Biodiversity as a priority
Even though there are fewer species of mammal living in the forest today than before, the fauna is extremely rich: the Brussels part of the Sonian Forest still has some forty indigenous species of mammal, including 18 species of bat. Bats are Europe’s most endangered mammals. There are also insects, spiders, birds, fish and amphibians, each of which has rare representatives in the Sonian Forest. The Sonian Forest is the only forest in Belgium where 7 different species of woodpecker can be found! The current managers and all nature lovers who are active there know this wealth and actively protect it. The defragmentation of the forest is an important thrust of the interregional structural vision. The aim is to reconnect large parts of the forest and connect it to green areas on the edge or even further away.
Deer are the biggest herbivores in the Sonian Forest, and Joe Public’s firm favourite. This species was reintroduced after it disappeared altogether, perhaps due to war-time marauding and a whole series of disturbances. Today, they are estimated to number around 150 throughout the forest. Deer love a herbaceous and shrub layers. They sometimes have a detrimental effect on sapling stocks because they eat the saplings.
You are most likely to see roe deer on early winter mornings. Then they keep themselves up in herds. Only the males have antlers, which they lose each autumn; in spring they grow back again. If a fawn crosses your path in May or June, do not jump to the conclusion that it is lost. The mother is probably nearby. Never touch the animal: your scent can scare the mother off and make her leave the young behind.
Occasionally, traces of wild boar are found. For the time being, only a few are resident in the forest. It is probably only a matter of time before this species re-establishes a viable population there.
The fox reappeared in the 1950s. Thanks to its extraordinary adaptability and intelligence, it is very successful. This omnivore eats small animals as well as forest fruits or insects and worms. It even comes looking for food in our bins! For public health reasons and to avoid overpopulation, it is better not to feed it.
Foxes are now penetrating into the city centre: they have already been spotted in the Cinquantenaire Park (which they reach via the Woluwe Park) and even in the Bruxelles-Luxembourg Station (which they reach via the railway embankments). Foxes are harmless to humans: rabies no longer occurs among wild animals. They fulfil an important ecological task by, among other things, keeping rat and rabbit populations under control and by eliminating all kinds of sick prey.
In spring 2017, for the first time in decades, a live badger was observed again in the forest. This magnificent animal is living proof that the defragmentation efforts are already bearing fruit.
The bat, a sympathetic and 100% natural insecticide The Sonian Forest is extraordinarily rich in bats: no less than 18 species out of the 19 that occur in Belgium are present here! This is due to the very high biological value of the forest, with its different types of forest vegetation and its numerous old hollow trees. Its favourite hunting grounds, mainly above and around the ponds of the Woluwe valley, the ponds of Groenendaal and the valley of the Argentine, are also an important asset.
The bat’s negative reputation is totally unjustified. This animal is a very useful and powerful ‘insecticide’: every night it devours insects equivalent to about half of its body weight!
In order for bats to survive and be preserved, it is especially important to preserve their day and night-time habitats, especially during winter.
The Siberian chipmunk: an established tourist. A number of exotic animal species, such as the Siberian chipmunk, feel at home in the forest. It is not a real competitor to our native squirrel because the latter is more likely to be found in trees.
Other mammals. With a bit of luck you may spot an ermine, a weasel or a polecat. Rabbit numbers go up and down, according to the year.
A hundred or so bird species nest here, including a number of notable ones: black woodpecker, common buzzard, European honey buzzard and hawk. Old trees with broad trunks are often excellent habitats, not only for these striking species, but also for the short-toed treecreeper and the nuthatch. Over the past few years, the wood warbler has been heard more and more frequently, and if you are lucky, you will come across a woodcock and see the sparrowhawk hunt between the beech trunks. The cuckoo and the golden oriole have unfortunately disappeared, and hopefully they will return one day. The kingfisher is a striking sight near all the ponds in the forest. In addition, you will find many water birds such as coot, moorhen, great egret, tufted duck, gadwall, pochard, mallard, little grebe and great crested grebe.
