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The Diemerzeedijk

http://www.euronet.nl/users/mbleeker/diemer_e.html

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Lawrence F. London, Jr. - Venaura Farm - Chapel Hill, NC, USA
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Title: The Diemerzeedijk

The Diemerzeedijk

Introduction Teasel Navigation Centre
History
Dioxin Botany Education
Vegetation types
The "Savanna" Species list
Epilog Flowering in July/Aug

Unless otherwise stated, the below links are photos (JPG, ± 35 Kb)

Preview

 The city is a home for lots of plants and animals. And especially at the city's rim, this ecosystem can be at its best. Even at one of the most polluted places in Holland, there's a magnificent flora and fauna at the moment. I am referring to the Diemerzeedijk: a real wild and rough terrain, and a botanist's paradise. July/Aug. is the 'peak' when the place looks like a giant blooming garden / [R]. If you've got a chance to see it, you better do it soon, because the town-council has plans to change it into a... (you'll never guess): recreation area.

Imagine: a savanna. Hills, with scattered bushes and also some larger thickets. The shrubs are mostly Willows / [R] and Elder (Sambucus) / [R], plus large patches of Bramble (Rubus fruticosus) / [R]. In high summer it is mainly Buddleja / [R] that gets all the attention - from butterflies mainly. Between those bushes is not so much grassland, but rather all kinds of flowering herbs. In July and the first half of August they are at their best, displaying wave after wave of bright colours. The opening act is done by the modest Sedum acre (Biting Stonecrop) / [R], soon followed by the many tall herbs, like Brassica nigra (Black Mustard) / [R] and Chamerion angustifolium (Rosebay Willowherb) / [R]. All this time the star performer was quietly preparing for the main act: Dipsacus fullonum (Wild Teasel) / [R] forms whole 'forests' that turn mauve.

And there are many more colourful plants in this landscape: here's a complete species list, or look at this flowering list for the months July and August. It's exemplary for other such ruderal vegetations in this part of Europe as well I suppose. More about the flora further down.

Animal life is not bad either. Lots and lots of rabbits are all around. In winter they eat the bark off many shrubs, and those shrubs that get 'ringed' this way will die. The whole aspect of the landscape might be largely determined by the rabbits. But foremost they are the basis of the ecological pyramid: there are 3 couples of foxes, I've seen weasels and there's even a Goshawk (Accipiter gentilis) nesting and fouraging here. Talking about birds: many small song birds and plenty waterfowl, enough to please birdwatchers from all around. The  Bearded Tit (Panurus biarmicus)* is a specialty here, and you can see  Cormorants (Phalacrocorax carbo)* resting, while drying their feathers with spreaded wings. Going down the taxonomical hierarchy: there's a population of ring-snakes, and in spring you'll hear the frog's concerts. Many insects as well: dragon-flies, bumble bees and butterflies to mention just a few groups.

* Both sound files courtesy of
ETI (CD-ROM "Birds of Europe")

History

The Diemerzeedijk is a very old dike, near Amsterdam and a town called Diemen (map). One reminder is a stately house built in the year 1609. It never changed owner since those Republican times: the Hoogheemraadschap, which is the high council that is responsible for the water-household in this region. I was almost honoured when I received a tax bill from them the other day. But apart from this house, Rembrandt would have a hard time recognizing anything now.

The zee-dijk part of the name Diemerzeedijk means 'sea-dike'. And yes, this dike once protected us from a real sea, called the Zuiderzee, or Southern Sea. In 1932 a large dam was built between the provincies North-Holland and Frysland, which closed this sea off from the North Sea (the Afsluitdijk). The water gradually became sweet, and the coastral vegetation also changed, but there are still quite a few relicts I believe.

Human interference has been the main theme since then. On the land-side a canal was dug out, connecting Amsterdam with the river Rhine. So there is water on both sides of the dike now, and it's only accessable from either Amsterdam, or the other end about 5 km south-east (as the Cormorant flies). The canal was dug right through two inland waters (one of them called the Diem), so it has always been a real wetland / [R].

