Primary succession is the series of
community changes which occur on an entirely new habitat which has never been colonized
before.
Examples of such habitats would include newly exposed
or deposited surfaces, such as landslips, volcanic lava and debris, elevated sand
banks and dunes, quarried rock faces.
A number of seral stages will take place in which an
initial or 'pioneer' community will gradually develop through a number of different
communities into a 'climax' community, which is the final stage.
Coastal Sand Dunes
An Example of Primary Succession
Where there is large supply of dry sand (for example,
on a sandy beach exposed by the tide and heated by the sun), together with winds having a
speed greater than 15 kph, the sand will be picked up and blown by the wind.
Plants growing on the shore-line, together with flotsam
and jetsam washed up by the tide, provide mini- wind breaks which slow the wind down in
their immediate vicinity. If the wind is slowed sufficiently, it will drop some of its
cargo of sand. A sand pile will begin to accumulate at this point, providing an ever
increasing wind break. Eventually the sand pile will grow into a dune at the back of the
beach.
With high wind speeds, the sand is continually pushed
over the crest of the dune, falling down the steeper windward slope. This has the effect
of causing the dune to move steadily inland. Pioneer plants will begin to colonize the
dunes, gradually holding the sand in one place with their root networks. New sand dunes
may build up behind the first dune, eventually forming a series of dunes from the
seashore, inland.
New sand dunes have the following environmental
features:
Continually moving sand.
This covers up pioneer plants, provides no anchor points for roots and is very abrasive,
damaging soft tissues.
Very little freely available freshwater.
Because sand is fairly coarse-grained, any rainwater rapidly percolates down through a new
dune. There is no humus or organic matter in the sand to help absorb and retain the water.
There is also a lot of salt derived from sea spray in the sand which may dissolve in the
water. This concentrated solution can make it physiologically difficult for plants to
absorb water.
However, some water is still available to plants colonizing the dunes.
The pioneer species will often have very long vertical roots which can reach down to
freshwater which has collected at the base of the dune. Freshwater is lighter than
seawater and so will tend to float on top of any seawater which is also present.
In addition, because the sand is very porous, with many air spaces, the sun heating the
upper layers of the sand will cause the air in the upper air spaces to expand. This draws
cooler, moister in air from the lower reaches of the dune. At night, as the air cools
again, the moisture in the air will condense out, leaving droplets of water in the sand.
Night dews will often also add water to the upper surface of the dune. This is enough for
the pioneer plants to be able to survive.
Very few nutrients.
The dune is made up of hard, inorganic grains of sand. In a young dune, there are
few nutrients available because there is not yet any dead organic matter being recycled
within the sand. Some dune pioneer plants are able to fix nitrogen in root nodules and so
are able to partially overcome this problem.
Characteristics of Sand Dune Pioneer
Plants
High vertical growth rates to keep up with continuous
sand deposition.
Extensive lateral root or creeping shoot systems to
bind the plants over a large area.
Deep vertical root systems to aid with accessing
water in the water table below.
Xerophytic adaptations to decrease water loss or
increase water storage capacity.
Examples include, small or rolled leaves which decrease the surface area
for transpiration, a waxy cuticle which prevents water loss
and succulent tissues which can store water.
Physiological adaptations to high salt content in the environment.
Continue to
Sand Dune
Stages
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