by Dr J Floor Anthoni (2000)
|Why would people want to plant the dunes and to stabilise them?|
|People think that root systems can protect them from the sea but this is not so.|
|It is a natural process for plants to invade the inhospitable zone of the beach, but should it be assisted? What does a healthy dune/beach system need?|
|In order to be able to repair themselves and to transport excess sand inland, dunes must be able to roll.|
|After dunes have been stabilised, they grow into a tall single dune, lifting the sea wind from the beach. Then permanent erosion sets in.|
|Motivations for planting
Planting and stabilising dunes, has been done for such a long time, that we collectively think of it as a good thing. It is taught at school; volunteer groups spend their free time planting dunes; governments have departments planting dunes and they allocate copious funds for doing so.
The botanist Dr Leonard Cockayne understood the beach/dune process when
he wrote in his book New Zealand plants and their story (first print
|"On many parts of the coast, sand is continually being brought on
to the shore by the advancing waves. In the neighbourhood of high-water
mark the shore soon becomes dry, and the sand is then borne landwards by
any wind coming from the sea. Where the sand accumulates faster than it
is being blown away, a hill, or a dune as it is frequently called, is formed.
Any obstacle in the path of the blown sand will also arrest its progress
and cause it heaping up . . . .
Frequently the dunes are very unstable, and in some places so much so that great areas of moving sand exist. These 'wandering dunes', insidiously advancing inland, do great damage - burying fertile fields, filling up flax swamps, choking watercourses, and overwhelming forests, plantations, pasture lands, and even dwellings. Originally nature had done much to stop such inroads, and the wandering dunes of New Zealand are chiefly the result of damage done by grazing animals through breaking the surface of the sand, and by fire destroying the natural safeguard, its plant covering."
The problems obviously started when people began interfering with the natural dune system. Runaway wandering dunes needed to be pinned down, usually with pine plantations and grass. From the back dunes, moving towards the sea, farmers managed to tame the dunes by planting shelter belts, seeding grass and by fertilising.
Cities grew where small beach settlements arose. The dunes were flattened and paved over as humans moved ever closer to the water line. Wherever humans moved in, the moving sand was stabilised. In the end hardly any dune system remained that functions today as it would naturally. The short-term gains were such that nobody could envisage anything wrong, but as we will see, the cost proved high in the long term.
Planting the dunes close to the sea was often
done, believing that nature would have done so anyway but is that true?
|In New Zealand the history of dune occupation had several phases. The
moment settlers cleared the land near dunes, these were grazed, which saved
a fence between pasture and dune. The boundary offered by the sea has always
been regarded as a cheap and natural one, perfectly secure for sheep and
cattle. At the time, also stock was transported by droving it along beaches.
In order to feed these wandering herds, they were set onto the dunes, resulting
in heavy grazing. As a result, the dune's cover disappeared and the sand
started moving inland.
This prompted the second phase, during which Cockayne and others portrayed the moving sand as evil, and capable of taking over the fertile pastures. Cockayne also considered dunes as potential productive pasture, waiting to be tamed, fertilised and seeded over. Thus started the phase of dune reclamation, on the one hand for forestry and on the other for pasture. One of the techniques used was 'capping', in which the moving sand was capped with a thin layer of clay, then fertilised and seeded with grass. It was a remarkably effective technique, still employed today.
The latest phase is that of dune erosion from the seaward side, and a belief that dune plants may somehow stop it. People living by the dunes are motivated to plant them in order to stop the sand moving indoors. They also enjoy walking in the dunes, and would like to turn these into botannical gardens. Very recently, the biodiversity issue made people believe that what is left of the dunes needs to be planted in as high a variety of (dune) plants as is possible.
|Myth: roots protect
In response to beach erosion, people plant deep rooting plant varieties, believing that their root systems will halt erosion. Salt tolerant trees such as New Zealand's Pohutukawa (Metrosideros excelsa) are planted in places where they would never have occurred naturally, even as close as 10-20m from the water's edge. But the force of moving water is rather unforgiving. Water is 800 times heavier than air and once it moves (by waves), is very destructive. Only sand that keeps moving, can remain an unsurpassable obstacle for the anger of the sea. Fixed roots cannot. But fixed roots can stall erosion temporarily.
Furthermore, salt water is fatal to any tree, excepting the mangrove tree (Avicennia resinifera), which thrives only in very sheltered, muddy bottoms. Once the tree's root system has been inundated with salt, the tree dies.
A lesser known fact of roots in dunes is that they form steep scarps
after large storms have eroded away the base of the scarp. By preventing
the dune from collapsing, these roots hinder the formation of a gradual
wind profile, necessary for the beach to repair its damage. As a result,
the eroded sand remains in the sea, laying the beach flat. Successive storms
keep eroding the steep bank further.