During the 'Rebuilding Period' after the war, a large part of this strip of land became a dump for industrial waste. This explains the hills I mentioned above: big piles of rubble, but now covered up with a sedimentary layer and a rich vegetation, because the dump has not been used as such anymore for about 25 years. Sometimes however, the subterrain does emerge, and you might have holes in the ground giving access to real dungeons... At he coastline, if you can reach it cutting yourself a way through brambles, stinging nettles etc, you might find yourself on top of a real cliff, enjoying a beautiful view over the former sea, now a large lake (IJmeer). Apart from this 'survivalist' aspect, this dumpsite (about 2½ km long and on average 500 m wide) became a unique piece of nature, perhaps not so much for the rarity of the species, but just the whole layout, the fields of colourful blooming flowers and shrubs are just magnificent. However...

Dioxin

In 1955 Philips-Duphar started to produce herbicides in a small factury near Amsterdam. They got a licence to dump their waste in the Volgermeer (north of Amsterdam) and at the Diemerzeedijk. Nobody had heared of dioxin in those days. This went on right until the factory exploded in 1963. By that time the dangers were known, and in fact this explosion was only one in a sequence of dioxin disasters occuring all over the world. Seveso 1976 is perhaps best remembered. In 1980 leaking barrels were dug up in the Volgermeer, and they were found to contain a relative high concentration of dioxin. The stuff was also found at a certain spot at the Diemerzeedijk. In fact, eye-witnesses of the explosion of the factory describe how the interior was covered with a layer of sort of a bad-smelling black mud. This eye-witness here, personally found a similar mud at the Diemerzeedijk. There is a small beach with inviting white sand, but when you step on it: "schlllupp", it's a quicksand with this black mud underneith. You won't get rid of the chemical stink for the rest of the day... This area has now been more or less closed off, and there are warning signs / [R].

Dioxin is a dangerous poison, and was part of 'Agent Orange', the defoliant used in Vietnam. It is one of the most poisonous chemicals we know: 10 milligrams is enough to kill a human. Luckily it hardly ever occurs in such lethal concentrations: it is mostly an inadvertent and much diluted side-product. Prolonged exposure to very low concentrations can also be dangerous however. The first symptomes of a dioxin poisoning typically show a few weeks after the fact. They include a skin-rash (chlorine acne) and neurological problems, resulting in depression or apathy. In a further stage the liver and the thyroid gland may be affected, and the victim will loose weight. Dioxin interacts with DNA (the heriditary material), which may lead to cancer or reduced fertility... The rabbits on the Diemerzeedijk seem to have no problem here tho...

During the 70-s and 80-s, dioxin was more notorious in the form of air pollution. It may come free with refuse incineration, especially when certain synthetics are being burned. This may rain out again, perhaps in grassland where it might be eaten by cattle. This way it comes back to man, not so much in the meat, but rather in the milk. A mother may also pass it on to her child with brest feeding. However, this problem has been largely solved in Holland now, with regulations (norms) and smoke filters on incineration ovens. Pollution of the soil is a larger problem. It is virtually impossible to clean it up. In the Volgermeer it was attempted with a strain of bacteria that could break the stuff down. It was not very effective. For the Diemerzeedijk there was talk about heating the soil with large electrodes, thereby capturing the chemicals in a sort of glazing process. Too expensive I presume. The plan is now to just isolate the polluted area by covering it up with plastic foil, and put a 2 meter thick layer of clean soil on top of it. This way, the area would not be fit for housing: they want to build a new residential area right next to it (IJburg), and the Diemerzeedijk would become a park. At the expense of the current flora and fauna of course.

A small bright spot is perhaps, that dioxin is hardly or not being absorbed by plants. Personally I did taste some of the many fruits you can find here (bramble, apples, cherries, berries), but I would not eat much of them. And this remains of course anybody's own responsibility. The real danger is more in the soil itself: children might get it in the mouth with dirty hands.

