You may already have looked at sorghum's adaptations as a xerophytic plant. Have a look at that page again, especially at the picture of marrum grass at the bottom of the page.

 

Ecosystems

Sand Dunes

Adaptations of Plants to sand dunes.

The picture below shows some of the essential items of equipment you will need to carry out a transect of a sand dune.

quadrat frameMr. Purvisleveling poleTransect lineplant ID sheets

 

make sure you record your results

 

SAND DUNE ZONATION – SUCCESSION AND BIOTIC FACTORS

The figure provides a summary of the process. Sand on the beach, just above the high water-mark, is an inhospitable place for plant growth because :

  1. It is often dry. Sand, especially when, as here, it lacks organic matter, has very low water retention properties, and so, as soon as it stops raining, it rapidly dries out .
  2. It is sometimes affected by seawater, at least at high tide when the sea is rough. The salts in the seawater cause a desiccating effect as a result of osmosis .
  3. It has an unstable surface which makes seedling establishment difficult.
  4. It is nutrient deficient because it lacks the organic matter which is present in most soils and provides a pool of nutrients that is continually liberated by micro-organisms. It also has low cation exchange capacity (the ability to retain cations) which, in most soils is provided by organic matter and clay.

Although few plants can grow here, certain species such as Sea Rocket (Cakile maritima) and Sea sandwort (Honkenya peploides) can. They have xeromorphic adaptations . Such plants stabilise the sand and add organic matter (as dead leaves and roots), so improving the soil’s nutrient status and moisture-holding capacity. They have brought about the first few changes which could lead to this area of foreshore becoming a forest – by succession. In many cases, Sea couch-grass (Agropyron junceiforme), becomes established. Small "foredunes" of sand accumulate around the grass as stability increases, but the permanent dunes only follow once Marram Grass (Ammophila arenaria) and/or sometimes Sea Lyme Grass (Elymus arenarius) make their appearance. Marram Grass is a species which flourishes when continually buried by sand. It responds by growing vigorously, and dunes which may reach over 25 m high are the result, stabilised by the Marram grass root system, raised above the reach of waves and with an ever-increasing organic matter accumulation with ever improving moisture retention and nutrient status. Leaching by rain starts to remove sodium chloride from developing soil. The foreshore species which helped to start the process are no longer present.

Numerous other species now become established, especially in the "slack", the "valleys" between the dunes where the (fresh) water-table is near or at the surface(small ponds may be present). Some of these species (such as Restharrow, Ononis repens) are legumes and add plant available nitrogen to the system. The Marram Grass "fixes" the growing mass of sand, whilst the smaller plants, including grasses like red Fescue (Festuca rubra), stabilise the surface. Once the dune stops increasing in size, the Marram Grass fades out. We now have a "fixed" dune. More species arrive. The community becomes grassland, whose species composition depends on whether the sand had a high shell content (that is, whether it is calcareous – high soil pH) or not (acid soil pH, 3.17). Such a community would be grazed by rabbits or domestic animals. Tree seedlings are very vulnerable to grazing, but the next stage, scrub, consists of "armed shrubs", such as Bramble (Rubus ssp.), Wild Rose (Rosa ssp.) and Hawthorn (Creteagus monogyna). These are protected from grazing by their spines, and under their protection, tree seedlings become established. Often, these are Pine (Pinus spp.), which has xerophytic adaptations . They cast a shade which reduces the scrub species. Over a period of centuries, other tree species and a variety of woodland plants and animal species will become established, and eventually, a stable climax will be reached complete with its own self-maintaining soil system and microclimate. By this time, continued succession to seaward will have pushed back the sea several miles. The dry, sandy nature of the soil may mean Pine will remain the dominant species.

British sand dune systems rarely show the complete series of successional changes (known as "sere" as far as climax forest, and where they do (for example, on the Fylde coast of Lancashire), trees have been planted to speed up the stabilising process – a biotic factor (although the introduced trees may be spreading to seaward as their seedlings begin to grown in the scrub, and thus they have become a real part of the process). Some, named "warren" on the map, may have had rabbits deliberately introduced and some are or have been used for grazing by sheep or cattle. Grazing pressure of domestic stock and introduced species like rabbits is much greater than that of the wild herbivores of prehistoric times, and tends to suppress scrub and forest development, causing the succession to "stick" at the grassland phase. Thus, so long as the artificial grazing pressure remains, the grassland is a permanent community, a biotic plagioclimax. Such grassland behind dunes used to be called "links", which in many places have become golf courses. Golf links too represent a plagioclimax, maintained by mowing. Another human influence on dune systems is trampling by tourists, which may destabilise the dunes themselves.