The Sonian Forest is much more than the most important green space in the Brussels Region, the largest deciduous forest in Flanders and the most extraordinary beech forest in Belgium. It is an ancient woodland, with an exceptional and very well-documented history. As a uniquely preserved landscape dating back to the end of the last Ice Age, it has an exceptional ecosystem with a fauna and flora that is miraculously rich for an urban forest. Partly located within the Brussels-Capital Region, home to over one million inhabitants, this green oasis suffers from intensive recreational pressures, but air and water pollution and the effects of climate change also threaten the ecological balance of the forest. The Sonian Forest has three main functions: ecological, social, and economic. Nowadays, the ecological, social (and landscape) functions are more important than its economic role.
These threats and functions are central to the new forest management plans for the coming decades. This medium-term vision for a forest existing for centuries must be seen against the backdrop of a much longer future time horizon. In a coordinated and coherent way, the structural vision creates an interregional framework to achieve common goals in terms of recreation and ecology. It responds to the need to address common challenges and problems affecting all the regions. This has a major impact on the management of the forest as a whole.
The forest in numbers
The Sonian Forest covers a total area of around 5,000 hectares! One very significant feature are the plentiful beech trees, which cover about 2,650 hectares – more than half the forest. In addition, the forest is home to an exceptionally large number of trees with trunks over 80cm wide – on average 5 such trees per hectare!
Belgium’s three administrative Regions are making similar efforts to protect nature, valuing and preserving their heritage whether biological, scenic, geological, archaeological, architectural and the very soil itself. All three are at the basis of the EU’s recognition of large parts of the forest as a Natura 2000 site.
There are three integral forest reserves in the Sonian Forest: Grippensdelle (83 ha) in the Brussels Region, Joseph Zwaenepoel (232 ha) in the Flemish Region and Ticton (23,5 ha) in the Walloon Region. Within these reserves, nature is allowed to simply take its course, with no human intervention. Since 7 July 2017, UNESCO has recognised these forest reserves as a world heritage site.
An eye on the fragile forest
In the course of its history, the forest has undergone major changes as a result of social developments. Still remaining today are some parts of the original ‘Charcoal Forest’ (Silva Carbonaria) dating from the time of the Romans: these include the actual Sonian Forest, Hallerbos and Meerdaalwoud. In order to avoid a further shrinkage of the area, since 1959 the Sonian Forest has been given legal protection as a “Preserved Landscape”. The forest managers are also attentive to other problems affecting the forest, such as motorised traffic, all kinds of pollution, etc. As a result, management measures taken over the last 20 years have already significantly reduced the impact of these problems, and future strategic plans will continue to take this into account.
New Management Plans
New medium term management plans for the Sonian Forest (one for each administrative Region) are being drawn up for the coming 20 to 24 years. They describe the demarcation of the Natura 2000 sites, the latest insights in forest management, the expected effects of climate change and the implementation of measures included in the Interregional Structural Vision. The management plans of the three Regions include the following main lines of action:
- Improving and enhancing the value of nature, with the following main objectives: forest defragmentation (reconnecting separated parcels of forest), increasing tree species diversity, improving forest structure, leaving more dead wood and trees (as a habitat for insects, fungi, birds and many other organisms) and paying attention to specific fauna and flora.
- Making the forest more resilient to the expected effects of climate change, such as extreme droughts in spring, very wet winters, severe rainstorms and windstorms.
- Finding a sustainable balance between recreation, protection of biodiversity and timber harvesting. Increasing knowledge about the forest.
- Preserving the landscape’s qualities and its geological, archaeological, soil and architectural heritage.
All these aspects are intimately linked with each other; they influence each other in a complex way. Typical landscape features such as the beech cathedral will be partially preserved. Open spaces, expanses of water and the edges of the forest will receive more attention. The forest will slowly but surely look more mixed and layered, a number of the avenues will be renewed; parts of the forest will no longer be managed, or at least managed less intensively, so as to create more opportunities for spontaneous, natural processes to develop. Sessile oak will be planted together with other rarer tree species such as hornbeam and small-leaved lime; more chances will be created for sessile oak trees to regenerate spontaneously. Natural regeneration of indigenous tree species will ensure that the forest rejuvenates itself as necessary. Imitating nature and directing or guiding this natural dynamic is the motto of the forest managers.
