impacts – the timing of grazing, and the stocking rate. High
stocking rates can result in a very short turf with low vascular plant diversity, but less intense grazing can result in
taller vegetation and greater diversity. In Britain, the most
diverse plant communities (Adam, 1981) are found at the
upper levels of sites with moderate grazing pressure.
In northern Europe, species such as Atriplex
portulacoides and Limonium spp. are grazing sensitive,
and on heavily grazed marshes are either absent or
restricted to microsites inaccessible to livestock, such as
creek sides. Grazing results in permanent changes to vegetation. If livestock is removed, the vegetation does not
revert to the community that would have been expected
on marshes which had never been grazed. In general,
when livestock grazing ceases the trend is for the development of tall, rank very species poor grassland communities
(Bakker, 1989; Adam, 1990; Lambert, 2000).
The effects of livestock grazing are not only reflected in
structure and composition of vegetation, but also by the
fauna, in utilization by birds, with herbivorous waterfowl
favoring shorter grass swards over taller vegetation, and
the composition of invertebrate communities. Spider and
beetle communities have been shown to be affected by
grazing; abundance of some spaces is increased by grazing, but under intense grazing, spider species richness
declines (Petillon et al., 2007).
Saltmarshes on the east coast of the United States were
heavily exploited as grazing land and haymaking from
early colonial times until relatively recently (Gedan
et al., 2009). However, the legacy of centuries of human
modification is not as obvious as it is in Europe. Why this
is so is not clear, but perhaps the structural and floristic
simplicity of Spartina marshes limited the changes
resulting both from agricultural use and its subsequent
decline. Ewanchuk and Bertness (2004) suggest that forb
panne habitat in upper marshes in New England was
reduced in extent by the effects of ditching which for three
centuries was used to facilitate livestock grazing,
haymaking, and mosquito control.
With the decline in direct exploitation of saltmarshes,
they are increasingly valued for their ecosystems services,
conservation, and for aesthetic experiences.
Threats
Despite increasing recognition of the values of saltmarsh,
the ecosystem is under increasing threat worldwide. The
range of threats facing saltmarshes has been identified in
a number of publications (Kennish, 2001; Adam, 2002;
Adam et al., 2008; Silliman et al., 2009b) and need not
be recounted here (see Anthropogenic Impacts).
The majority of the world’s human population is now
urban, and many of the world’s largest cities are on or
close to estuaries. Even if environmental regulations are
in place, there is a continuing increase in environmental
degradation caused by accidental spills of chemicals
in the catchment, the accumulation of pollutants
from industrial discharges and domestic sources,
eutrophication from general catchment runoff,
stormwater inputs, and greater pressures for recreational
use. As the world population grows and becomes more
affluent, there is increasing need for greater development
of ports and construction of airports, oil and gas terminals, and other industrial facilities. Ports, of necessity,
must have waterfront access, while for many other forms
of development, availability of level land created by
marsh infill close to cities is seen as a major economic
benefit. Construction of major airports on fill is viewed
as a means of securing locations where noise impacts
on residential suburbs can be minimized. The importance
of saltmarsh habitat loss from major infrastructure development is recognized, but the economic and social benefits (at least in the short-term) of development prevail
over environmental concerns.
Development pressures are localized in their impacts,
even if cumulatively the effects are large, but the growing
human population, increasing living standards, and development will result in continuing release of greenhouse
gases and consequent climate disruption. One of the consequences of global warming will be an increase in the
volume of the oceans due to thermal expansion and, in
the longer term, the melting of ice caps and glaciers.
A potential consequence of sea-level rise will be “drowning” of saltmarsh and “coastal squeeze,” where natural
topography or artificial constraints prevent compensation
for loss of the seaward parts of marshes by retreat landward. On a site-by-site basis, what is important will be relative change in sea level; the eustatic rise may be
compensated for by sedimentation or isostatic or tectonic
change in land level. In some circumstances, the increase
in atmospheric CO 2 may lead to increased plant production and greater incorporation of organic material into
saltmarsh soils (Langley et al., 2009) (see Sea-Level
Change and Coastal Wetlands).
The role of saltmarsh in sequestering carbon in soil is of
global significance (Chmura et al., 2003, Chmura, 2009).
The importance of “blue carbon” in ameliorating the
effects of greenhouse gas emissions is likely to be of
increasing importance in the future.
Increased temperature may affect plant distribution
and evapotranspiration and hence soil salinity. Changes
in other aspects of climate such as rainfall patterns or
the incidence of major storms will also have impacts on
saltmarshes, some local and some more widespread.
The full range of climate change impacts is difficult to
predict except in general terms (Ross and Adam, 2013),
but it is clear that there will be changes in both the physical and biotic components (Semeniuk, 2013). Increase
in CO 2 will affect the relative competitiveness of
plants, with C 3 species likely to have an advantage over
C 4 (Mayor and Hicks, 2009) species. Hence, changes
in the floristic composition of saltmarshes in temperate
and subtropical marshes can be anticipated, with
concomitant changes in fauna. Spartina spp. are currently the most widespread C 4 species on saltmarshes
(see Climate Change).
