First of all it's necessary to consider that salt marshes are not easily accessible, considering its fragility.
t's an environment that it's not part of our daily experience, and it's easy to have a stereotyped
picture of it, or anyway a picture that doesn't correspond to reality.
In the salt marshes zonation of vegetation is very clear and constant (Spartina / Limonium / Puccinellia).
Vegetation is steady: three perennial plants association (spartinetun, limonetum e artemisietum)reach the best exploitation
of the available resources and don't show any tendency to change in more complex associations.
The succession of vegetation is always the same, and associations can be considered to be in equilibrium with the environment.
This means that high salinity, that is the real limiting factor in this environment, doesn't permit the evolution of the vegetation toward its natural climax,
that is represented by mixed forest of Quercus
peduncolata and Carpinus betulus.
In this environment it's possible to put in evidence the difference with the coastal environment where, going from the water's edge to the forest behind the dunes, life condition become always less difficult
growing in the number of species and specimen both vegetable and animal.
So we can observe how vegetation aims to evolve where conditions permit it.
n salt marsh this kind of evolution it's not possible because prohibitive conditions due to high salinity
don't permit vegetation to evolve from its initial stage, and the sequence spartina-limonium-puccinellia
will always go on recurring untilconditions remain the same.Owing to slight disturbances
it's possible that Salicornia veneta makes its appearance,
a pioneer plant that is present where, owing to erosion, the typical associations of low salt marshes edge disappear.
However this evolution is temporary, considering that, as soon as the situation stabilizes, perennial plants association happen again.
Therefore in this environment it's possible to introduce the following topics:
· Limiting factor
· Evolution of vegetation
· Climax
· Adaptation:
§ Sea lavender: it suceeds in defend itself form salinity eliminating it through secretory cells
taht are on the stem and leaves.
§ Glasswort: the problem for the plants that live in this kind of environment
it's the water absorption that happen owing to the osmotic pressure: water moves from an environment where concentration of solutes
is lower towars one where it is higher.
In salt marshes salts concentration is high enough, and any plant couldn't absorb water from the soil.
In the tissues of halophyle plants salt concentration can reach even 10%, and this permits water and nutrients to be absorbed.
In branches salt accumulates and photosyntesis takes place, replacing in this function leaves, taht in glasswort are very short or even absent.
§ Sea purslane: on the leaves it has hairs to eliminate salts excess
accumulated inside them. The accumulation of dead hairs and salts on leaves surface increases the reflection of solar radiation
reducing heat and limiting evaporation.
§ Vegetative part ittle developed and near to the ground in order to stand the wind.
§ ARoots system developed in order to absorb most of the nutrients from this arid and salty soil.
§ Flowers inserted in little niches to avoid pollen to be dispersed by the wind.
Besides this, it's still unknown the link between salt marshes disappearance
and sea level increasing due also to sea and ocean water expansion, caused by increasing of temperature.
Regarding this topic, it's possible
to carry out some didactical activities that can help students to
understand better global change and greenhouse effect. (see in the
section Didactics -The
Greenhouse Effect and Global Warming , Thermal
Expansion and Sea Level Rise)
Another cause of progressive
decrease of salt marshes surface is subsidence, that causes the soil
to become lower compared to sea level. In the "Didactics"
section there is a didactical activity that can help to understand
better subsidence Loss
of Wetlands: Subsidence).
It can be interesting to analize how the appearance of the lagoon of Venice has changed during the centuries. You can have a look at the maps of territorial changes heppend between 1810 and 1992 and the map showing salt marshes and fish farms location.