Forestry
New priorities in the forest
The recreational and ecological functions of forests have become increasingly important in Western Europe in recent decades, and rightly so. As a result, the economic function, which relates to timber production, has somewhat faded into the background. Many people think that a forest grows by itself and therefore does not need any management. While that remains true, spontaneous forest development however does present a number of drawbacks. For example, open spaces get overgrown with trees and water surfaces are lost. Forest avenues disappear or become unrecognizable as such, dead and unstable trees pose a risk to forest visitors, and the dominance of shadow-loving species constantly supplants others that prefer more light. Without human intervention, beech is becoming dominant in most parts of the forest, especially now that this species is rejuvenating spectacularly and abundantly. In the medium term, this means the end of the oak and the light-loving coniferous and other species, to the detriment of tree diversity. This, in turn, will diminish the diversity of species in the herbaceous and the shrub layers.
Logging is about harvesting a perfectly renewable and sustainable raw material. This is important in order to control the tree species composition and the amount of light that penetrates into the secondary layer (middle tree layer), the shrub layer and the herbaceous layer.
The FSC label, which applies to the Flemish and Brussels parts of the forest, together with the PEFC label, which applies to Walloon territory, is a positive reinforcement of the sustainable management of the forest. This means that the wood from the forest is certified as sustainable, and that it is recognisable on the market as sustainably produced timber.
A dynamic forest
Since the beginning of the 21st century, very good beech seed years (also known as ‘mast’ years) have become increasingly common. Since 2005, for the first time in living memory, this phenomenon, in combination with other, more equivocal factors, has caused this species to rejuvenate en masse, and to actually grow into the shrub layer and the secondary layer. However, just as we are witnessing this natural regeneration, a number of scientists are now questioning whether beech trees can still play an important role in the Sonian Forest in the 21st century, given the species’ sensitivity to the effects of climate change (drier springs, hotter summers and wetter winters).
Both climate change and our own ecological objectives are pushing us towards a more mixed forest. Mixing indigenous trees with all the shrubs and plant species that we can naturally expect to grow in these places is the long-term objective. In this way, the forest of the future will not only be less disease-prone and less exposed to the risks of severe storms, but will also be better equipped to withstand the effects of climate change. This is how we will make the forest more robust.
This is a long-term task in which the forest manager plays a crucial role.
Forest management aims to stimulate and accelerate natural processes as much as possible, by carrying out thinning and planting works, and harvesting individual logs instead of large-scale tree clearance. The guiding principle will be to bring in other tree species, enriching the forest with species that are now rare but suitable in the light of climate change. Important goals include increasing the proportion of dead wood, creating a more open forest with more daylight to allow a mix of new and old, broadening forest edges and clearings, and preventing, reducing or even excluding disturbances and pollution. Forest management has evolved with the times. To fight against nature is no longer part of the forest manager’s vocabulary: the leitmotiv is to imitate nature and to steer and speed up nature’s own work.
Rejuvenating the forest
It may be stating the obvious but trees do not enjoy eternal life. It is a fact that trees are often underestimated, even by forest managers. Real-life examples of beeches still growing even after 250 years do exist and oaks can live even longer. So why think about rejuvenation when the oldest trees have not yet reached their maximum age? The following considerations play a part in the reflection:
- Today, the forest consists largely of homogenous, even-aged wooded areas. This means that one tree species (usually the beech) dominates and that all trees in a plot are equally old. The storms of 1990 have clearly shown that this is not a stable situation, especially when the trees are old, very tall and with small crowns. That is a situation the forest manager wants to tackle. This is achieved by rejuvenation, not by large-scale clearing. Rejuvenation is best carried out on a small-scale, creating – over the long term- a mixed forest landscape with trees of varying ages and shapes, resulting in greater stability and greater biodiversity. Forest management will then become more dynamic.
- The fact that a desirable species of tree is not present everywhere also plays a role. If, for example, we want more (sessile) oaks in the forest, we will usually have to plant them because mother trees of that species are rare and seedlings don’t easily establish themselves. The same applies to lime trees and a number of rarer species such as wild pear and wild apple, elm, aspen, and a lot of shrubs that did not get a chance in the beech forest, where there has been so little daylight for such a long time.
In order to rejuvenate the forest under dynamic management, approximately 50 hectares of young forest should germinate or be planted year on year. Preference is given to natural rejuvenation from the spontaneous germination of seeds produced by the forest itself. That is a cheap way, ensuring a natural look for the forest; often, it happens en masse, which later on gives the forest manager a wide range to choose from, if they want to select individual trees. Planting manually is expensive, but has the advantage that the forest manager can decide on the species of tree. The rejuvenation process is therefore not at the mercy of the ‘goodwill’ of mother nature, who sometimes makes weird jumps.
