7.1
Succession
How science works
Name ……………………………………………………….
Date ………………….
Saltmarsh Succession
Aims
 To appreciate the factors that affect ecological succession
 To learn how plants cope with saline environments
 To understand how salt-tolerant plants could boost world food supplies
Background information
Halophytes are salt-tolerant plants. This activity looks at the succession on an English
saltmarsh. About 7% of the world’s land surface and 5% of cultivated land are affected by
salinity (saltiness). High concentrations of salt are toxic to most plant species, including most
crops. Salinity affects a plant’s ability to absorb water (so-called ‘physiological drought’) by
osmosis from the soil. There is also an effect of excess sodium and chloride ions on cellular
processes. In particular this can affect the function of stomata, which use potassium ions to
control opening. Sodium and potassium may compete for uptake into the guard cells. A
potential crop needs not only to be able to cope with salinity, but also to be able to achieve a
good growth rate and yield in saline conditions.
Saltmarsh succession
Halophytes have evolved adaptations to help them cope. Saltmarshes occur on the gently
sloping shores of estuaries, where salt and fresh water meet. Their natural limits are the
lowest and highest tide marks during the lowest tides of the year (at the spring and autumn
equinoxes).
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7.1
Succession
How science works
Figure 1 A typical plant succession on an English saltmarsh
At the lower end of the marsh, waterlogging and instability of the soil cause problems for
plants. Waterlogging causes lower oxygen levels in the soil and a high level of salinity.
Lower light levels, while they are submerged, is also a problem to which plants have adapted.
The middle levels of the marsh will suffer considerably from wave action (the lowest levels
will be less affected by this as they will be submerged for much more of the time). Soils will
not have time to dry out as much as the upper levels, so extremes of salinity may not be so
great here.
At the upper levels of the saltmarsh, evaporation rates are high, as they are more exposed to
the wind and rain. Salinity will fluctuate as the soil will be flushed by heavy rain (fresh
water). Temperature fluctuations will also be more extreme, since temperatures will be
reduced while submerged at high tide, but increased at low tide. At, and beyond, the upper
limit of the marsh, a salt-tolerant grassland dominated by saltmarsh grass (Puccinellia
maritima) develops, and is often grazed by sheep or horses.
Plants adapted to growing in a saltmarsh must be able to tolerate the environment described
above. But first they must get there, so seed dispersal, and the ability to germinate in unstable
soil that is waterlogged and saline all the time, is important. Plant root systems help to
stabilise the mud and encourage further deposition of silt which builds up the soil level,
allowing other species to thrive. As the plants die and decay, they add organic material
(humus) and minerals to the soil. Later in the succession they will also provide shade and
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7.1
Succession
How science works
shelter for the seedlings of other species. Plants which follow in the succession need to be
able to cope with more shading and competition for nutrients and minerals in the soil.
Saltmarsh species differ in the depth and spread of their root systems. Some, such as sea
lavender (Limonium), have very deep roots that avoid surface competition.
As the height of the marsh increases, there is a reduced frequency of flooding and of wave
action, but an increased exposure to sun, wind and rain. There is also a reduction in water
content of the soil and an increase in the content of organic matter. This leads to a variety of
halophytes that are adapted to salinity.
Adaptations to salinity
Most halophytes are either salt tolerators or salt avoiders.
Salt tolerators have physiological and biochemical adaptations for keeping their metabolism
working despite accumulating salts in their cells. They tend to keep salt in particular
compartments of the cells, such as the vacuole. This involves genes that control the actions
of protein channels/transport proteins in membranes. Certain alleles may be activated by
salt stress.
Salt avoiders have structural adaptations to minimise salt concentration in their cells, to
prevent salt uptake by the roots, or to get rid of salt by excreting it through special glands on
their leaves, where it is washed away by rain. Halophytes like cordgrass (Spartina) and sea
lavender (Limonium) actively excrete salt through special salt glands on the surface of the
leaves. The salt is washed away by rain. The leaves of some Limonium species can excrete up
to half their weight in salt in 24 hours.
Some plants produce chemicals (‘osmoprotectants’) in their cell sap that lower their water
potential so that they can still take up water by osmosis. This helps to protect cellular
structures such as chloroplasts and prevents macromolecules being deformed by water stress.
Others control the water content of their tissues, either by becoming succulent, or by
changing the numbers of water-forming channels (aquaporins) in cell membranes and
perhaps by adaptations in their ability to control water movement in or out of cells.
Succulents have the added advantage of a reduced surface area, which reduces water loss by
transpiration and evaporation. An example of this is the pioneer species glasswort
(Salicornia).
Some halophytes actually take in salt to reduce water potential. These ions are usually stored
in the vacuole so that they do not interfere with cell metabolism. A few species accumulate
salt in their leaves, then shed them from time to time.
Some genes affect the loading of sodium into the phloem in roots and shoots. Certain alleles
can discourage loading of salt into the phloem at the roots, so keeping the root sodium
content high for water uptake, but preventing the shoot tissues being exposed to too much
sodium.
The seeds of some halophytes remain dormant in very salty conditions, but break dormancy
when conditions become less salty, for example, after heavy rain. This ensures the seeds
germinate when conditions are favourable for the uptake of water by the embryo.
Many halophytes actually grow better in non-saline conditions, but they are at a great
competitive advantage in saline conditions.
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Succession
How science works
Questions
1 What are the most challenging environmental factors for plants at:
a) the lower part of a saltmarsh
b) the middle of a saltmarsh
c) the top of a saltmarsh?
2 What features make a plant a good coloniser of bare mud?
3 How do plants at the lower end of the marsh change conditions so that the habitat becomes more
favourable for other plants to invade?
4 The frequency of different species changes as you progress up the marsh. How would you sample a
saltmarsh to get reliable figures for the changing frequency of the different species in relation to
elevation above sea level?
5 How do several different species manage to coexist at the same level of a saltmarsh?
6 Compare species in the early and late stages of saltmarsh succession. What environmental factors
vary at the different levels? Take into account seed dispersal and germination, the relationship
between photosynthesis and light intensity, respiration rates, transpiration rate, and stomatal
behaviour.
7 How can saltmarshes protect the coast? Why might this be better than sea walls or dykes?
8 a) What metabolic/physiological processes are involved when plants excrete excess salts?
b) How can plants prevent salts from damaging cell components?
c) What adaptive features of halophytes might be under genetic control?
9 Considering the practical and economic advantages and disadvantages, which do you think would
be most successful, genetically engineering existing crops to tolerate saline soils, or developing new
crops from existing halophytes?
Sources
The background information will give you some ideas.
You will find information on the basic science in Oxford University Press A2 Biology Chapter 7.1
Ecological succession. You may also find Oxford University Press AS Biology Chapter 3 (Cells and
movement in and out of them) useful.
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This resource sheet may have been changed from the original.
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