The Dutch version of this text deals more with the hazards, in case you're actually going to visit this area. It is more of a guided (survival) tour.

Introduction Navigation Centre
History
Dioxin Botany Education
Vegetation types
The "Savanna" Species list
Epilog Flowering in July/Aug

Vegetation types

 There are roughly 5 types of vegetation. One of those is a forest, with the White Willow (Salix alba) / [R] as dominant tree. Other, smaller willow species are more at the fringes of this forest. A lower stratum of shrubs is formed by the Elder (Sambucus nigra) / [R]. Botanically, this forest is rather poor in species, not at all like the marsh forest it ought to be (see document about Slovakia). The Small Balsam (Impatiens parviflora) is worth mentioning perhaps, and in April you might find some bundles of Daffodil (Narcissus pseudonarcissus major) / [R]. Part of this forest was destroyed last year, to clear an area for dumping mud from the canal. Birdwatchers were able to stop this destruction, because it turned out the authorities did not have a felling licence. The hawk's nest was saved.

Then there are the real marshes, mainly reedlands / [R]. Also very important for the birds, but again rather poor in plant species. Typha latifolia, the Bulrush is quite common here, and of course the Reed (Phragmites australis) / [R] itself. Looking down from the dike on these reedlands with small lakes between them, there's a lot to appreciate. It is in essence as Dutch as can be. And it has something for every season: in spring and autumn migrating waterfowl, in summer the marsh warblers and other singers. In September the reed starts to bloom, with beautiful dark purple plumes. Later in the season they die off, but the dead stalks remain upright, and keep on propagating the feathered seeds, right through winter, into next spring. And winter can be the most beautiful of all: imagine a little snow or glazed frost on the reed, a pale sun on it, preferably in contrejour... splendid! A little bit higher up the dike, but also rather wet, you can find Petasites hybridus (Butterbur) / [R]. It flowers in March, but has no leaves then. These come later, and in summer they reach giant proportions. Whole fields full of larger-than-life umbrellas: you can almost walk underneith them, and imagine you're in a tropical jungle. Very characteristic is also the Yellow Iris / [R], that blooms end May and June.

The flanks of the dike are in fact a gradient from the waterside to the dryer verges of the road on top. It is mostly a grassland vegetation, not much different from other road-sides and polders in Holland, except that it's left alone here. Heracleum spondylium (Hogweed) is quite common here, different kinds of Vicia (Vetch), and much more. A bit less common perhaps are Convolvulus arvensis (Bindweed) / [R] and kinds of Silene (Campion). And don't underestimate the common Thistle (Carduus crispus) / [R]: the flower-head is really beautiful and much appreciated by butterflies.

The fourth terrain type is a very recent one: the place where mud from the canal was dumped. This happened in 1994. Spring '95 it was a desert-like bare sandy plain. But the next summer it was already covered with a yellow jungle of Brassica nigra (Black Mustard) / [R]. There are many yellow Crucifers, and this one is recognizable by the fruits, that are pressed against the stem. That year, there was some work done in the area, something related to drainage, and a kind of 'natural' creecks were grinded out in the muddy/sandy soil. A whole new vegetation is starting to settle now in these gradients (wet/dry, sun/shade). Last autumn I counted 5 new species for the area, like Senecio inaequidens (Broom Ragwort), a recent invader from South Africa, by way of Belgium (it has been known in Belgium since the beginning of this century, but somehow it decided to wait with its invasion of Holland till the years 60 and 70). Another peculiar species here is Atriplex patula bracteata (Common Orache) with its large, a bit fleshy bracts. Short, it's very interesting to see this pioneer vegetation develop and reclaim the bare soil. A fast succession of vegetations rather. I wonder what this year will bring (if not bulldozers).