Insects and spiders
According to a digital species database, there are at least 168 different species of beetles living in the Sonian Forest, including the golden ground beetle, a rare species. A striking butterfly that, with a bit of luck, you will find during the summer months is the purple emperor. A typical beech forest species is the tau emperor, which is in fact a moth whose beautiful, coloured males go in search of a female during the day. In May and June, you can see it racing erratically through the forest. Spectacular insects such as (some rare) dragonflies can be found in and around the waters of Rouge Cloître / Rood Klooster, Groenendaal, the park of Tervuren and the domain of the castle of Terhulpen / La Hulpe.
At least 170 species of spiders can be found in the Sonian Forest, including some that are rare or even unique to Belgium, for example a colony of the purseweb spider, a species related to the imaginative tarantula. No need to be afraid though – this is a very small and totally harmless offspring of that particular group! Their decline in numbers is probably due to the disappearance or degradation of their habitat due to soil compaction and erosion caused by walkers, cyclists and mountain bikers leaving the trails. On top of this, there is a clear lack of undisturbed open spaces.
The European stag beetle is the largest and undoubtedly most spectacular beetle in our country. The adult male has an impressive jaw and can grow up to 8 cm in size. The larva is also very large and lives mainly in decayed oak stumps. During the mating season, the males have real gladiator fights to please the females… Fortunately, these fights usually end without injuries. To preserve the ‘breeding places’ in the vicinity of the forest and to allow them to grow, the forest rangers provide more upright and lying dead wood.
The wetlands and ponds of the forest are important not only for the more general amphibian species (European common frog, common toad, alpine newt…), but also for the rarer ones (fire salamander, northern crested newt…).
Some ponds, such as those of the Rouge Cloître / Rood Klooster and Groenendaal, contain remarkable fish species such as European bitterling, protected by the European Habitats Directive.
There are only two native reptiles in the forest: the slowworm and the viviparous lizard. These cold-blooded animals love open spaces where they warm up in the sun: forest edges and meadows are their preferred biotope.
The red-eared slider is an exotic species that unfortunately was dumped in the forest. They are originally aquarium animals that people want to get rid of, and which are eventually left behind in the ponds.
All indigenous amphibians and reptiles in the forest are under threat. In addition to the traditional causes (habitat destruction, human interference, water pollution, competition from exotic species, diseases, etc.), the systematic and excessive release of fish can also be disastrous. It eats the larvae of amphibians, drastically reducing their numbers.
The beech: emblem of the Sonian Forest
Impressive, upright beeches reaching high into the sky: this is what the Sonian Forest is known for. The Sonian Forest is aptly nicknamed ‘the Beech Cathedral’.
Today, 70% of the Sonian Forest is made up of beech forest. This dates to the time of the Austrian Habsburgs (1714-1795). The young Austrian landscape architect Joachim Zinner had beech trees planted en masse. Today, they still make up the majestic beech cathedral. Beech trees over 200 years old are no exception. Beech wood used to be used to make charcoal. Moreover, it is very suitable for furniture and toys because the timber is hard and does not splinter.
Beechnuts are a delicacy for many animals in the forest: deer, squirrels, and mice. Yet a beech forest also has disadvantages. The dense foliage of the beech lets little light through. This makes it difficult for young plants to germinate. In addition, the fallen leaves do not easily rot. As a result, the un-rotted humus accumulates and acidifies the soil.
Superman of the forest: the oak tree
The oak tree is one of the most impressive trees in the forest. It is the walker’s favourite because of its powerful appearance, and at the same time the lumberjack’s favourite because of its fixed value on the timber market.
Even those who are not nature specialists will recognise the oak tree from its graceful yet commanding majesty. It is a majestic tree that can grow over 30 metres high and up to hundreds of years old. The crown of the oak tree is widely branched; its trunk is stocky. The grey-brown bark turns black with crevices in older trees. The oak tree is a natural world in itself: not only oak processionaries are fond of its trunk. Between the ridges of the bark and in the foliage, it is teeming with caterpillars and larvae, beetles, ants, and spiders. The oak is also loved by squirrels and jays, deer, and wild boar, all of which are attracted by the acorns.
