Environmental Engineering Reference
In-Depth Information
found on ungrazed than on heavily grazed seminatural
marshes with sedimentation fi elds in the Dollard
(Esselink
et al
. 1998). Dense, tall vegetation positively
affects the rate of sedimentation (Leonard
et al
. 1995 ).
Detailed measurements have revealed vertical accre-
tion during winter periods, but shrinkage during the
dry summer period (Erchinger
et al
. 1994 ).
The vertical accretion results in succession from
pioneer communities, via low-salt marsh communities
to high-salt marsh communities. This successional
sequence, derived from aerial photographs, is mirrored
in the spatial zonation from pioneer to low and high
marsh in mainland estuarine marshes in the south-
western Netherlands (de Leeuw
et al
. 1993 ). In con-
trast, long-term permanent plot studies revealed
different successional patterns at the low and high
marsh of the barrier island of Terschelling, the Neth-
erlands, where low-marsh plant communities did not
transform into high-marsh communities (Leendertse
et al
. 1997). Hence, the zonation does not mirror
succession in back barrier salt marshes, but refl ects
the underlying
geomorphology
of the sandy base
elevation (de Leeuw
et al
. 1993 ; Olff
et al
. 1997 ). At
many older ungrazed marshes in north-western
Europe, the tall grass
Elytrigia atherica
is taking over
gradually. With increasing age of the salt marsh,
E.
atherica
is spreading downwards along the elevational
gradient. It appears that the grass can cope with the
salinity stress at lower elevations when more nutrients
are available (Olff
et al
. 1997). Salt marshes are
nitrogen-limited systems (see references in Davy
et al
.
2009). Nitrogen supply can affect the competitive
relations of marsh plants and hence has important
consequences for the abundance and distribution
across marsh landscapes (Levine
et al
. 1998 ). The
lower tidal boundaries of marsh plant distributions are
generally set by physical stress, whereas the upper
boundaries of plants are set by competitive exclusion
(Snow & Vince 1984 ).
The infl uence of fresh water in mainland estuaries,
discharged by rivers from the hinterland, creates a gra-
dient of decreasing salinity inland. The vegetation fea-
tures
Scirpus maritimus
and especially
Phragmites
australis
, as in estuarine marshes of the Wadden Sea
(Esselink 2000) and the Baltic Sea in Europe (Dijkema
1990), and the Atlantic coast in the United States
(Bertness 1999 ).
In tropical regions,
mangrove forests
and
salt marshes
compete for space along a latitudinal gradient. In
north-east Queensland, Australia, mangroves may be
up to 30 m, but in southern Victoria they take the form
of low shrubs about 1 m tall (Thomson
et al
. 2009 ).
Salt marshes can occur where mangrove growth is
inhibited, which often occurs by cutting the trees.
Mangrove cutting results in increased soil salinity and
hence bare intertidal fl ats or herbaceous vegetation
(Costa
et al
. 2009 ).
19.2.2
Plant-animal interactions
The barrier island of Schiermonnikoog, the Nether-
lands, extends eastward, thus featuring a chronose-
quence from east to west. This phenomenon provides
an opportunity to study salt marsh development
over a period of some 200 years. A positive correla-
tion is found between the thickness of the clay layer
and both the nitrogen pool (Olff
et al
. 1997 ) and the
availability of nitrogen for plants (Bakker
et al
. 2005 ).
Hence, the
chronosequence
represents a
productivity
gradient
featuring low-statured plants in early stages
and tall grass in later successional stages with thicker
clay layer, and a decrease in the number of plant
species (Bakker
et al
. 2002b). As a result, the forage
quantity for natural herbivores such as spring-staging
geese and resident hares increases, but the quality,
expressed as the leaf-stem ratio, decreases (van de
Koppel
et al
. 1996). Hence, geese and hares are 'evicted'
by vegetation succession, namely dominance of the
tall forb
Atriplex portulacoides
and the tall grass
Ely-
trigia atherica
. Exclusion experiments revealed that
geese can forage on the salt marsh only when hares
occur and eat the tall plants, hence hare facilitate
for geese, and they can retard the succession with a
few decades (Kuijper & Bakker 2005). Indeed, back
barrier island marshes without hares - such as on
Mellum, Germany - appeared to be completely domi-
nated by tall stands of vegetation (Kuijper & Bakker
2003 ).
Salt marsh ecosystems have long been considered as
being controlled by physical bottom-up factors such as
salinity, tidal inundations and soil nutrient concentra-
tions. The above-mentioned studies with natural her-
bivores stress the importance of consumers and food
web characteristics of salt marshes. Hence, top-down
control is an essential element in understanding salt
marsh ecosystems (Silliman
et al
. 2009 ), including
human exploitation of salt marshes by livestock
grazing. The role of livestock grazing is further dis-
cussed in section 19.3 .
