Geoscience Reference
In-Depth Information
organic particles. In more acidic situations earthworms
are inhibited, and the undecomposed organic matter
accumulates as moder humus.
is still not covered by vegetation. Plants must be able to
withstand stress from both heat and drought. Sands hold
little moisture, and the water table will be several metres
below dune surface. Spring annuals flower and set seed
before the higher soil temperatures of summer. Nitrogen
is considered to be critical for marram grass and sea
buckthorn, and most is fixed from atmospheric nitrogen
by nitrogen-fixing bacteria living in the rhizosphere of
these plants. Beyond the yellow dune zone, the
grey dune
zone
is marked by the appearance of sand-binding mosses
and lichens. These older and more stable dunes have a
larger cover of plants, with a more diverse flora of grasses,
heaths and shrubs. Non-maritime species like Scots pine
(
Pinus sylvestris
) start to make their appearance.
The course of the sand dune succession depends upon
three factors: (1) the type of sand, (2) the position of the
freshwater table, and (3) the nature of animal grazing.
Acid sands favour dune heath vegetation, as they have low
cation exchange capacities and few nutrients. Sand with
calcium carbonate content
Psammoseres
The psammosere is a succession which starts its develop-
ment on bare, loose sand, either on sea or lake shores. The
vegetation of sand dunes is shaped by a combination of
physical, chemical, biotic and human factors. Even within
a small dune system there are gradients of instability, soil
pH, moisture content, grazing pressure and trampling.
The beach itself has no plant cover, as the waves
continually move the sand, whose abrasive action will
destroy any rooted plants. Flora are restricted to micro-
algae and diatoms, often attached to sand grains. Some
organic matter will be brought in on each tide, and
decomposition will be carried out by organisms such as
lugworms living in the sand. A considerable population
of micro-organisms, nematodes, cocepods and worms
provides food for large predatory worms, and many
species of filter feeders are found within the sand.
In coastal regions with consistent onshore winds, sand
is moved inland and deposited as dunes. The first
deposition of
embryo dunes
is initiated by the deposition
of sand around pioneering plants such as saltwort (
Salsola
kali
) and sea rocket (
Cakile maritima
). Continued growth
of the dune depends upon the ability of certain plant
species to grow in, and stabilize, the wind-blown sand by
growing up through it. The grasses are, first, sand couch-
grass (
Elymus farctus
), which has only a modest ability to
withstand burial, but is so tolerant of salt water that it
initiates embryo dunes close to the strand line. Second is
sea lyme-grass (
Leymus arenarius
), and thirdly is marram
grass (
Ammophila arenaria
) the main dune-building
species in Europe. This species is salt-tolerant, drought-
tolerant and thrives in loose sand, where it spreads
vegetatively by laterally growing underground stems called
rhizomes. It can keep pace with up to 1 m of fresh sand
deposition per year. Rhizomes and extensive root systems
help to bind the sand, and to convert the mobile dune into
a fixed dune which is more favourable to a larger number
of less xerophytic and less hardy plants. By binding the
dune together, and by maintaining the aerodynamic
roughness of the surface, marram allows dunes to build
up to a considerable height.
Actively growing dunes are an extremely hostile
environment for most plants. However, as the rate of sand
deposition declines a larger range of grasses and annual
and perennial herbs are able to colonize these
yellow
dunes
; they are semi-fixed dunes, much of whose surface
3 per cent from shells are
alkaline and favour grassland. Wind erosion frequently
creates deflation blowouts of marshy depressions or
'slacks' in the dune zones. There is a marked annual
fluctuation in water level. Water levels in slacks normally
reach a peak in early spring, then fall sharply through the
summer, reaching up to 2 m below the surface before
rising again in autumn. The slacks pass through salt marsh
and swamp phases, giving local hydroseres or haloseres,
where unique vegetation has adapted to these unusual
conditions. These are rushes (
Juncus
spp.), sedges (
Carex
spp.) and willows (
Salix
spp.). Moving inland, soil
properties change rapidly. Acidity increases and pH
declines as the influence of seashells and salt spray lessens,
and leaching remains strong through the coarse sand.
Organic matter in the topsoil increases with distance
inland, as the more varied and more abundant plant cover
gives a larger litter input to the soils. Micro-organisms are
more abundant and produce more humus, which in turn
is able to hold more moisture, to provide more nutrients,
and to give greater stability to soil surfaces. Eventually a
deep, humus-rich soil with a thriving faunal population
of earthworms, snails and insects will result.
Hydroseres
Hydrosere successions occur around lakes which are
gradually being infilled by sediments. Silting occurs
independently of plant succession, but the process is
considerably speeded up on lake margins by the
development of hydroseres. The littoral zones of ponds
and lakes are shallow sediment-receiving zones where the