Before the steep bank above, dune grass has been planted, trapping the sand successfully. Because of the steep scarp along the entire beach, sand moves along the beach, around this corner. It is hoped that a foredune will form. However, this sand remains within reach of the water at spring high tide.
|The myth of succession
Anyone visiting the dunes, can observe that there exists a kind of habitat zoning running parallel with the waterline. When going landward from the beach, one first traverses a stretch of bare sand, followed by a zone with advancing dune grasses, behind which comes a zone with low plants. At the very back of the dune, the shrubs growing taller and there may be a forest. It is clear that there exists a kind of 'succession', but how is it brought about? What is 'natural' ?
1) The common thought is that the zones are succeeding one another while they march up towards the beach. First come the brave and hardy pioneering grasses with their running roots. They bind the soil, nutrients and moisture for the next wave of plants, hardy dune weeds and small shrubs. These pave the way for the forest. So eventually the forest will arrive at the water's edge. Dune plantings, it is thought, just help this natural process. But there's something wrong with this idea because if it were true, all our dunes would have been covered in lush native forest many thousands of years ago.
2) Another explanation is that each habitat zone is suitable only for the type of vegetation which is able to germinate and live there, even during times of adversity. Natural dunes have sparse vegetation that has evolved in time to exist, yet without adversely affecting the dynamics of the dune/beach system. In other words, the dunes were able to keep moving, while uprooting or burying vegetation in the process. When humans interfere by planting or fertilising, the balance swings in favour of vegetation and against the dune's dynamics.
3) From an evolutionary viewpoint, a different story emerges. The recent
ice age some 10,000 years ago lowered the sea by some 100m, moving the
shoreline many kilometres, almost to the edge of the continental shelf.
The land between our present shoreline and the shore line back then, became
covered in lush native vegetation. Dunes and beaches must have formed as
usual at the edge. As the climate warmed, the sea level rose, and the shoreline
with its beaches moved up. One would thus have expected the beaches to
be bordered by lush native forest, but this is not so. As the water rose
and beaches moved landward, they also destroyed the forests ahead of them
by a zone of sand. One would thus expect to find remnants of forest
underneath the dunes, but this is not so either (but true in some places).
It follows that beaches oscillated inland in a series of destructive and
constructive phases. During the destructive phase, sand is blown into the
forest, and a tall dune forms, obstructing its own maintenance, and inviting
the sea to move in and take it all out: sand, soil and vegetation. Then
a constructive phase follows with dune formation as we know it, followed
by a long period of stability.
This destructive capacity also allowed the 'living' beach/dune system to influence the natural selection of those plant species that could live in harmony with it, some native dune grass species, but only in densities allowing the sand to move freely. The natural state of a dune system is thus one of sparse vegetation, growing sparser towards the water's edge.
What we are doing by planting the dunes, either too densely or with inappropriate species, is to start its natural destructive cycle, resulting in permanent erosion until all the dunes and all our work has been annihilated. The situation is made more acute by centuries of excessive human-induced erosion, during which too much sand and mud was deposited in the coastal environment. The effect of this will be discussed in a separate chapter.
Let's begin by examining how dynamic dunes move naturally and how dune
plantings interfere with it.
The sand of which dunes and beaches are made must possess four conflicting properties:
The saltating sand comes to rest immediately in the lee of any object. So it drops out of the air immediately behind the foredune (top drawing), giving it a steeper slope to lee-ward. It makes the foredune roll landward in a peristaltic movement, eventually joining the second dune, which is also rolling inland to join the third dune and so on (second drawing). In order to stay healthy, dunes must be able to move (roll) inland. They can do so only when sparsely vegetated, which is the natural situation. If humans replant dunes in order to stabilise them, the dunes can no longer roll, which has serious long term consequences.
|Effects of dune planting
The first drawing shows a healthy situation with a steep wet beach, an area of dry beach, low fore dunes and rolling dunes further inland. The dune vegetation is sparse. Sand moves.
Once dunes have been planted for stabilisation, the dense vegetation
starts to trap the sand. Once trapped, the sand can no longer move. The
dunes grow and become a single large dune (second drawing). The plan appears
to work. But once the dune has grown sufficiently tall, it lifts the sea
wind from the beach, impairing its self repair mechanism. The sea starts
to eat into the tall dune, carving a steep bank (scarp). Sand can no longer
saltate up this bank and the dune stops growing.
|The Dutch have been fighting the sea for thousands of
years but it is a fight that is never won. Holland's entire easterly flank,
its border with the sea, consists of dunes and beaches.
The photo on left shows an aerial view of a typical Dutch dune system near Haamstede. Houses are well set back. There are no high-rise buildings. The dunes have been planted densely and they have become very tall. The beach has flattened. Sand banks appear where they never were before. The beach needs regular infusions of sand to combat erosion. The cost is high.
The photos below were taken in the top left corner of this aerial view.
At Port Waikato (NZ) where the Waikato River enters the sea (in the far distance), the dunes have been stabilised by densely planted native spinifex. As a result, the dunes have grown too tall. Notice the flat beach and the sand banks extending far into the sea. There is too much sand in the sea.