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Health of the Brussels Sonian Forest

Since 2009 the Brussels-Capital Region has been monitoring the vitality of the 3 main tree species of the Sonian Forest (beeches and native oaks) on an annual basis. In 2012 the average defoliation – which reflects the general health of the trees – was 20% for beeches and 24% for oaks. This average defoliation dropped in the period 2009-2012. However, this evolution (over a limited period thus far) turns out to be difficult to interpret because so many factors could play a role.

The Sonian Forest covers nearly 10% of the Brussels territory and constitutes a natural and social heritage which is of great importance for the Brussels-Capital Region.

Vulnerability factors

However, there are various factors that render the forest vulnerable: visitor pressure, the characteristics of the soil (fairly dry soil on part of the slopes, compaction of the soil at the surface, a fairly shallow hardpan in various places, etc.), dominance of often old beech populations, unbalanced structure of the populations where age is concerned, air pollution, etc. In addition, the climate changes that are expected to occur in the coming decades could noticeably affect the functioning of the ecosystems, for instance when it comes to the growth of the forest population or the development of crop-destroying populations. In this context an exploratory survey has been performed at the request of Bruxelles Environnement - Leefmilieu Brussel (Daise et al., 2009). This study has shown that beeches and, to a lesser extent, common oaks in the Sonian Forest risk being affected if a climate change takes place according to the middle scenario (cf. Synthesis 2007-2008, file 'Zoniënwoud en risico‘s verbonden aan de klimaatwijziging' - Sonian Forest and the risks associated with climate change). More generally, signs of forest dieback have been observed – to a greater or lesser extent – practically all over Europe over the past thirty years.

Observation system

In this context the Brussels-Capital Region decided to set up a permanent observation system in order to map the vitality of the 3 main tree species of the Sonian Forest, concretely beeches and native oaks (sessile oak and common oak). These tree species make up 84% of the Brussels forest, in pure or mixed populations. The method – which is carried out according to a scientific protocol which was perfected at the European level – is based on the visual observation of trees located in 'test parcels' (parcels of 400 m x 400 m) in summer. The number and distribution of the trees within the forest were chosen in such a way that they constitute a representative sample. The observation refers to trees of a sufficient size (based on their diameter) and height (crown accessible to light) and takes into account different criteria, such as defoliation, discolouration, fructification and damage and symptoms. Defoliation – defined as the loss of leaves in the top part of the crown compared to a healthy tree – is an integrating criterion that reflects the influence of climate, soil quality, parasite attacks or the age of the tree, among other things. Hence, it provides a general idea of the health of the tree.
These observation campaigns have been performed in the Brussels-Capital Region since 2009.

Results of the observations

In 2012, of a total of 186 trees observed, 18 native oaks (or 28% of the total number of oaks) and 34 beeches (or 28% of the total number of beeches) were affected by a defoliation that was higher than 25% (in the Walloon Region a defoliation of more than 25% is considered abnormal). The average defoliation was 20% for beeches and 24% for oaks.
The average defoliation of both oaks and beeches dropped in the period 2009-2012.

Evolution of the average defoliation of all beeches and oaks included in the monitoring (2009-2012)
Sources: Delente. et al., 2012
Evolution of the average defoliation of all beeches and oaks included in the monitoring (2009-2012)

If we only look at the trees that were studied in each of the monitoring campaigns between 2009 and 2012, we also find that the average defoliation decreased for beeches (38% in 2009, 31% in 2010 and 28% in 2012, based on a sample of 58 trees) and oaks (30% in 2009, 24% in 2010 and 25% in 2012, based on a sample of 10 trees). This evolution, over a limited period for now, turns out to be difficult to interpret, given that there are so many factors that can affect the defoliation of a tree (growth and climate circumstances, fructification, diameter, location with respect to nearby trees, etc.).
In fact, the researchers have noted a clear improvement in the crown structure of the two species studied: 62% of the oaks and 54% of the beeches had a well-branched crown in 2012, compared to 0% and 8% in 2009. To get a better idea of this evolution, the causes of which are still unclear (falling of dead branches, differences in interpretation by the different observers, etc.), from 2014 onwards the monitoring campaign will also comprise a photographic follow-up of the crowns of the trees.
Where the discolouration of the foliage is concerned, the share of beeches with a discolouration between 10% and 60% rose from 6% in 2011 to 16% in 2012. For oaks the increase was also 10%, from 32% in 2011 to 42% in 2012. However, it is very difficult to determine which factors cause the discolouration of the foliage. The most common causes are mineral deficiencies, air pollution, the presence of parasites or periods of drought in summer or spring.
These figures are difficult to compare to the data recorded in the adjacent regions because the populations described there are different (age and density of the populations, soil conditions, the (micro)climate, the relief, etc.) and the quality of the observations may vary depending on the network. However, based on the defoliation values recorded in the nearby regions it is possible to determine orders of magnitude. For instance, average defoliation at the European level (30 countries), according to the observations in 2009, was 19.4% for beeches and 23.7% for oaks, while the values recorded in the Brussels-Capital Region were 37% and 28%, respectively. In Flanders the respective defoliation percentages for that same year were 15.7% (beech) and 21.7% (common oak) (Fischer et al., 2010).  When we look at all species combined, overall defoliation in Europe is increasing (Fischer et al., 2010 quoted by Braem et al., 2010).
In the Walloon Region a defoliation higher than 25% was observed for 25.9% of beeches and common oaks and 17.3% of oaks in 2011 (in 2012 this percentage was 28% in the Brussels-Capital Region, both for beeches and for oaks).

Management measures

The regional management plan for the Sonian Forest, which was approved in 2003, is currently being revised. These revisions are intended to take into account, in particular, new elements that have appeared over the past decade: risk of dieback for certain species (in particular beeches) in light of the expected climate change, the award of the natura 2000 status to the Sonian Forest, approval by the 3 Regions of a 'structural plan' that determines the major orientations and common management principles for the Sonian Forest, increasing demand for green recreational areas as a result of the growth of the population, etc.


Date de mise à jour: 29/05/2020