Oaks come in many varieties. Typical for our regions are the pedunculate oak and the sessile oak. In southern Europe there is much more variety, with the Turkey oak, Hungarian oak, Iberian cork oak, holm oak and kermes oak. The Americans traditionally divide their many oak species into a white and a red group. The latter includes the northern red oak, which has also been planted in many places in Europe.
The waning popularity of the northern red oak tree
The northern red oak was imported into our regions from the north-east of America at the beginning of the 19th century because of its rapid growth – and hence its profitable timber yield in a short space of time.
Today it is a less welcome guest in the woods. Its fallen leaves form a thick layer of litter, which does not easily rot. As a result, the soil acidifies, and many native species can no longer grow there. Under the broad crown of the northern red oak, for example, you often only find … its own seedlings. When it is cut, it will therefore be more likely to get the axe than a native tree. Yet northern red oaks do not deserve to be completely eradicated either. Squirrels and bats particularly appreciate a cosy nesting cavity high up in a tall, old northern red oak tree.
The prickly husk of the sweet chestnut
Originally, the sweet chestnut is found in the Mediterranean. Since the 16th century – as Brussels tapestries testify – the sweet chestnut has grown in small numbers in the Sonian Forest. Its wood is particularly tough, elastic, and durable. Tanning agents arm the wood against moisture and mould. Chestnut wood lasts particularly long. Even untreated, it remains unaffected for years. This makes it particularly suitable for grazing posts or fencing, but also for roof trusses and parquet.
Its fruit, the sweet chestnut, is easily confused with that of the horse chestnut. To be on the safe side, remember that the sweet chestnut has three pointed chestnuts in its prickly husk, close together. In the horse chestnut’s fruit, you will only find one perfectly round chestnut.
An elegant representative of the olive family: the ash
The ash is a tall and impressive tree, with slender ascending main branches. It carries opposite compound leaves, consisting of nine to thirteen irregularly toothed, almost sessile petals. In winter it is easily recognisable by its black, velvety buds in the shape of a mitre. Fruit-bearing ash trees retain their winged nuts, ‘samaras’, for a long time after the leaf fall. These hang in dense bundles all over the crown.
The ash seeds easily and grows quickly in richer, moist soils. It holds up well in the forest as soon as it has taken its place in the upper canopy. As an adult tree, ash does need light. More often you find it in hedgerows, and it also thrives in forest and urban avenues. Typical for parks and gardens is the beautiful weeping variant, which is propagated by grafting. Ash wood is bendable and resistant to shocks. It is therefore used to make axe and spade stems and gymnastic equipment.
Maple trees feel at home in the Sonian Forest
How can one recognise a maple tree? By its hand-shaped leaf, its smooth, thin bark, and the fruits that you can spin like a helicopter thanks to their wings.
In our region, the common maple is particularly widespread. The Norway maple, too, which with its pointed leaf lobes can easily be confused with a plane tree, can be spotted in our forests. The smaller field maple on the other hand, with smaller leaves, is much rarer. In the arboreta, especially in that of Tervuren, you can find many other exotic maples. In autumn they guarantee a beautiful play of colours.
In the Sonian Forest, which is 70% beech, common maple (2%) is the fourth most deciduous tree species, after pedunculate oak (13%) and sessile oak (2%). It will be seen even more in the future, as it naturally reproduces easily and, as a young tree, tolerates shade well. This is good news for biodiversity in the forest, as the dead wood of the maple attracts many rare fungi, lichens, and mosses.
About alder cones and alluvial forests with alder
In winter, the alder can be recognised from afar by its dense, somewhat confused profile, with fine branches, adorned with many long, pendulous male catkins and smaller, upright female ones. After pollination, the female catkins form a kind of cones, which become somewhat woody and remain hanging on the tree for a long time after the seed has fallen. The black alder, which is most common in our country, has an inverted, ovate, double serrated leaf that is stumped at the top and often U-shaped incised. The leaf of the rarer grey or speckled alder is pointed at the top and have felt-like hairs at the underside.
The black alder does well in wet places. It is a typical species for brook and spring areas. It also does well on nutrient-poor and peaty soils thanks to its nitrogen-fixing root nodules. You often find it there together with the soft birch. On loamy soils the alder often grows together with the ash.
