Environmental Engineering Reference
In-Depth Information
sluices. From 1990 onwards a 0.3-m tidal amplitude
was established with stormflood simulations of 0.8-m
tidal amplitude twice a month in the non-breeding
period of birds. After the introduction of reduced tidal
influence, the invasion of glycophytes stopped. A
typical salt-marsh zonation established, including a
Salicornia
spp. zone, a
P. maritima
zone with some
Ph. australis
and an
E. athericus
zone, whereas the
highest parts did not change except for the estab-
lishment of shrub species, in the absence of livestock
grazing (Wolfram
et al.
2000).
banks for regeneration. Loss of vegetation and hyper-
saline soil conditions also had a deleterious effect on
the soil seed bank (Chang
et al.
2001). Even with the
application of mulch and fertilizer, attempts at reve-
getation using soil plugs and intact plants are dif-
ficult under these conditions (Handa & Jefferies
2000). To prevent expansion of the overgrazed area
in the breeding sites, the governments of the USA and
Canada in 1999 introduced a spring hunt to reduce
numbers of snow geese in the mid-continent popula-
tion. The number of geese actually harvested since the
introduction of the expanded seasons has increased
from 0.6 million in 1998 to 1.4 million in 2001 (Kruse
& Sharp 2002).
13.6.4 Changes in grazing regime
The resumption of livestock grazing in abandoned back-
barrier salt marshes revealed that the changes in the
vegetation after abandonment are reversible within
10 years at a relatively high stocking rate. The number
of plant communities along the elevational gradient
increased again. At different elevational levels the num-
ber of plant species per unit area increased, mainly
because of the removal of the tall grass
E. athericus
(Bakker 1989, J.P. Bakker
et al.
2003). However, at
the low marsh the number of species decreased. The
soft bottom is trampled by cattle and only annual
pioneer species can be maintained (Bakker 1989). It
is seen as a problem to restore artificial brackish
marshes by resumed grazing, once
Ph. australis
has
established (Esselink
et al.
2002). It is estimated that
the numbers of brent geese in the entire Wadden Sea
in May can be a factor of 4-8 higher when all salt
marshes are grazed by livestock, than in the absence
of grazing (Bos
et al.
2005b).
The overgrazed sub-arctic marshes might be
restored by excluding them from grubbing by geese.
However, exclosure experiments showed that once
the sediment is hypersaline, it is still devoid of
vegetation in the absence of geese even after a period
of 20 years (F.H. Abraham, R.L. Jefferies and R.T.
Alisauskas, unpublished observations). The success
of restoration depends on the way species may sur-
vive the hypersaline conditions for a longer period.
Puccinellia phryganodes
and
Carex subspathacea
spread vegetatively. In contrast, dicotyledenous spe-
cies associated with these graminoids depend on seed
13.7 Concluding remarks
Understanding the historical context of the develop-
ment of intertidal flats and tidal salt marshes during
a period of several centuries is a prerequisite for any
fruitful discussion about the perspectives of nature
conservation and restoration in these systems. The
intertidal flats, unless subject to long-term impact
by human exploitation, can still be characterized as
near-natural systems, because they were capable of
recovering as long as humans use them as natural
resources. As soon as these ecosystems were con-
sidered as areas for economic exploitation, mainly
during the past few decades, the degree of disturbance
included the risk of irreversible change. Restoration
will then take a period of decades, if it is possible
at all. Tidal salt marshes, on the other hand, have
experienced centuries-long land use, and the major-
ity nowadays are artificial ecosystems. This implies that
these systems require continued management, livestock
grazing being an inherent part of the scene. It has
become clear that de-embankments, which aim at
restoring salt marshes from polder areas, can quite
easily result in re-establishment of the former abiotic
conditions, but that the plant communities can become
different from the reference communities due to
species-specific dispersal of propagules, which may have
consequences for small herbivores as well. This pheno-
menon will be further discussed in Chapter 16 in this
volume, in terms of newly emerging ecosystems.