SALTMARSHES
529
stocking rates can result in a very short turf with low vascular plant diversity, but less intense grazing can result in
taller vegetation and greater diversity. In Britain, the most
diverse plant communities (Adam, 1981) are found at the
upper levels of sites with moderate grazing pressure.
In northern Europe, species such as Atriplex
portulacoides and Limonium spp. are grazing sensitive,
and on heavily grazed marshes are either absent or
restricted to microsites inaccessible to livestock, such as
creek sides. Grazing results in permanent changes to vegetation. If livestock is removed, the vegetation does not
revert to the community that would have been expected
on marshes which had never been grazed. In general,
when livestock grazing ceases the trend is for the development of tall, rank very species poor grassland communities
(Bakker, 1989; Adam, 1990; Lambert, 2000).
The effects of livestock grazing are not only reflected in
structure and composition of vegetation, but also by the
fauna, in utilization by birds, with herbivorous waterfowl
favoring shorter grass swards over taller vegetation, and
the composition of invertebrate communities. Spider and
beetle communities have been shown to be affected by
grazing; abundance of some spaces is increased by grazing, but under intense grazing, spider species richness
declines (Petillon et al., 2007).
Saltmarshes on the east coast of the United States were
heavily exploited as grazing land and haymaking from
early colonial times until relatively recently (Gedan
et al., 2009). However, the legacy of centuries of human
modification is not as obvious as it is in Europe. Why this
is so is not clear, but perhaps the structural and floristic
simplicity of Spartina marshes limited the changes
resulting both from agricultural use and its subsequent
decline. Ewanchuk and Bertness (2004) suggest that forb
panne habitat in upper marshes in New England was
reduced in extent by the effects of ditching which for three
centuries was used to facilitate livestock grazing,
haymaking, and mosquito control.
With the decline in direct exploitation of saltmarshes,
they are increasingly valued for their ecosystems services,
conservation, and for aesthetic experiences.
Threats
Despite increasing recognition of the values of saltmarsh,
the ecosystem is under increasing threat worldwide. The
range of threats facing saltmarshes has been identified in
a number of publications (Kennish, 2001; Adam, 2002;
Adam et al., 2008; Silliman et al., 2009b) and need not
be recounted here (see Anthropogenic Impacts).
The majority of the world’s human population is now
urban, and many of the world’s largest cities are on or
close to estuaries. Even if environmental regulations are
in place, there is a continuing increase in environmental
degradation caused by accidental spills of chemicals
in the catchment, the accumulation of pollutants
from industrial discharges and domestic sources,
eutrophication from general catchment runoff,
stormwater inputs, and greater pressures for recreational
use. As the world population grows and becomes more
affluent, there is increasing need for greater development
of ports and construction of airports, oil and gas terminals, and other industrial facilities. Ports, of necessity,
must have waterfront access, while for many other forms
of development, availability of level land created by
marsh infill close to cities is seen as a major economic
benefit. Construction of major airports on fill is viewed
as a means of securing locations where noise impacts
on residential suburbs can be minimized. The importance
of saltmarsh habitat loss from major infrastructure development is recognized, but the economic and social benefits (at least in the short-term) of development prevail
over environmental concerns.
Development pressures are localized in their impacts,
even if cumulatively the effects are large, but the growing
human population, increasing living standards, and development will result in continuing release of greenhouse
gases and consequent climate disruption. One of the consequences of global warming will be an increase in the
volume of the oceans due to thermal expansion and, in
the longer term, the melting of ice caps and glaciers.
A potential consequence of sea-level rise will be “drowning” of saltmarsh and “coastal squeeze,” where natural
topography or artificial constraints prevent compensation
for loss of the seaward parts of marshes by retreat landward. On a site-by-site basis, what is important will be relative change in sea level; the eustatic rise may be
compensated for by sedimentation or isostatic or tectonic
change in land level. In some circumstances, the increase
in atmospheric CO 2 may lead to increased plant production and greater incorporation of organic material into
saltmarsh soils (Langley et al., 2009) (see Sea-Level
Change and Coastal Wetlands).
The role of saltmarsh in sequestering carbon in soil is of
global significance (Chmura et al., 2003, Chmura, 2009).
The importance of “blue carbon” in ameliorating the
effects of greenhouse gas emissions is likely to be of
increasing importance in the future.
Increased temperature may affect plant distribution
and evapotranspiration and hence soil salinity. Changes
in other aspects of climate such as rainfall patterns or
the incidence of major storms will also have impacts on
saltmarshes, some local and some more widespread.
The full range of climate change impacts is difficult to
predict except in general terms (Ross and Adam, 2013),
but it is clear that there will be changes in both the physical and biotic components (Semeniuk, 2013). Increase
in CO 2 will affect the relative competitiveness of
plants, with C 3 species likely to have an advantage over
C 4 (Mayor and Hicks, 2009) species. Hence, changes
in the floristic composition of saltmarshes in temperate
and subtropical marshes can be anticipated, with
concomitant changes in fauna. Spartina spp. are currently the most widespread C 4 species on saltmarshes
(see Climate Change).
SALTMARSHES
529