Over the centuries, alder bushes were mostly managed as coppice because they easily sprout again after felling and thus provide continuous firewood and coppice. That is why you see so many multi-trunk alders. Alder is not very durable in contact with the air, but it is very useful for constructions under water. It is therefore often used for walls of wells. It is peculiar that the freshly sawn wood immediately turns a bright orange.
The virgin white birch
In summer and winter, you will immediately recognise the birch by its bright white bark, often speckled with black rhombic dots. Birch is a pioneer species par excellence. In July-August the seeds detach themselves from the catkins to be carried away by the wind on wings. Birch germinates in any place and holds up well everywhere, even in the gutter or between cobblestones.
There are four species of birch in Europe. The silver birch feels particularly at home in the Sonian Forest.
If there is one tree that is truly indigenous, it is the hazel. It colonised our regions long before there was any mention of Belgium. After the ice age, it was one of the first trees to form forests here again. By digging up large quantities of shells near settlements, archaeologists have shown that even our ancestors sometimes liked a hazelnut. No wonder: the nuts are full of fats and proteins. They certainly helped prehistoric man to get through the harsh winters.
The hazel is our earliest spring bloomer. Ornamental hazels distinguish themselves by curled twigs or red tinted leaves. The multi-stemmed shrub likes some shade, but under a dense foliage it gets a hard time. That is why it is better off in a copse.
You can recognise the hazel by its large, almost round leaves, which are somewhat reminiscent of the large-leaved lime. But thanks to the double serrated leaf edge and the short tip at the top of the leaf, it is easy to distinguish. The malleable brushwood lends itself well to the heavier wickerwork, as in wickerwork walls.
The elm: an endangered tree species?
In prehistoric times, elms were an important part of the Atlantic primeval forest in the Low Countries. With the rise of agriculture, their share among the native trees gradually decreased. The foliage was used as fodder; the hard wood was particularly popular with furniture makers and cartwrights. Since elm disease struck at the beginning of the 19th century, the number of elms has been greatly reduced.
The leaf of the elm looks a bit like that of the hornbeam, but the leaf edge is double serrated. The elm leaf has a typical oblique leaf base, with one side set lower on the short stalk than the other. You can also easily recognise the fruit: a flat nut, embedded between two membranous wings. The three native species – field elm, wych elm and European white elm – are slightly more difficult to distinguish from each other. Twigs of the field elm often have cork strips. The flowers and fruits of European white elm are stalked.
The elm disease is caused by a fungus, which is carried by the elm bark beetle. The above-ground part of an affected tree dies quickly. The field elm can grow out again through root sprouts. The wych elm reproduces almost exclusively through seed and is therefore extra vulnerable to the disease. Because of its firm bark, the European white elm is best able to withstand beetle damage. The elms are not really threatened with extinction, but large, mature trees have become very rare.
The medlar: a rare delicacy
Like the chestnut, the medlar is a fruit tree that was introduced into our regions from the east in Roman times and which has survived in our forests. Unlike the chestnut, the medlar is rare. It produces flower and fruit, but the seeds are very difficult to germinate. It also grows very slowly. Usually it is shrubby, but it can also grow into a small tree. You can find the medlar in old, light forests, forest edges, hedgerows, thickets, and hedges.
The leaf of the medlar is undivided, lanceolate to inverted egg-shaped, smooth-edged, and covered with soft hair on the underside. You can recognise the wild medlar most easily by its flower and fruit. The flower is quite large, with five cream-coloured petals and five styles that are fused over half of their length. The ripe fruit is brown and round, slightly flattened and 2 to 3 centimetres in size.
You can only eat the fruit when it is overripe. Some nature lovers think that you have to let medlars rot before you can suck the tender flesh out of the rough skin. Not everyone is fond of this, but at least you can make a nice jelly from the flesh. Medlars were especially popular in the 19th century. In our regions there are four known landraces. Domesticated medlars can be distinguished from the wild species by their larger leaves and fruits. The landraces were often grafted on rootstocks of the wild medlar or related hawthorn.