The beech cathedral
This nationally and internationally acclaimed forest landscape risks falling prey to climate change. Nevertheless, the forest managers want to preserve part of this historical landscape as well as possible. They are taking a calculated risk with the beech. Originally, the beech cathedrals were not an end in themselves, but rather a consequence of resistance to large clearing operations, which is how forest management was carried out from the beginning of the 20th century: this allowed pure beech stocks to stand for a long time. It is hoped to continue with several hundred hectares of this type of forest and its characteristic management. Fine examples of these cathedrals can be found in the valley of the Vuyl brook, in the vicinity of the horse training track of Groenendaal, near the chapel of Drève de la Bonne Odeur / Welriekende Dreef and in Jezus Eik.
The forest
We’ve already mentioned the beech cathedral. The forest managers will maintain and rejuvenate them on a few hundred hectares so that the look of that typical landscape, including its species of tree will be maintained at an age of around 120 years.
B
eech will continue to be the most important tree species in the future, but will no longer be found in homogeneous, even-aged forest areas. Since 2005, it has been rejuvenating en masse and for the first time it is developing from a seedling into the intermediate layer of the forest. Over the next few decades, young beeches will change the aspect of the forest in places where they thrive. In groups, combined with other species, and whether planted or not, they will, with appropriate management, create a forest both diverse and uneven-aged.
In addition, the sessile oak, together with a number of other accompanying tree and shrub species, will gradually claim a more important share in the Sonian Forest. This species is better equipped to cope with the expected effects of climate change with its drier springs and wetter winters. It will usually grow together with the natural rejuvenation of beech and other native tree and shrub species. If the latter do not emerge spontaneously from seed, they will be planted in a targeted manner.
The racetracks of Groenendaal and Boitsfort / Bosvoorde, the ponds of Enfants Noyés / Verdronken Kinderen, the Grasdelle, the Blankedelle, the ponds of Groenendaal and Koningsvijvers, the Hoefijzervijver and the Pinnebeek pond, the ponds of Rouge Cloître / Rood Klooster and the grasslands in the arboretum of Tervuren are all important open spaces. These oases of light provide opportunities for water surfaces, often with very special flora and fauna. Bats and insects in particular need these open spaces, but light-loving plants such as the iris, the symbol of the Brussels-Capital Region, also prefer to make themselves at home there.
The old oak massifs in Boendaal, around Rouge Cloître / Rood Klooster and along Drève des Bonniers / Bundersdreef, are impressive because of their many thick tree trunks, but also because of their exceptional biological importance and spring flora.
Often, these old coniferous plots provide variety in the landscape, together with the typical fauna and flora that occur there.
The arboreta of Groenendaal and Tervuren are world-famous because of their age (early 20th century), the variety of species and their rationale. In Groenendaal, the aim was to assess hundreds of tree and shrub species regarding their suitability for our forests. In Tervuren, geographical forest massifs of different origins were planted: the so-called Nieuwe Wereld (‘New World’) part is simply out-of-this world spectacular, the sloping landscape and open grasslands making this geographical arboretum a feast for the eyes.
Forest edges
The gradual broadening of the edge of a forest (moss, herbaceous, shrub and tree layer) marks a transition to the closed forest area. Ideally, this border zone is about the width of one and a half trees, being both more species-rich and more visually attractive. In the Sonian Forest, however, these transitional spaces are usually still too abrupt and offer too little added value for fauna and flora. This will certainly change over the coming years.
Forest edges as transitions from open spaces to the closed forest will receive more attention. A network of tens of kilometres of forest edges will be created and maintained in the forest over the coming decades. Along roads and built-up areas, these forest edges not only provide more biodiversity and a more attractive view, but they also increase safety (falling trees). Cattle grids at the edge of the forest along the R0 and E411 motorways should prevent animals from becoming victims of traffic accidents or from causing them. The forest edges guide roe deer and other animals in the direction of the wildlife crossings on railway line 161 in Bosvoorde / Boitsfort and on the R0 motorway in Groenendaal.
The avenues
These lines of trees, some of which still bear witness to their past use as hunting grounds, undeniably determine the attraction of the landscape. A number of avenues will be renovated in the coming decades. This drastic local intervention is a necessary evil in order to ultimately achieve the desired result: straight-lined, cathedral-shaped views, often kilometres long.
Specimen trees
Countless specimen trees adorn the forest. In the Brussels territory, they are marked with a blue polygon visible from the footpaths. Elsewhere you can also discover for yourselves unusual shapes and trees with strikingly broad trunks such as the Schone Eik (‘Beautiful Oak’) in the Zwaenepoel forest reserve, the peculiar hornbeam in the Grasdelle and the Cosyn Oak in Rouge Cloître / Rood Klooster nature reserve. Most specimen trees need to be specially managed. Leave them intact and don’t step on their ‘toes’. Soil compaction, even by light human feet, is very bad for the trees.
