Environmental Engineering Reference
In-Depth Information
3(99)
2(99)
1.2
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)
4(91)
0.4
2(80)
2(85)
1(99)
1(80)
3(75)
1(75)
0.2
4(94)
3(80)
5(80)
4(85)
2(75)
4(80)
4(75)
0.0
0.0
0.5
1.0
1.5
2.0
2.5
5(75)
Axis 1
Figure 14.4
Pathway of species composition under fi ve different management regimes from 1975 to 1999 in steps of 5
years 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. At the start of the experiment in 1975, species composition was similar in all plots, but
deviated strongly afterwards under the different management regimes. Largest differences occurred between grazing and
burning (at the left side) and grazing (at the right side). Treatments: 1, grazing; 2, haymaking; 3, mulching; 4, burning; 5,
laissez faire. Years are given in parentheses. (Modifi ed from Kahmen
et al
. 2002. Reproduced by permission of Elsevier.)
Similar homogenizing effects were found in a long-
term experiment on calcareous grasslands in Germany
(Kahmen
et al
. 2002). The initial results seemed prom-
ising. However, after 25 years the grass
Brachypodium
pinnatum
became so dominant that the plant commu-
nity resembled that of a species-poor abandoned fi eld
(Fig.14.4; Kahmen
et al
. 2002). In contrast to grass-
land communities, burning is successful for a domi-
nant heathland species. Prescribed burning is used
routinely in the management of upland heaths within
the United Kingdom, where it is successfully applied in
rotation. The aim is to remove the vegetation and allow
the dominant species, common heather (
Calluna vul-
garis
), to regenerate for the stem bases. Usually a 6-12-
year rotation is practised (Gimingham 1992).
because the former induces spatial differences whereas
the latter produces a uniform effect across the whole
fi eld. Where extensive grazing occurs on areas with
plant production exceeding herbivore utilization, veg-
etation compositional and structural patterns are pro-
duced at different scales (Bakker 1989) that cannot be
mimicked by cutting. It was also found in North Ameri-
can tallgrass prairie that grazing by bison can generate
small-scale heterogeneity (Veen
et al
. 2008 ).
Grazing by sheep with low stocking rate is often not
enough to prevent adverse successional change. For
example, in a successional sequence following topsoil
removal, ruderal communities with tall forbs estab-
lished after 3 years in a site with bare soil after topsoil
removal in Germany (Stroh
et al
. 2002 ). Sheep grazing
could not stop this trend. Despite their suppression
by sheep, many ruderal species were maintained and
represented a latent potential for recolonization (Stroh
et al
. 2002 ).
14.4.4
Grazing
After abandonment of old fi elds, the stocking density
adopted is often the one used in mesotrophic or oligo-
trophic communities. Such a grazing regime is more or
less successful in sites that have not been heavily ferti-
lized in the past. Examples show that the development
of mesotrophic communities is possible. Apart from
the lower costs, grazing is often preferred over cutting
14.4.5
Sod cutting
To enhance the restoration of oligotrophic grasslands
and heathlands, sod cutting (up to 10 cm in depth) is
practised. This technique does remove nutrients with
