Environmental Engineering Reference
In-Depth Information
3(99)
1.2
2(99)
Fig. 8.4
Development of species
composition under five different
management regimes from
1975 to 1999 presented in an
ordination diagram of a detrended
correspondence analysis (DCA).
Total number of species is 84.
Eigenvalues: Axis 1, 0.3; Axis
2, 0.1. Treatments: 1, grazing;
2, haymaking; 3, mulching; 4,
burning; 5, laissez faire. Years
are given in parentheses. After
Kahmen
et al
. (2002). Reproduced
by permission of Elsevier.
5(99)
4(99)
1.0
1(85)
0.8
3(94)
5(91)
1(94)
3(91)
0.6
5(94)
1(91)
2(94)
3(85)
5(85)
2(91)
0.4
4(91)
2(80)
2(85)
1(99)
1(80)
3(75)
1(75)
0.2
4(94)
3(80)
4(85)
5(80)
2(75)
4(80)
4(75)
0.0
0.0
0.5
1.0
1.5
2.0
2.5
5(75)
Axis 1
(Schiefer 1981) and France (Chabrerie 2002) seem
promising. However, after 25 years the grass
B. pin-
natum
became so dominant that the plant community
in Germany resembled that of abandonment (Fig. 8.4;
Kahmen
et al
. 2002, Moog
et al
. 2002).
results in patterns on different scales (Bakker 1998)
that cannot be mimicked by cutting. Only when the
starting position is good (not very intensive previous
exploitation), and the soil seed bank harbours target
species, will the establishment of such species be
recorded.
Grazed areas often include a mosaic of eutrophic/
mesotrophic sites to be restored and still-existing dry
grassland and heathland communities on oligotrophic
soils. This is done to replace the sharp artificial
boundaries by more natural boundaries and allow
dispersal of seeds into the area abandoned from
former agricultural exploitation. Unfortunately, her-
bivores may enrich the sites with oligotrophic soil by
depositing their dung, while the seeds they introduce
in this way seem to originate mainly from species in
the sites with the higher nutrient levels (better forage
quality for the herbivores; Bakker 1989, Mitlacher
et al
. 2002, Bokdam 2003).
Spontaneous succession in a former debris-disposal
site with bare soil resulted in ruderal communities
including tall forbs after 3 years. Grazing by sheep could
not stop this trend. Target species of oligotrophic dry
grasslands were not available in the soil seed bank,
and hence did not establish. Inoculation with target
species by the introduction of mown material, raked
material or intact sods from reference sites turned
out to be successful for the establishment of target
species. Despite their suppression by sheep, many
ruderal species maintained and represented a latent
ruderalization potential (Stroh
et al
. 2002).
8.6.5 Grazing
Grazing by livestock is mainly practised for cattle and
sheep. After abandonment of intensively exploited
fields, the stocking density adopted is often that used
in mesotrophic or oligotrophic communities. Such a
grazing regime is more or less successful in areas not
too heavily fertilized in the past. Examples show
that the development of mesotrophic communities is
possible, but it seems that the transformation stops at
a dominance of the grasses
F. rubra
and
A. capillaris
(Bakker 1989). Grazing at low stocking rates implies
differentiation within the fence. Indeed, locally
mesotrophic species establish. In other places, where
the grazers tend to rest and to ruminate, accumula-
tion of dung takes place and stands with tall forbs
develop (Bakker & Grootjans 1991). The higher the pre-
vious fertilizer application, the larger the area with no
grazing at all in large areas within the fence, which
leads to tall forb stands including
Urtica dioica
.
Apart from the lower costs, grazing is often preferred
above cutting because the latter produces a uniform
effect across the whole field. Extensive grazing, when
plant production exceeds utilization by the herbivores,
