Dune flora

The dunes contain many different kinds of vegetation. The flora of the wind-blown beach ridge, consisting of marram grass, lyme grass and sea holly, cannot be compared to the flora found in young dune valleys behind this ridge, where elderberry trees and sea buckthorn bushes flourish. In older dunes, there is a large difference between the vegetation of the northern slopes (moderate climate, lots of crowberry and ferns such as common poylpode), the southern slopes (strongly varying climate, lots of lichens) and the plains (primarily fields of heather).

Dune formation

Succession occurs in the dunes. The series begins with sea rocket and sand couch both of which can germinate on the flood-mark. The plants hold down the sand thereby allowing a dune to form. When fresh water is trapped in the dunes, marram grass will follow. Even lyme grass can then be found. As more and more sand is retained, sea holly and sand evening primrose will appear in sheltered spots.

In the shelter of the first row of dunes, one finds sea buckthorn. All the sea buckthorn bushes bunched together form impenetrable thorny woods. Sea buckthorn is an important plant for the dunes because it sets nutrients (nitrogen) via its roots into the bottom. These nutrients are used again by the sea buckthorn itself, however enough remains over for helping other plants to grow. In the area around sea buckthorn, one often finds thick woods with elderberry trees and a ground growth of stinging nettles and blackberries.

Because of the presence of all the plants, humus is also formed in the ground. All the decayed leaf refuse means even more food for other plant types as well. Eventually, grass and moss species will start growing well and will fix the dunes. At this point, an end to the drifting sand will have arrived.

Heather boundary

A remarkable boundary is found by Bergen aan Zee in North-Holland. The dune sand south of this boundary is rich in calcium and nutrients while the dune sand north of this boundary is much poorer in nutrients. Larger bushes overgrow the heather in the richer soils. Spindle, poplars, oak and beech grow in the dunes south of Bergen while large fields of heather strive well in the dunes to the north. This boundary is therefore referred to as the heather boundary.

The sea sand in the young dunes contains so many minerals that heather cannot grow. Only when they dunes grow older and the minerals are washed down to greater depths by rainwater will heather get a chance. Therefore, heather is only found in older dunes.

There is less difference between the northern and southern dune regions in the younger dunes lying close to the sea, where the sea wind dominates. There is also calcium present in young dunes in the north, which the sea itself has contributed in the form of shells. this calcium washes out after several centuries. One finds plants along paths made from shells that are normally characteristic of calcium-rich dunes in the south, such as vipers bugloss and sand-hill screw-moss. This is made possible thanks to the calcium in the shell paths.

The old dunes north of the heather boundary are thickly overgrown. The high clumps of crowberry are interchanged with low thickets of creeping willow and dewberries. The tough tendrils of the honeysuckle wind between the low bushes.

In addition to the crowberry, other heather species are found more inland: common heather. Heather bushes die off after tens of years. In the open areas that have been created, the moss and lichen get their chance to grow until the heather plants again begin to germinate. The most beautiful plants grow in the damp places, such as sundew, marsh gentian and common tormentil. This is also where a third heather species is found: bog heather.

Northern and southern slopes

When the dunes grow older, and leaching and decalcifying occurs, interesting differences can be observed between the northern and southern slope of a dune. Heather species establish themselves on the northern slope. Plant communities including crowberry and Adder's tongue are especially typical. Common heather is found more often on northern slopes more inland. The plants on the southern slope have a very tough time. The temperature varies strongly. On sunny days, it can reach up to 50 degrees Celsius. There can also be very dry periods. The plants that try to establish themselves there must be well adapted to this extreme environment. The vegetation consists of mosses and lichen with clumps of grey hair grass in between. The heath dog violet is also found on this side of the dune. Once rabbits have found the dune, lots of herbs will follow, among which lady's bedstraw and dune milkwort. It appears that from the difference in the growth between the two slopes, the strength of the dynamics has influence on the final climax situation.

wind-blown beach ridge	young dune valleys	older dunes
marram grass lyme grass sea holly sea rocket sand couch sea sandwort	elderberry sea buckthorn	heather common polypode lichen orchids silver birch heath dog violet	mountain ash creeping willow hawthorn burnet rose honey suckle moss

Source: de Vleet, Ecomare