Restoration of Salt Marshes - Restoration Ecology: The New Frontier - page 262

of seeds on mangroves into more elevated zones, and
mangrove forests will migrate landward at the expense
of salt marshes (Costa et al . 2009 ).
In future, global temperatures and CO 2 levels will
also rise, just as sea levels will. In Europe, temperate
and northern salt marshes might change through
changing competition between plant species. Interspe-
cifi c competition experiments revealed that Puccinellia
maritima responds more positively to combined
increase of temperature and CO 2 than Spartina anglica .
Although both species will extend their range north-
ward from 57°N, Puccinellia will outcompete Spartina .
It is also suggested that Puccinellia under the new
regime of both higher temperatures and CO 2 enrich-
ment will extend its lower limits further downshore
in existing marshes at latitudes below 57°N (Gray &
Mogg 2001 ). Note that the broad - leaved Spartina
anglica was introduced to trap more sediment than the
native species (Davy et al . 2009 ). Mangroves are
restricted to (sub)tropical regions because their leaves
cannot survive frost. Increasing temperatures may
result in spreading of mangrove forests at the expense
of salt marshes towards higher latitude (Saintilan &
Willems 1999 ).
It is not clear how to deal with the trade-offs between
vertical accretion and biodiversity issues. Highest verti-
cal accretion can be accomplished on salt marshes
with tall and dense canopy, hence without livestock
grazing. However, the diversity of plant and animal
communities including characteristic halobiontic
organisms is better preserved or restored in grazed
marshes with lower vertical accretion than in aban-
doned salt marshes. Highest diversity will be achieved
with stocking densities that create patterns of short
grazing lawns alternating with patches of tall vegeta-
tion. Particularly long-term ungrazed marshes have
low diversity of plant and animal communities. Short-
term abandonment may result in fl owering plants
attracting pollinating insects, breeding birds foraging
on them, and plants setting seed and attracting grani-
vorous birds.

There are more general trade-offs between functions
of salt marshes, and hence targets for restoration.
Long-term monitoring on different spatial scales in res-
toration sites ( Long-Term Ecological Research sites ) of
changes in geomorphology, hydrology, vertical accre-
tion, plants, fi sh, birds, mammals and invertebrates, in
a foodweb context, will be helpful as decision tools for
future projects. An important issue is to learn at what
time scale the different elements of the ecosytem can
be restored.
In parts of the world with intensive grazing such as
South America and Australasia, experiments with
removal of cattle are needed to assess their impact on
salt marshes. In north-western Europe, abandonment
of livestock grazing and mowing of salt marshes has
already taken place with negative results for diversity
of characteristic plant and animal species. Recovery of
mono-species stands (resulting from abandonment
from agricultural exploitation) is successful after
reintroducing livestock grazing. Experiments are
needed with respect to stocking density and rotational
grazing for the benefi t of various groups of organisms
in their food web context such as plants, invertebrates
and breeding and migrating birds. In general, the sci-
entifi c basis for management based on experiments is
largely missing.
Summarizing, restoration of salt marshes, and
hence their functioning with respect to sediment
trapping and coastal defence, is possible after de-
embankment of (summer) polders, but to a limited
extent after the increase of tidal amplitude through
sluices. Natural drainage systems are hard to restore in
salt marshes with intensive ditching. The area of salt
marshes may decrease by the spread of mangrove
forests as a result of future increase of temperature.
The effects of sea level rise depend on the local condi-
tions such as tidal range, suspended sediment concen-
trations and density of the vegetation. The composition
of the community and the structure of the vegetation
depend on both the elevation of the salt marsh and its
management, especially livestock grazing.