MORE LIKE BIRKDALE…
Hummocks – clumps of sand accreting around grass – are becoming much more common in the late Spring and early Summer each year, because as the beach level rises they are increasingly protected from the sea and have more time to establish between high ‘Spring’ tides.
They often establish in straight rows, as a consequence of seeds being collected by the sea from the nearby saltmarsh and dunes such as at Red Rocks, and deposited in long strandlines of flotsam; largely dead, black, twiggy vegetation, often including Skate eggs (black pouches), shells and feathers, during the highest tides.
These strandlines may appear messy to those whose idealised view of a beach is pure, golden sand. But they serve a very important purpose.
With more and more time before a subsequent high tide, the seeds have time to establish. They grow very quickly indeed, becoming extremely efficient at accreting sand on the next high winds, quickly enhancing the profile of the beach, and soon begin to join together to form a more resilient dune ridge.
A clearly visible, active embryo dune system can evolve in less than five years, such as the newest one at Birkdale, as shown below.
- Dune succession would start where the Puccinellia is growing now.
- A dune ridge would grow until equilibrium is reached with the tide. The vertical rate of growth slows down as the dune grows longer and wider and eventually moves towards the sea; it is a mobile system.
- At Birkdale, the newest dune ridge reached about one metre tall, 250 metres long, and 30 metres wide, within about three years.
- Grasses would be quickly succeeded with dune species such as Sand Couch (Elytrigia juncea) and Marram (Ammophila arenaria) (Figure 9).
- It is worth noting that Common Cord Grass (Spartina anglica) does not grow on dunes but can survive in adjacent saltmarsh, such as at Red Rocks, where it has not ‘taken over’ but is soon out-competed and succeeded by other plant species.