Duinstruwelen in het Deltagebied

Samenvatting

The main part of this study on dune scrub communities in the Delta region of the Netherlands, especially the dunes of Voorne, was carried out from 1965-1967. It is part of the inventories of the State Institute for Nature Management which started these long-term projects in 1959 in this region. The actual charge for the study has been given by the Stichting Wetenschappelijk Duinonderzoek (a foundation for dune research) and it has been financed by the Netherlands Organization for the Advancement of Pure Research (Z.W.O.). It is connected with similar studies on dune grassland and dune valley communities. Besides the question of classification and typology, these studies are in particular concerned with the behaviour of the communities in space and time in relation to their environment in a broad sense. The main purpose of this approach is to create the best conditions for their conservation and their management. Also the methodological problems with the analysis and synthesis of very complex vegetation types demand much attention.

Dune scrubs are (vegetation) communities in which woody, scrubby plants form the bulk of the vegetation. The species of scrubs occur mostly in narrow zones along forests and in hedges and form a so-called mantle. In their most complete expression they form a part of a grassland-outskirt-mantle-forest complex, but often one or more of these elements is lacking. In some landscape types one may find extended mantles, as today for instance in sea and river dunes and on abandoned chalk grasslands and fields in many parts of Europe.

On a smaller scale they form more or less clear band-type, ribbon-type or lattice- type patterns, sometimes around 'islands' of forest. Mostly they are surrounded by herb communities (outskirt) and grassland elements. In young or strongly disturbed dune areas they have a less complicated structure.

Although we suppose that in the so-called young dune landscape, formed after 1200 AD (and also in earlier periods of dune building) dune scrub communities have always existed, in our time they have come to occupy a relatively large area. Until the beginning of the 20th century the dunes were intensively grazed by cattle. When grazing was stopped, the scrub developed rapidly. In the last decades the vegetation has become impoversished by catchment of drinking water and all kinds of urban activity.

The dune area of Voorne has the greatest variety in dune scrub development for the following reasons:
a) large areas of the dunes have been formed after 1910 by extension of the coast line. These areas have never been grazed, except by rabbits and therefore give the most complete picture of the primary succession in dune scrub communities to be found in the Netherlands.
b) large parts of the area were grazed until 1910, but since then they have been protected as nature reserves, so that they are comparatively undisturbed. In these more inland dune areas secondary succession developed and on yellow dunes also primary succession occurred.
c) Voorne has been used comparatively little as a drinking water catchment area. Therefore the wetter and moister scrub communities are well represented.
d) the islands in the Delta region are surrounded by river estuaries. This results in a characteristic chain of isolated areas, different from the rest of the Dutch dune area on the mainland and in the Wadden district (in which the dune sands are poor in lime).

The dune scrub descriptions consisted of about 750 relevees in total, together with more than 360 species. 76 relevees are not scrub vegetations in a pure sense: the coverage of the scrubs was less than 30% and therefore they were called outskirt-communities. Although they were handled as a separate group, they have been integrated in the study because of their close connection with the scrub communities, in space as well as functionally.

All 750 relevees have been noted on hand punch cards. They have been divided into 13 'primary groups' mainly based on structure and predominance of the scrub species. Also simple calculations have been made with these punch cards, worked out in a former publication. They have served as a basis for the computer cards and in the final stage of the study they were used again to test the computer results. They are easy to consult and can be used in the future for different purposes.

In the Department of Mathematics of the Agricultural University, Wageningen experiments were carried out to arrange the material, to calculate presence and frequency, rarity and groups of species. Also calculations were done on parts of the groups of species in groups of relevees and the possibilities on classification were examined. The 13 primary groups served as a basic classification.

With 670 selected relevees, arranged in 85 basic 'types' two ordinations were carried out. The first with a rather heavy accent on the predominance of the scrub species; in the second the scores of the species (0-5) were transformed in a 0-9 scale, which gives a more balanced expression of the complete species composition. The computer program Ordina, applied by the Department of Geobotany at Nijmegen University gave sufficiently detailed results. These were interpreted roughly.

Because the main species, the scrubs, are very important for the composition and interrelations of the communities in the field as well as in the ordination models, their behaviour in space and time was described. The patterns in space of the reference points of 4 axes in the ordination model (Hippophae rhamnoides; Crataegus monogyna; Salix repens; Sambucus nigra; Ligustrum vulgare, Rubens caesius) were analysed.

