3C Vegetation Stabilisation of Sediment

Vegetation is important in stabilising sandy coastlines through dune successional development on sandy coastlines and salt marsh successional development in estuarine areas. 


​General

  • Vegetation can stabilise unconsolidated sediment and protect it from erosion. 
  1. Plant roots bind sediment together, making it harder to erode. 
  2. Plant stems and leaves covering the ground surface protect sediment from wave erosion and erosion form tidal or longshore currents when exposed at high tide.
    1. They also prevent sediment from wind erosion at low tide. 

In addition vegetation increases the rate of sediment accumulation:
  1. Plant stems and leaves interrupt the flow of wind and water, reducing their velocity and encouraging deposition. 
  2. When the vegetation dies it adds its organic matter (hummus) to the soil. 

​ Unconsolidated sand, silt and clay, freshly deposited at the coast are a very harsh environment for plants. 

The coast is an incredibly harsh environment for plants because:
  • They're exposed to high wind speeds at low tide. 
  • Lack of shade produces a high diurnal (daily) temperature range.  
  • They're submerged in salty water for half the day. 
  • The evaporated sea spray makes the sediment saline. 
  • Salt is highly porous and permeable, so rain water drains quickly- so plants have little fresh water. 
  • Submerged sediment has its pores saturated with salt water - there's no oxygen for plant roots to respire with. 
  • Sand lacks nutrients.  

Pioneer Plants
These are the first plants to colonise freshly deposited sediment. 
They modify the environment:
  1. Stabilising sediment
  2. Adding organic matter that retains moisture, contributes nutrients and provides shade. 
  3. Reduce evaporation in sand. 

Now, slightly less hardy plants can colonise the sediment. They add more organic matter, stabilise existing sediment and trap more. 

Each step in plant succession is called a seral stage. 

The end result of plant succession is called a (climatic) climax community. 

Plant succession: the changing structure of a plant community over time as an area of initially bare sediment is colonised. 
Plant succession: the process by which a series of different plant communities occupy an area over time.  


​Dune Successional Development

  • Xerophytic plants are specially adapted to dry conditions to colonise bare sand. 
  • Plant succession on sand is called psammosere.

Psammosere:
  1. ​Embryo dunes form when seaweed driftwood or litter provides a barrier or shelter to trap sand. 
  2. As the embryo grows, it is colonised by xerophytic pioneer plants, like sea couch grass, lyme grass, saltwort and sea rocket. 
    1. the embryo dunes alter the conditions to something other plants can tolerate, allowing other plants to colonise and forms a fore dune
  3. Pioneer plants stabilise the sand allowing marram grass to colonise. 
  4. Marram grass is marvellous because it:
    1. has waxy leaves to limit water loss through transpiration and resist wind-blown sand abrasion. 
    2. has roots that can grow to 3m to reach down the water table and the stem can grow 1m a year to avoid burial by deposited sand. 
    3. allows the dune to grow, rapidly forming a yellow dune
      1. it's called this because the surface is mainly sand, not soil
  5. As the marram grass and sedge grass dies, it adds hummus to the sand, creating soil. A grey dune develops, with plants such as gorse. 
    1. grey dunes and dune slacks (see bottom) are fixed dunes
    2. examples of plants are: red fescue, heather, creeping willow
  6. The dune is now above high tide level, so rain washes salt from the soil, making it less saline. 
  7. The soil now has improved nutrients and moisture retention, allowing non-xerophytic plants to colonise the dunes until a climax plant community is reached, in equilibrium with the climate and soil conditions. 
    1. e.g. bramble, pine, birch


​Salt Marsh Successional Development

  • Halophytic plants are specially adapted to saline conditions to colonise mud. 
  • Halosere is plant succession in salty water. 
  • Estuarine areas are ideal for salt marshes because:
    • they're sheltered from strong waves (so sediment like mud and silt can be deposited) 
    • rivers transport a supply of sediment to the river mouth, which may be added to by sediment flowing into the estuary at high tide

Halosere
  1. The mixing of fresh water and sea water in the estuary causes clay particles to stick together and sink - called flocculation. 
  2. Blue-green algae and gut weed colonise mud, exposed at low tide for only a few hours. 
  3. The algae binds mud, adds organic matter, and traps sediment. 
  4. As the sediment thickens, water depth is reduced, and the mud is covered by tide for less time. 
  5. Halophytic glasswort and cord grass colonise as the next seral stage - the marsh is still low, and covered by high tide each day.
  6. An accumulation of organic matter and sediment raises the height of the marsh until it is only covered by spring tides. 
  7. The higher marsh is colonised by less hardy plants 
    1. sea aster
    2. sea lavender
    3. sea thrift
    4. scurvy grass
  8. Rainwater washes salt out of the high marsh's soil, allowing land plants to colonise. 
  9. This continues until climax community is reached. 
    1. In most of the UK, the climax community would be deciduous oak forest, or coniferous pine forest in north Scotland. 


​Other stuff

  • Blowouts are gaps created when storm events erode sections of the yellow dune through wind or wave action. However, over time, deposition of sediment and recolonisation of vegetation will rebuild the dune's damaged areas. 
    • However, the wind may have blown away the dry sand, reaching the heavier wet sand at the water table. A marshy dune slack may form with fresh water plants, like marsh orchids. 
  • In a salt marsh, the river distributaries run through the marsh as creeks.
  • In tropical latitudes, mangrove swaps stabilise marine sediment rather than salt marsh.