Environmental Engineering Reference
In-Depth Information
marshes feature undisturbed
geomorphological
con-
ditions and have no history of direct management.
They show a natural drainage system with meander-
ing creeks and levees with higher elevation than the
adjacent depressions and no agricultural exploitation
(Plate 19.1). Grazing can occur with natural grazers
such as geese and hares. Natural or pristine salt
marshes are very rare, and occur in Europe in sandy
back-barrier conditions or in parts of former Wadden
Sea bays along the mainland coast. Two groups of
seminatural
marshes can be considered. Some semi-
natural marshes are similar to natural marshes with
natural draining creeks, but include livestock grazing
or mowing (Plate 19.1). An other group of semina-
tural marshes features artifi cial ditches, sedimenta-
tion fi elds, defences against erosion and often land use.
The sedimentation fi elds are areas surrounded by
200 m × 200 m brushwood groynes that reduce water
velocity, with subsequently higher sedimentation of
suspended material. These marsh types are affected in
their geomorphological conditions by artifi cial drain-
age and/or by measures to enhance livestock grazing
or cutting (Plate 19.2). Seminatural marshes are found
on barrier islands, in foreland clay marshes and in
marshes with sedimentation fi elds and an artifi cial
drainage system (i.e. ditches) (Table 19.1). Character-
istic salt marsh plant species can be present in all three
salt marsh types. However, their abundance in typical
salt marsh communities and their spatial arrangement
in the vegetation structure can be affected by
land use
.
Intensive livestock grazing results in homogeneous
short sward, whereas abandoned salt marshes are
often characterized by uniform tall vegetation. Moder-
ate grazing features most variation in vegetation struc-
ture by combining short and tall canopy.
Understanding the historical context of the develop-
ment of tidal salt marshes along intertidal fl ats is a
prerequisite for any fruitful discussion about the per-
spectives of nature conservation and restoration in
these systems. In this chapter, I focus on concepts,
which makes it easier to understand salt marsh systems
and the possibilities for restoration than by presenting
many examples of restoration from all over the world.
Because of my familiarity with western Europe, many
cases will be from that continent.
19.2 CHARACTERISTICS OF
SALT MARSHES
Abiotic conditions, especially elevation, affect the
duration and frequency of inundation by sea water.
Hence, they affect the
zonation
along the
elevational
gradient
of salt marsh ecosystems. The
succession
of
the communities on estuarine marshes is driven by ver-
tical accretion. However, the succession of the com-
munities on sandy marshes is nutrient-driven (de
Leeuw
et al
. 1993 ; Olff
et al
. 1997 ). However, salt
marsh vegetation is governed not only by bottom-up
physical factors, such as tidal inundations, salinity and
soil nutrient concentations, but also by top-down proc-
esses such as grazing animals.
19.2.1 Abiotic conditions, zonation
and succession
The driving bottom-up control in salt marsh develop-
ment is the tidal amplitude, causing inundation and
subsequent sedimentation of silt. The mean spring-
tidal range in Europe varies from 12.3 m in estuaries to
1.6 m in the Wadden Sea (Allen 2000), to nearly zero
in the Baltic Sea. Transplant experiments in Alaska
have demonstrated that the abiotic conditions drive
plant communities at low elevations and higher inun-
dation frequency and duration, whereas at higher
elevation interspecifi c competition drives plant com-
munities (Snow & Vince 1984). Therefore, grazing may
strongly affect plant communities at higher elevation
by removal of biomass and subsequent spreading of
communities of the low marsh towards higher on the
elevational gradient (Bakker 1985).
The pioneer zone of salt marshes consists of annual
plant species, and they do not trap sediment. The per-
ennial grasses
Spartina anglica
and
Puccinellia maritima
at the low salt marsh catch sediment, whereas
erosion
can take place of unvegetated soil (Langlois
et al
.
Table 19.1
Outline of the different landscape types from
natural to seminatural landscape features and examples for
land use.
Drainage
type
Erosion
protection
Type of
landscape
Land use
Creeks
No groynes
Natural
No
Creeks
No groynes
Seminatural
Grazing/
cutting
Ditches
Groynes
Seminatural
Grazing/
cutting/no