Spring blooming plants
The Sonian Forest is in most places rather poor in species because of the dark, acidic loamy soils under the beeches. In a number of places, the soil is more open, richer and less acidic at the surface. Spectacular flower carpets appear there in the months March to May. Massive growth of wood anemones and bluebells is most striking, but more modest plants such as wood violets, greater stitchwort, cuckoo pint, wild garlic, and yellow archangel complete the colour palette.
This spring flora is best developed in the oak massifs of Rouge Cloître / Rood Klooster and the surroundings of the racecourse of Boitsfort / Bosvoorde, in and near the forest reserve Joseph Zwaenepoel and in the surroundings of the Drève des Bonniers / Bundersdreef.
In humid valleys, golden-saxifrage forms beautiful ribbon-shaped carpets. Spiked rampion, bird’s-nest orchid, herb-paris and common spotted orchid are rare plants. In the valley of the Vuyl brook the spectacular greater tussock-sedge and the striking pendulous sedge occur, together with the graceful great horsetail.
The ponds in Groenendaal and the adjoining meadows are very interesting in terms of plants. The important vegetation types in the Sonian Forest often have a striking spring flora on moderately nutrient-rich loamy soils with often a mixed tree species composition.
The most striking spring flora consists of wood anemone or bluebell, or a combination of both. In humid to wet places, such as at the foot of slopes, you can find wild garlic in spring. The plant even gave its name to the ‘Lookdelle’ (literally Garlic Dale) in the Walloon Region.
On poorer soils the spring flora is hardly present or even totally absent. There, wood sorrel is sometimes the most striking spring blooming flower. Broad buckler-fern, eagle fern and brambles often dominate the herb layer and on some poor soils there is well-developed vegetation with greater wood-rush.
Nettles, thistles, and brambles sting, and that is good!
Nettles grow mainly along road edges thanks to the regular supply of dolomite used as road cover and… dog excrement! Brambles, and sometimes also thistles, often occur in large numbers in places that suddenly receive a lot of light, such as after trees have been cut or blown over. They are excellent food for roe deer and many insects. People do not like them because they sting…, but in this way they make sure that walkers don’t leave the forest paths, and especially that a richly laid table is ready for all kinds of organisms that depend on it. They disappear again as the shade in the forest increases. They are typical pioneer plants (plants that appear first in open and light-rich places) that play their role in the life cycle of the forest.
Mosses, lichens, and mushrooms
Apparently, the Sonian Forest is the place where most mosses are found in Benelux. Some twenty lichens and over a thousand species of mushrooms have been described, most of which are unfortunately rare and endangered. The causes are clear: intensive picking and soil compaction due to excessive access and air pollution. Mushrooms are extremely important as an essential link in the natural recycling of organic material (such as branches and leaves) and as food for the animals in the forest. Many species live in symbiosis with trees and are therefore important for their health. Mushrooms, mosses, and lichens are strictly protected in the Sonian Forest and must not be picked!
Exotic species, especially invasive ones, can be difficult. They multiply explosively at the expense of naturally occurring species. They threaten local biodiversity and disturb the natural balance in various ways. They supplant indigenous species, eat them or make them diseased. By mixing genes (hybridisation) typical characteristics of local (sub)species disappear. This concerns both animals and plants. Once established, invasive exotic species can be almost impossible to eradicate. Therefore, prevention is better than cure! The most striking exotic animals in the Sonian Forest are the rose-ringed parakeet, the Siberian chipmunk, the mandarin duck, the Carolina duck, and the red-eared slider. For the time being, their effect on the native fauna does not seem to be as bad as expected. A number of exotic plants can spread en masse, especially if there is disturbance in combination with increased light supply. Black cherry and the northern red oak may become troublesome in the absence of competition from other shade-providing species. Thanks to appropriate management, these species will never be a real problem in the Sonian Forest. The Japanese and Sakhalin knotweed are plants that proliferate extensively. Especially in open areas they can be impossible to remove. These species originate from gardens or come along with earthmoving during infrastructure projects. The Canadian goldenrod pops up near open spaces after tree felling works but disappears over time due to shade pressure from growing young trees. Himalayan balsam appears as a striking blooming plant in wet places. The giant hogweed is very competitive in wet, nutrient-rich places. When touched, it can cause nasty burns. So it should be avoided at all costs.