"The Savanna"

 The main attraction however, is the actual (former) dump site. It is an un-dutch mountainous landscape, with a rocky soil. The open vegetation with scattered shrubs reminds of a savanna. The herbs, typically very tall ones, are often grouped in fields, valleys or hillsides, which gives the impression of a planted garden. Also stinging plants like nettles, brambles and thistles often confine themselves to such 'spots', which makes it easier to avoid them. As a result, there's a beautiful colour display, whith dynamic changes in space and time.

Spring, early Summer

Winter and early spring are not very colourful yet. The flowers you could find are mostly in the other vegetation-types mentioned above. The nice silvery catkins of different Willow species are the first announcements of spring, but it is still a long time then till the end of April, when the shrubs and trees become green. There are a few Ash trees (Fraxinus excelsior) / [R] with their peculiar bundels of male or female flowers, that seem to sprout from a sort of black warts. A large tree like the Wild Cherry (Prunus avium) will also flower in April/May, accompanied by the smaller Bird Cherry (P. padus) / [R]. Soon after, the beginning of May, it is the population of beautiful small Apple trees (Malus sylvestris cv) / [R] that attracts the attention, the flowers always a bit messy, and like an aquarel changing from red to white. In places the ground is now blue: Ground Ivy (Glechoma hederacea) is giving us a prelude to what is to come. June is the month when things really start going here. The Elders (Sambucus nigra) / [R] set the tone now, with their soft white pancake-like inflorescences, together with shades of pink produced by at least 6 species of botanical roses. Like this Rosa rubiginosa (Sweet Briar) / [R], which has oil glands on the green parts: you can smell the plant sooner then you see it, and not only when it blooms - a nice sweet-sour apple-like smell. Rubus fruticosus (Bramble, Blackberry) / [R] is blooming now too, and allthough more appreciated for its fruit, the flowers are rather nice as well. They are of a very pale pink, not as white as its nephew R. caesius, the Dewberry, which grows more pressed against the ground. The first of the tall herbs to bloom is the tallest of them all: the majestic Heracleum mantegazzianum (Giant Hogweed) / [R]. Don't touch: it might give a skin rash in combination with sunlight. When Sedum acre (Biting Stonecrop) / [R] starts to fill spots with a bright yellow colour, you know summer is really starting off now.

High Summer

The pink-violet fields are characteristic now, being Chamerion angustifolium (Rosebay Willowherb) / [R]. Later in the season it will carry its fruit with hairy seeds / [R]. The plant lookes a lot like Epilobium hirsutum (Greater Willowherb) / [R], but the latter has the flowers more between the leaves, which are hairy. A third member of this family has larger yellow flowers: Oenothera biennis, the Evening Primrose / [R], a beautiful invader from across the ocean, that opens its flowers in the evening and attracts night-moths. A bit warmer yellow, and also covering whole fields, is Senecio jacobaea dunensis (Dune Ragwort). This is a form of S. jacobaea that has no ray flowers. There is a border between the two forms, that runs east-west at the latitude of The Hague, so in Amsterdam most plants are without rays, but sometimes you see some individuals of the type species. Both carry the orange-black zebra-striped caterpillars of St.-Jacob's Moth (Tyria jacobaeae). Want something else ? How about blue, we did'nt have that one yet: Echium vulgare (Viper's Bugloss) / [R], like the previous, a plant more afilliated with the coastral dunes in Holland. At the end of July the feast reaches its peak when Buddleja davidii (Butterfly Bush) / [R] comes into bloom. Two varieties have struck root here: a red-violet, and a blue-violet one, which blooms a bit later. At about the same time the majestic Dipsacus fullonum (Wild Teasel) / [R] displays its intriging mauve flowers. They start to bloom in the middle of the egg-shaped flowerhead, this ring of flowers then splits up, and one grows to the top, the other to the bottom of this flowerhead. The lower leaves of the plants are connected at the base, and catch rainwater, just like with Bromeliads. Perhaps this is a defence against ants, or according to another theory, the plant can absorb amino-acids from drowned insects: it is a carnivore. It is a protected species in Holland, but a very common, characteristic plant of this vegetation. Another rare plant in Holland is Verbascum nigrum (Dark Mullein) / [R], which has a small but healthy population here (not a garden variety). On the photo seen between Willow Herbs. And, last but not least, have look at this overview / [R] with six flowering herbs.