The changes in the environment, in history as well as those that are operating at present are discussed extensively. The former consist of the changes in the coast line, the influence of grazing and its sudden cessation in the beginning of the 20th century, the changes in the free water table, the influences of recreation activities and the plantations of woody species in the 19th and 20th centuries. The present changes are on a much smaller scale and - as it turns out - very complex and difficult to analyse. Despite all attention paid to different types of succession it is not easy to understand the present distribution of the scrub species mainly because of the extremely complicated and dynamic environment of the dune landscape. Many of the scrub species appear to be characteristic for more or less severe degradation of organic material in vegetation - and soil structure. Hippophae rhamnoides and Crataegus monogyna are species, although quite different in their starting point, both characteristic of an environment where the instability is high and diminishes suddenly (which - in turn - causes an increase of instability inside the ecosystem). Ligustrum vulgare is supposed to be found in regressive successions as well as certain Rosa and Rubus species and to a certain extent, Salix repens. This species, however, is considered to be characteristic for environments where slow mineralisation (by blowing sand or fluctuating groundwater) occurs in an organic soil layer that is already present. On the other hand Sambucus nigra occurs in mineral environments where a surplus of organic material is deposited.

They form a 'kap'-'sluier' contrast (in which the 'kap' environment on the one hand is characterized by degradation of organic material, already present in or above the ground. 'Sluier' (shroud) on the other hand is characterized by (rapid) mineralisation of organic material supplied from elsewhere). All other scrub species are considered from their position to the reference points mainly in terms of different degrees of mineralisation of organic material, either locally present or supplied from elsewhere.

With this background the main groups and basic types of scrub communities are compared with each other.

Although the study was concentrated on Voorne, the dune scrub communities of Goeree, Schouwen, N. Beveland and Walcheren were studied in a comparative way.

On Goeree the Hippophae-Ligustrum-Sambucus scrub on the Punt is the most important. Unfortunately a motorway, connected with the Delta works has recently been built right through the area. This is an extensive, relatively homogeneous and dense, dry scrub on younger dune ridges on Voorne. In some places a pioneer Hippophae scrub is growing at the seaside; on the inland side some Crataegus and other higher developed scrub occur. The Hippophae-Sambucus scrub on the higher parts of the Kwade Hoek are rather young, they are usually wetter and have many higher grasses and herbs.

On Schouwen, the variety is somewhat greater because the inland dunes are poorer in lime. The Verklikkerduin and the Zeepe have an extensive Hippophae- Ligustrum - Sambucus scrub with Epilobium species and lower Salix repens Ligustrum vulgare plus Hippophae - Ligustrum scrub, respectively. On the landward side of the coastal ridge in the Western and Northern part of the island Hippophae scrub with many grasses can be found. In the innermost part also Sarothamnus scoparius scrub can be found. On Goeree, Walcheren, Schouwen, Rubus ulmifolius is present.

On Noord-Beveland very few scrub communisies have developed, namely in the south of the dunes near the dam to Walcheren. This is HippophaeSambucus scrub with some Ligustrum.

On Walcheren, in the water catchment area near Vrouwenpolder, a well developed Ligustrum vulgare - Crataegus monogyna scrub is growing. These dunes are dried out and disturbed by eradication of the hawthorns because of fire- blight. In the outer zone there is a rather grassy Hippophae - Sambucus scrub. The rest of the dune ridge of Walcheren is rather narrow and in general the dune scrub is not so well developed. It mostly consists of Hippophae Sambucus scrub and is frequently disturbed by human activities.

Some remarks were made on the possible application of this study for the ecology and management of other Dutch scrub communities.

The classification in 16 main groups and 85 basic types has been compared with the existing system of Dutch plant communities and with more local classifications of dune vegetation. Some of the questions raised have not been answered, especially the connection with the outskirt communities and the dune valley vegetations. (Research is being done at the moment on these vegetations by others). All scrub vegetation types are supposed to belong to the Berberidion Br. Bl (1947) 1950.

A Rhamno - Crataegetum and Ligustro - Betuletum were described, the former as a new association, the latter as a replacement of the Crataego Betuletum BOERBOOM 1960. Quite a few degradation stages have been described under the neutral name 'community'. Their exact place in the syntaxonomical system is still uncertain. In the near future a more detailed publication on these problems will be prepared.

The lower Salix repens scrub and Salix repens - Ligustrum vulgare scrub also have been included in the Berberidion because of their species composition. Thus the Salicion arenariae belongs to the Berberidion or has to be connected with other dwarf scrub communities. The Salicetum arenario - purpureae is not a part of the Salicion albae. Until the moist and wet mantle communities have been studied in more detail it is also included in the Berberidion.

Finally some remarks are made on the management of dune areas where scrub communities are present. Very little attention has been paid to this problem until now. It is suggested that certain parts of the dunes, especially in favour of the (future) scrub development, should be grazed very extensively. The area of forest and scrub should be much larger than the area of grassland.