Sand Couch Grass
Marram - and its adaptations as a pioneer
Dune plant succession
Definition: the sequence of plants that occupy the different habitats within dune systems
Coastal sand dunes or psammoseres provide a range of habitats for plants and animals. The habitats nearest to the beach are harsh - the plants may suffer stress from lack of moisture and nutrients, exposure to salt spray and wind abrasion and ground instability. Only the toughest pioneer plants can colonise the embryo dunes at the back of the beach. Marram tends to dominate the large dunes facing the beach. With increasing distance inland, the number of species and the amount of ground cover also increases, reflecting the time available for soil development and the increased availability of moisture, humus and nutrients. Shrubs and woodland are found only on the oldest, stable dunes farther inland. At sites with limited human disturbance this woodland may approach the climax vegetation for sandy soils in an area.
At Aberlady Bay there is little vegetation at the back of the beach. Beyond the strand line pioneer species include sea rocket and sand couch but the ground cover is <10%. The pioneers act to stabilise the sand and their decay slowly adds humus to the sand. No soil profile is developed and Ca contents are high. These hardy plants may have to cope with high sand temperatures, high salt contents, drought and very unstable ground.
There is one set of large fore-dunes at Aberlady. The vegetation cover is 40-100% and dominated by marram grass. Marram is adapted to this harsh environment in several ways:
The fore-dune area remains unstable, with lots of blow-outs. Shell contents are lower and there is a thin (0.5 cm) humic layer. Soil formation has begun, with some leaching of calcium but soil moisture content remains low.
The older, stable dunes are around 5 m high and covered by vegetation. Marram grows less vigorously and forms a sward rather than separate tussocks, while other grasses, such as Sand Fescue (Festuca arenaria) become more abundant and continue the stabilising process. The ground flora is more mixed, with open areas remaining, often after grazing and burrowing by rabbits. Dune mosses are important here. T. ruraliformis tolerates the high temperatures (>60 ºC) that may exist at the sand surface. The twisting and untwisting of the leaves in response to dry and wet conditions acts to throw off loose sand off the colony surface. Other species include daisies and dandelion. Soil depth here is ~2 cm and the humic layer is up to 1 cm thick. More CaCO3 has been leached so that shell content of the sand is now reduced. Soil moisture and humus content also increase. The rate of organic build-up in soils on dunes initially is very slow, being below 1% for the first 100 years, and then speeds up. The acceleration of humus formation may be a consequence of the calcium carbonate being leached from the uppermost part of the soil profile, producing a drop in soil pH which encourages humus formation.
The older dunes are being invaded by shrubs. These included hawthorn and sea buckthorn. This latter shrub is an aggressive coloniser. Sea buckthorn is a legume – it has nitrogen-fixing bacteria on its roots - and also provides leaf litter. The soils on the older dunes are therefore richer in nutrients and thicker. The shrubs provide cover and act as wind breaks. This means that these areas provide habitats for a range of animals, including snails, and birds, including finches and other song birds.
The climax vegetation (definition: the final stage of plant succession where there is a stable environment and vegetation is in equilibrium with the local climate) in this area should be oak woodland. None occurs here because the oldest part of the dune systems is managed intensively for recreation. All woodland was removed to create Gullane golf course. The many human influences on the vegetation succession means that Aberlady provides a good example of a plagiosere.