Environmental Engineering Reference
In-Depth Information
atmosphere. Colonization by flowering plants commences as soon as there is a stable soil
surface. The rate of deposition increases as vegetation cover increases, together with the
binding effects of their root systems. The pattern of vegetation communities reflects the
frequency and duration of tidal flooding, and the effects which these two factors have on
sediment accumulation. The principle of competitive replacement occurs in response to
these environmental changes. As salt marsh plants have to survive periodic inundation by
sea water, the vertical zonation reflects relative tolerance of salinity. Salinity causes
problems to plants in nutrient absorption and in water absorption, owing to the high
osmotic pressure of soil water, which makes it less available to plant roots. Some species,
e.g.
Spartina
, overcome this by excluding sodium from root uptake; other species, e.g.
Suaeda
, move potassium out of older leaves before they are shed in order to maintain
internal ionic balance. Halophytes, plants tolerant of high salt content, also require an
increase in tissue solute concentrations so that the osmotic potential of the plant exceeds
that of the external soil solution.
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 herbacea
) and seablite (
Suaeda maritima
). Both are
resistant to high concentrations 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 possible 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
).
The ecology of haloseres has been transformed in the past two centuries by the
colonization of European marshes by cord grass,
Spartina anglica
, which is able to raise
marsh levels by silt accretion much more rapidly than other marsh species.
Spartina
anglica
(previously known as
Spartina x townsendii
) is a recently evolved hybrid of
Spartina maritima
(a West European species) and
Spartina alternifolia
(an introduction
from North America in the 1820s).
Spartina anglica
has spread rapidly, both naturally
and also through planting programmes, and has become the dominant species in many
salt marshes. It is a large, vigorous grass which has quickly colonized low mudflats and
open salt pans. These areas were previously unvegetated. Thus the accidental
transhipment of a species from America to Europe has resulted in a new hybrid with
distinctive properties. The processes of succession have been dramatically speeded up by
this hybrid. In turn
Spartina anglica
is replaced by sea poa (
Puccinella maritima
),
especially where the grass is intensively grazed.
Puccinella
withstands the grazing and
trampling of sheep and cattle much better. Extensive die-back of
Spartina
has also been
reported because of accumulated plant litter and toxic conditions in very fine-textured
sediment.