Late Summer, Autumn

After August, most of the spectacular flowering is over. Some plants carry on however, and there are some newcomers too. Matricaria maritima (Sea Mayweed) / [R], a Camomile-like plant, is such a persevering bloomer, just like this Polygonum aubertii (Russian Vine) / [R], although it's difficult to tell here when exactly the fruit replaces the flower. The related Polygonum cuspidatum (Japanese Knotweed) / [R] flowers late in the year. It is considered a pest in some countries, but the bamboo-like branches can be quite ornamental too... Humulus lupulus, Hop, also blooms late. The green 'bells' have a very characteristic smell. At one place, ironically, the plant is climbing in the signpost with a warning sign for the pollution... And there are the fruits: Brambles are plentyfull, and another nicety are these small, very red apples / [R], allthough the equally small green variety tastes better. Also very decorative are the Rose-hips / [R] (in Holland we call them "Rozebottels": the "bottle" part of this name is apparently English - or is it? Hips?). On the photo they are of Sweet Briar (notice the hook-shaped thorns). Those of Rosa rugosa ("Wrinkle Rose" ?) are more flat (wider than long) and become beautiful deep red. The hips of the "Dune Rose" (R. pimpinellifolia) are nearly black. Autumn is of course also the time for mushrooms, like this Coprinus comatus / [R], and there are some beautiful 'flames' of autumn-colour in the foliage too. Many of the tall herbs are summer-annuals, that is, the seed germinates as soon as it touches ground. So in autumn, you already see the rosettes of next year's plants. Same goes for 'monocarpous' plants, that do not flower the first year. The leaves of Dipsacus have characteristic small humps. The rosettes of the Spear Thistle can be quite artistic too (here its flower head / [R]). But the most beautiful are the rosettes / [R] of Verbascum thapsus, the Great Mullein, in late afternoon or early morning, when they are completely covered with glistering droplets of dew.

Epilogue

What's so special about this area ? Should it be protected ? The whole matter is of course very ambiguous. On the one hand there is pollution in the ground, and this should be cleaned up. Housing is a problem, building a new residential area must have high priorities. However, the way the authorities are planning to deal with it now is rather half-heartedly: not cleaning it up, just isolating the polluted area, not building on it, but next to it, and turning the area into a park or other recreational area. Given this situation, I think it should not be a matter-of-fact that the current vegetation must disappear, there should still be some room for discussion. Why should this park be a neat lawn with neat rows of trees, can't something of the currently established vegetation remain ?

Let me just put forward some botanical arguments. Point is, I've never seen a vegetation like this. It is a climax of a ruderal vegetation (if that's not a contradiction in terminis), with elements, perhaps relicts, of a coastral (dune) vegetation, and species that have 'escaped' from cultivation. When you think of it, a sort of a mix, not unlike the cultural diversity of people in the nearby urban area's :-), anyway, it's quite a unique ecosystem, and beautiful, so one should think twice before destroying it. Although Dipsacus is a protected plant, and Verbascum nigrum is rare, it is not so much the individual species that make this area of interest, it is more the combination of them, the dynamics, also in relation to the poison in the ground. How is this vegetation evolving? Are species adapting? Are species of the genus Salix hybridising here? What's the long term effect of dioxin poisoning? These questions could be studied here. And to have such a beautiful piece of real rough and wild nature just near town, is a value on its own. Perhaps we could think of an alternative kind of recreational use instead of a park... Fishing, hunting, survival tours, botany and ornithology education, ...

Introduction Navigation Centre
History
Dioxin Botany Education
Vegetation types
The "Savanna" Species list
Epilog Flowering in July/Aug

Last update: June 30, 1996