Static zonations are widespread; for example, on rocky
intertidal shores or as the bands of vegetation related to
altitude on mountains. The position of boundaries
between species, or between communities, is determined
by species’ responses to environmental factors and by
interactions between species. The patterns could be
redrawn by climate change, major tectonic events, or the
invasion of introduced species but, in the absence of these
external forces, will remain stable. There are also many
well-documented examples of zonations which reflect
succession, for example, the plant communities on
moraines deposited by retreating glaciers.
Are the zonations on saltmarshes expressions of succession? The zonation may continue beyond the tidal
limit, for example, the zones of swamp forest
(Casuarina, Melaleuca, and Eucalyptus) found inland of
saltmarsh in eastern Australia (Pidgeon, 1940). While
Pidgeon (1940) suggested that this zonation reflected succession, there is little evidence for progression from
saltmarsh to nontidal communities. In many parts of the
world, the most seaward saltmarsh communities today
are dominated by introduced species of Spartina. The
zonation of communities above the Spartina zone reflects
succession from the original (pre-Spartina) pioneer, but, if
the marshes continue to prograde and succession from
Spartina occurs, it is unlikely that there will be
a recapitulation of historic succession, rather a new range
of communities may develop (Adam, 1990; Davy, 2000).
During the course of marsh development, management
regimes may change. The introduction or the removal of
livestock, or changes in the abundance of native grazing
animals, can change the occurrence and abundance of
plant species, so that, even if zone boundaries remain at
the same elevation, the species composition within zones
may change, so the communities now occurring are not
those present at the same elevations earlier in the marshes’
development.
In northern temperate regions at the end of the last glaciations, the newly exposed land surface would have been
prone to erosion, but sea level was much lower than at present so deposition would have occurred at locations now
beneath the sea. When sea level reached its present position about 6000 years ago, the landscape was well vegetated and rates of erosion would have declined.
Clearance of forests and the development of agriculture
would have initiated a new phase of increased erosion.
In Europe, this would have occurred more than 1,000
years ago, but in North America, major clearing following
European colonization was only a few centuries ago
(Kirwan et al., 2011). In North America, the increased sediment input into estuaries stimulated a phase of saltmarsh
growth (Kirwan et al., 2011). Recognition of the damage
caused by erosion in catchments and the initiation of soil
conservation measures has reduced sediment input into
rivers. Construction of dams and other water management
works has reduced sediment transport into estuaries
throughout the world (Walling, 2006; Walling, 2008).
A consequence of this is that in a number of major estuary
systems (e.g., Mississippi, Nile, Yangtze, and Venice
lagoon), saltmarshes are now suffering “sediment starvation,” compromising the ability of saltmarshes to respond
to disturbance or to future rises in sea level. Mudd (2011)
has suggested release of pulses of sediment into estuaries
to preserve marshes.
Succession on saltmarshes
The development of saltmarshes is affected by a range of
physical processes and interactions between these processes and the biota, both flora and fauna (Allen, 2000;
Reed, 2000). The position of saltmarsh relates to that of
sea level, which over geological time has varied considerably. Following the last glacial maximum, sea level rose to
reach approximately its present position 6000 years ago.
Since then, there have been considerable changes in the
occurrence and extent of intertidal marshes. There is
ample empirical evidence that saltmarshes can develop,
or erode away, in short periods (Oliver, 1906; Oliver,
1907; Packham and Liddle, 1970; Pringle, 1995; Adam,
2000; Davy, 2000). As a marsh develops, it can both prograde (extend farther seaward) and accrete (increase in
surface elevation). The two processes frequently
co-occur, but accretion can be maintained, or even
increased, if, when, a marsh front is eroding, sediment
released by erosion is deposited on the remaining marsh.
Low- and mid-marsh zones can develop over short
periods – decades (Adam, 2000) or one or two centuries
(Pethick, 1980; Pethick, 1981). Upper marshes may be
stable for very much longer – 2,000 + years (Pethick,
1980; Pethick, 1981) in East Anglia, United Kingdom,
and 4,000 + years in northeastern United States
(Redfield, 1972). Upper marsh zones are frequently not
homogeneous, but are complex patterns of species and
communities reflecting the operation of a range of ecological factors rather than simple succession.
Accretion requires an increase in surface height, generally as a result of accumulation of sediment, either
minerogenic (allochthonous) – sand, silt, or clay carried
in by the tide – or autochthonous accumulation of organic
material (from plant roots or incorporated stems and
leaves) or varying combinations of the two. Marsh growth
commences with the establishment of pioneer plants,
which promotes sedimentation. In the earlier stages of
marsh development, sediment accumulation is not necessarily continuous in space or time. Sediment deposited
may be eroded. However, as the density of vegetation
increases, more sediment is retained, and the marsh surface rises. With increasing elevation, the rate of accretion
will decline as the number of flooding tides decreases
and the flooding water will have already passed through
the vegetation of the lower marsh where sediment will
have been deposited. The rate of allochthonous sedimentation would be expected to fall close to zero at the tidal
limit. Studies in eastern England (The Wash – Kestner,
1975; North Norfolk – Pethick, 1981) showed that, while
accretion in the lower marsh declined as elevation
518
SALTMARSHES
intertidal shores or as the bands of vegetation related to
altitude on mountains. The position of boundaries
between species, or between communities, is determined
by species’ responses to environmental factors and by
interactions between species. The patterns could be
redrawn by climate change, major tectonic events, or the
invasion of introduced species but, in the absence of these
external forces, will remain stable. There are also many
well-documented examples of zonations which reflect
succession, for example, the plant communities on
moraines deposited by retreating glaciers.
