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Plate 20.5 Grey dunes in a psammosere near Denia, Alicante Province, Spain, show severe erosion due to urbanization and
trampling. Some dune stabilization is given by marram grass (Ammophila arenaria) and an alien surface creeper from South Africa,
the yellow-flowered Hottentot fig (Carpobrotus).
Photo: Ken Atkinson
shallow water allows concentric zones of aquatic vegeta-
tion to develop, with one community replacing another
as the depth of water changes in space and time. Figure
20.11 shows a transect of vegetation zones which one
would meet in passing from the open water of the centre
of the lake on to surrounding dry land.
The pioneer community of water lilies ( Nymphaea
alba ) establishes itself when the lake is reduced to about
a metre in depth. This plant has the effect of accelerating
silting, both by reducing water velocity and by adding
organic debris to the lake bottom. When the depth is
further reduced, a second-stage community of bulrushes
( Scirpus lacustris ) and reeds ( Phragmites communis )
develops. Silting continues and the water shallows enough
to allow a zone of pond sedges ( Carex spp.). The emer-
gence of a marsh surface above the water surface allows
tussock sedges to develop, especially the cotton grasses
( Eriophorum spp.). Eventually higher ground supports a
deeper soil, with a better drained surface which allows tree
seedlings to survive. Alder ( Alnus glutinosa ) and willows
( Salix spp.) can tolerate wetness and form carr woodland.
In turn drier conditions allow Scots pine ( Pinus sylvestris )
to grow which eventually will be supplanted by the climax
deciduous woodland.
Each stage in the hydrosere is characterized by a
vegetation zone where the plants have specific tolerances
of waterlogging and the degree of soil wetness. Plants
which can tolerate waterlogged soils are hydrophytes .
Excess water excludes air (oxygen) from the pores of
waterlogged soils, and this shortage of oxygen causes
problems with root respiration. In well drained soils
oxygen enters the root by diffusion from the soil atmos-
phere. However, the rate of oxygen diffusion in water is
about 10,000 times slower than in air. The hydrophytes
contain much more spongey tissue ( aerenchyma ) with
thin walls and large air spaces that permit air to diffuse
 
 
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