Geoscience Reference
In-Depth Information
Plate 20.6
Primary succession or prisere in a National Nature Reserve on Esthwaite Water, English Lake District. The
Photo: Ken Atkinson
The halosere shows a distinct vertical zonation,
reflecting decreasing flooding as the level of the marsh is
built up through accretion, and the higher parts are
flooded only by occasional high spring tides. The
characteristic species of the lower marshes are summer
annuals such as saltwort (
Salicornia europea
) and seablite
(
Suaeda maritima
). Both are resistant to high concentra-
tions of sodium, although many seedlings are destroyed
by unstable substrates. When the mudflat has achieved a
certain height above the high-tide mark, rainfall will start
to leach out the salt. Thus a less salty and more stable
surface will allow a more diverse collection of plants to
become established. These include the grass
Puccinella
maritima
, sea lavender (
Limonium vulgare
) and sea aster
(
Aster tripolium
). Marsh soils are typically waterlogged
and anaerobic, with problems associated with oxygen
deficiency and potential chemical toxicity. Some species,
however, are restricted to the better-drained banks of
creeks or to areas of upper marsh; these include the dwarf
shrub sea purslane (
Halimione portulacoides
) and sea
wormwood (
Artemisia maritima
) (
Plate 20.7
).
The ecology of haloseres was transformed in the early
nineteenth century by the invasion of European marshes
by the American cord grass,
Spartina alterniflora
, which
was introduced into Southampton Water, where it
hybridized with the local
Spartina maritima.
The hybrid,
Spartina
townsendii, is sterile, but in the 1880s its
chromosomes doubled, leading to the creation of a new
species,
Spartina anglica.
This grows more rapidly and is
able to colonize more unstable areas of salt marsh than
its parents. It raises marsh levels by silt accretion much
more rapidly than other marsh species, and has become
the dominant species in many salt marshes. It was planted
extensively in Europe, Australia, New Zealand, China and
the United States to assist in the reclamation of intertidal
land. It is a large, vigorous grass which has quickly
colonized low mudflats and open salt pans. There has been
some opposition to its use, as an 'aggressive invader', but
studies in Morecombe Bay, north-west England, indicate
that by helping salt marsh to spread it has provided cover
for waders and wildfowl and increased local biodiversity
(Pennington 2007). Processes of succession have been
