Environmental Engineering Reference
In-Depth Information
8.5.2 Reversion to earlier successional
stages
1200
1000
The following examples deal with restoration succes-
sion of forest after abandonment. Calcareous grass-
lands that have been overgrown by scrub and later
forest lose the majority of their characteristic plant
species. The 170 characteristic species of central
European calcareous grasslands were classified ac-
cording to life-history traits such as seed longevity,
wind dispersal and dispersal by herbivores. After
removal of the forest, initially a small number re-
established, most likely from the long-living soil seed
bank. Another small number re-established later and
was supposed to originate from elsewhere by wind dis-
persal. Finally, a relatively large number re-established,
most likely dispersed by sheep that connected still-
existing fragments of calcareous grasslands with the
area where the forest was cut (Poschlod
et al
. 1998,
Knevel
et al
. 2003).
A wooded meadow in Sweden was partly overgrown
by forest. Many of the characteristic wooded meadow
species did not occur when compared with an adjac-
ent, still-present wooded meadow. A third previously
wooded meadow overgrown by forest was cut, and after
36 years a great deal of the wooded-meadow species
were present. The cut part and the forest were
connected by grazing livestock. Assessment of the con-
tents of dung revealed that dispersal of the charac-
teristic wooded-meadow species seems unimportant:
the majority of the species found in the dung were
trivial grassland species growing on more-fertile soil,
such as
Poa trivialis
(Mitlacher
et al
. 2002). It is likely
that the animals prefer these species, having a better
forage quality. Hence species of eutrophic habitats have
more chance of being dispersed by herbivores than the
target species of mesotrophic or oligotrophic habitats.
800
600
400
200
0
1973
1977
1981
1985
1989
Year
Fig. 8.3
Dry weights (means
10)
of standing crop in July, from 1973 to 1990, in
N-fertilized (•; 50 kg of N ha
−1
yr
−1
) and unfertilized
(
) control plots that were cut for hay annually in
early August. After van der Woude
et al
. (1994).
±
S.E.M.,
n
=
8.6.1 Growing crops
An experiment was carried out with cultivating barley
in pots with soil from a previously intensively used
arable field or oligotrophic soil. The nitrogen content
of the crop grown on fertile soil was higher than that
of the crop grown on oligotrophic soil. This suggests
that growing crops can help to enhance the process
of nutrient removal after abandonment of intensively
exploited agricultural fields (Marrs 1993).
8.6.2 Adding nutrients and carbon sources
After abandoning intensively exploited agricultural
fields, nitrogen will be depleted first (Marrs 1993). This
will lead to low production and hence little removal
of phosphorus and potassium. Adding only nitrogen
annually in a July cut grassland system on sand
resulted in a higher standing crop than the control
without nitrogen addition. After 17 years the stand-
ing crop of both treatments was similar (Fig. 8.3).
A factorial design with nitrogen, phosphorus and
potassium added individually revealed that potassium
had become depleted to such an extent that it became
8.6 Restoration after reclamation and
intensification
An important point in the restoration of communities
on mesotrophic or oligotrophic soils from intensively
exploited agricultural fields is removal of the surplus
of nutrients, especially nitrogen. Several techniques will
be discussed.