Are the zonations on saltmarshes expressions of succession? The zonation may continue beyond the tidal
limit, for example, the zones of swamp forest
(Casuarina, Melaleuca, and Eucalyptus) found inland of
saltmarsh in eastern Australia (Pidgeon, 1940). While
Pidgeon (1940) suggested that this zonation reflected succession, there is little evidence for progression from
saltmarsh to nontidal communities. In many parts of the
world, the most seaward saltmarsh communities today
are dominated by introduced species of Spartina. The
zonation of communities above the Spartina zone reflects
succession from the original (pre-Spartina) pioneer, but, if
the marshes continue to prograde and succession from
Spartina occurs, it is unlikely that there will be
a recapitulation of historic succession, rather a new range
of communities may develop (Adam, 1990; Davy, 2000).
During the course of marsh development, management
regimes may change. The introduction or the removal of
livestock, or changes in the abundance of native grazing
animals, can change the occurrence and abundance of
plant species, so that, even if zone boundaries remain at
the same elevation, the species composition within zones
may change, so the communities now occurring are not
those present at the same elevations earlier in the marshes’
development.
In northern temperate regions at the end of the last glaciations, the newly exposed land surface would have been
prone to erosion, but sea level was much lower than at present so deposition would have occurred at locations now
beneath the sea. When sea level reached its present position about 6000 years ago, the landscape was well vegetated and rates of erosion would have declined.
Clearance of forests and the development of agriculture
would have initiated a new phase of increased erosion.
In Europe, this would have occurred more than 1,000
years ago, but in North America, major clearing following
European colonization was only a few centuries ago
(Kirwan et al., 2011). In North America, the increased sediment input into estuaries stimulated a phase of saltmarsh
growth (Kirwan et al., 2011). Recognition of the damage
caused by erosion in catchments and the initiation of soil
conservation measures has reduced sediment input into
rivers. Construction of dams and other water management
works has reduced sediment transport into estuaries
throughout the world (Walling, 2006; Walling, 2008).
A consequence of this is that in a number of major estuary
systems (e.g., Mississippi, Nile, Yangtze, and Venice
lagoon), saltmarshes are now suffering “sediment starvation,” compromising the ability of saltmarshes to respond
to disturbance or to future rises in sea level. Mudd (2011)
has suggested release of pulses of sediment into estuaries
to preserve marshes.
Succession on saltmarshes
The development of saltmarshes is affected by a range of
physical processes and interactions between these processes and the biota, both flora and fauna (Allen, 2000;
Reed, 2000). The position of saltmarsh relates to that of
sea level, which over geological time has varied considerably. Following the last glacial maximum, sea level rose to
reach approximately its present position 6000 years ago.
Since then, there have been considerable changes in the
occurrence and extent of intertidal marshes. There is
ample empirical evidence that saltmarshes can develop,
or erode away, in short periods (Oliver, 1906; Oliver,
1907; Packham and Liddle, 1970; Pringle, 1995; Adam,
2000; Davy, 2000). As a marsh develops, it can both prograde (extend farther seaward) and accrete (increase in
surface elevation). The two processes frequently
co-occur, but accretion can be maintained, or even
increased, if, when, a marsh front is eroding, sediment
released by erosion is deposited on the remaining marsh.
Low- and mid-marsh zones can develop over short
periods – decades (Adam, 2000) or one or two centuries
(Pethick, 1980; Pethick, 1981). Upper marshes may be
stable for very much longer – 2,000 + years (Pethick,
1980; Pethick, 1981) in East Anglia, United Kingdom,
and 4,000 + years in northeastern United States
(Redfield, 1972). Upper marsh zones are frequently not
homogeneous, but are complex patterns of species and
communities reflecting the operation of a range of ecological factors rather than simple succession.
Accretion requires an increase in surface height, generally as a result of accumulation of sediment, either
minerogenic (allochthonous) – sand, silt, or clay carried
in by the tide – or autochthonous accumulation of organic
material (from plant roots or incorporated stems and
leaves) or varying combinations of the two. Marsh growth
commences with the establishment of pioneer plants,
which promotes sedimentation. In the earlier stages of
marsh development, sediment accumulation is not necessarily continuous in space or time. Sediment deposited
may be eroded. However, as the density of vegetation
increases, more sediment is retained, and the marsh surface rises. With increasing elevation, the rate of accretion
will decline as the number of flooding tides decreases
and the flooding water will have already passed through
the vegetation of the lower marsh where sediment will
have been deposited. The rate of allochthonous sedimentation would be expected to fall close to zero at the tidal
limit. Studies in eastern England (The Wash – Kestner,
1975; North Norfolk – Pethick, 1981) showed that, while
accretion in the lower marsh declined as elevation
518
SALTMARSHES