The forest is more than just trees
As mentioned earlier, the forest landscape will slowly but surely change over the coming decades. The homogeneous evenly-aged forest areas in the Sonian Forest are relatively species-poor and vulnerable. Species diversity must be increased in order to achieve the Natura 2000 objectives and to make the forest less vulnerable. This will demand specific measures: more admixture of species, more dead wood, trees of different ages, attention to permanent and temporary clearings, preservation of ecologically important centuries-old trees (often in groups) … Together with the designated forest reserves, these increase biodiversity.
The nature reserves and their permanent open spaces consist of high-quality nature in the form of species-rich grasslands. In combination with the other habitats in the area, they form an extremely important nature network in the Sonian Forest. The many different microclimates that prevail there make for great species variety and a web of biodiversity.
Natura 2000 and the Sonian Forest
Natura 2000 is a coherent European ecological network of protected areas, established for the conservation of habitats of high ecological value and of rare fauna and flora. The sites have been designated on the basis of two European directives: The Birds Directive and the Habitats Directive. The Birds Directive protects wild bird species and their habitats, nests and eggs. In addition, the Habitats Directive provides for the conservation of special habitats and wild species (flora and fauna, except birds) of European interest. No less than 9 habitat types (acidophilous beech forests, beech forest with spring flora of bluebell, oak-hornbeam forest, alluvial forest of alder, tall herb fringe, nutrient-rich ponds, Brabant heather, oak-birch forest on poor soils and dry heath) are present or are being developed and protected in the forest. The presence of certain species such as the European stag beetle, the European bitterling (fish) and especially four very rare bat species (greater mouse-eared bat, Geoffroy’s bat, Daubenton’s bat and barbastelle bat) was decisive in this respect.
The Natura 2000 status of the Sonian Forest guarantees the conservation or improvement of these habitats with their characteristic animal and plant species. It obliges the respective administrative Regions to take all necessary protection and conservation measures to ensure the long-term survival of these species. However, this does not yet make the Sonian Forest a closed nature reserve. Human activities such as soft recreation (walking, cycling) and forestry remain possible if they do not compromise the conservation of protected habitats and species. Certain measures taken to protect Natura 2000 sites such as the Sonian Forest are financed by LIFE +, a European funding programme for environmental and nature projects.
The origins of today’s forest
The current look of the forest was created under the influence of human beings. Neolithic man left his mark 5000 years ago, for example in the remains of charcoal burning and iron ore mining. These activities have had an impact on forest cover and tree species composition. The development of the city of Brussels has strongly influenced the development of the forest since the 11th century AD.
The beech cathedral
At the end of the 18th century, during the Austrian and later the French domination, the Austrian landscape architect Joachim Zinner was charged with the ‘restoration’ of the largely ‘plundered’ forest. Beech was the tree of choice to restore the forest to its former allure. Initially, hundreds of hectares of largely fallow areas of the Sonian Forest were planted with this tree species. The current dominance of beech finds its origin there. The typical management system in the plots to be felled at the time was “tire et aire” (freely translated as “felling and clearing”). The large-scale clearance of dozens of hectares of trees around 100 years old in one fell swoop was then a normal practice, retaining 30 to 50 trees per hectare. The relics of this felling method can still be found everywhere in the forest: scatterings of thick or very broad beeches and oaks, beneath which grow a second layer of evenly-aged beech trees. At the end of the 19th century, public opinion began to oppose large-scale and drastic felling as a means of woodland management, and it was phased out at the beginning of the 20th century. Those plots of land originally considered ready for felling at the age of 100 years were allowed a longer lease of life and were thinned down. This is how the cathedral ‘look’ came into being. Today these cathedrals are 170 to 220 years old. At that age they start to decay. Even without further human intervention, this type of landscape would slowly but surely disappear.
The great changes of the 19th century
In the 18th century, the Sonian Forest comprised some 12,000 ha. Today, however, only 5,000 ha remain. The Sonian Forest has become much smaller in a short period of time. This woodland had always been in the hands of the governing authorities, but under the United Kingdom of the Netherlands (1815-1830) the Sonian Forest was privatised. It was transferred to Société Générale and from 1830, after the independence of Belgium, Société Générale sold about 60% of the forest. Many new owners developed their property and in less than 15 years the forest lost about 2/3 of its surface area. From 1843 onwards, hectare by hectare, the Belgian State purchased the forest. Management was transferred to the Administration of Waters and Forests. In 1983, Belgium was regionalised and the Sonian Forest and its management was divided between the three administrative Regions of the country. Since then, the forest complex has come under three jurisdictions, each with their own management system.