitrogen content of the crop grown on
fertile soil was higher than that of the crop grown on
oligotrophic soil (Marrs 1993). This suggests that
growing crops can help to enhance the process of
nutrient removal under fi eld conditions. Marrs
et al
.
(1998) started an experiment to remove especially
phosphorus to create the right soil conditions for the
re-establishement of heathland in a large, fi eld - scale
restoration project in Minsmere nature reserve in
Suffolk, United Kingdom. They added only nitrogen (as
ammonium sulphate) and potassium as inorganic fer-
tilizers, and grew crop plants (mainly cereals) on sandy
