focus : health of the brussels sonian forest

Annual monitoring campaigns on the vitality of the 3 main species of the Sonian forest (beech and indigenous oaks) have been carried out by the Brussels Region since 2009.  In 2014, average defoliation - an overall reflection of the health of trees - was 23% for beech and 19% for oak.  Over the period 2009-2014, the average defoliation of indigenous oaks showed a downward trend whereas defoliation for beech appears to be relatively constant.  This evolution, observed over a limited timescale, is difficult to interpret given the high number of factors likely to come into play.

The Sonian Forest covers nearly 10% of the Brussels territory and constitutes a natural, social and cultural 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 ”monitoring plots” (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. Although it provides a general idea of the loss of tree vitality, this indicator alone is insufficient to establish a complete diagnosis of the state of health of trees. However, a reduction in foliage biomass can represent a tree's temporary regulatory mechanism when confronted by stress.

These observation campaigns have been performed in the Brussels-Capital Region since 2009.

Results of the observations

In 2014, of a total of 143 trees observed (88 beeches, 48 common oaks and 7 sessile oaks), 45% of the beeches and 22% of the oaks (or 36% of the total sample) showed a level of defoliation higher than 25% (this indicator is commonly used, at the European level in particular; a tree with a level of defoliation higher than 25% is considered as damaged).

With regards to average defoliation, it is 23% for beech and 19% for oak (20% for common oak and 13% for sessile oak). These particularly low percentages, compared to those observed in previous years, can probably be explained, at least in part, by the favourable climatic conditions which prevailed in 2014 (heavy rainfall from June to August). It should also be specified that these data are estimates with a significant margin of error.

Evolution of the average defoliation of all beeches and oaks included in the monitoring (2009-2014)
Source: Earth and Life Institute Environmental Sciences (UCL), 2014

According to the researchers, the high average defoliation observed for beeches in 2009 can be explained by difficult climatic conditions and high levels of fruiting, but also by the researchers' lack of experience.  If we ignore 2009, the defoliation average for beeches appears to be relatively constant over time. However, the average defoliation of oaks has tended to decrease since 2010.

By only considering the trees studied in each of the monitoring campaigns carried out between 2009 and 2014, the average defoliation of oaks appears to be constant (the sample, made up of 6 trees, is nevertheless too limited to draw any reliable conclusions) whereas the level for beeches has shown a slight downward trend (29% in 2010, 25% in 2012 and 25% in 2014 of a sample of 72 trees).

This evolution, observed over a limited timescale, is difficult to interpret given the number of factors likely to influence the defoliation rate of a tree (growth conditions and climate, fruit-bearing rate, circumference of the trunk, its position in relation to surrounding trees, shape of the branches at the top, etc.).

However, the researchers observed a clear improvement in the structure of the crown in the two species studied: respectively 71% and 47% of the oaks and beeches were densely branched in 2014 versus 0% and 8% in 2009. So as to better understand the evolutions of this defoliation and crown structure, for which the causes are still poorly attributed (falls of dead branches, differences of estimates between observers, etc.), a photographic monitoring of the crowns of trees has been included in the system of observation since the 2014 campaign.

Evolution of the percentage of oaks and beeches which are densely branched (2009-2014)
Source: Earth and Life Institute Environmental Sciences (UCL), 2014

Since the last 2-3 monitoring campaigns, a clear decrease in the discolouration of foliage in beeches and oaks has been observed. In 2013 and 2014, practically all the oaks examined had less than 10% foliage discolouration, which had never been observed since the implementation of the monitoring campaigns. For beeches, the percentage of trees with less than 10% foliage discolouration amounted to 84%, 100% and 87% in 2012, 2013 and 2014 respectively. The factors behind this improvement are nonetheless difficult to identify. The most common causes of discolouration are mineral deficiency, atmospheric pollution, parasitic attacks or droughts in summer or spring.

It is difficult to compare these figures with those observed in neighbouring regions insofar that the colonies described are different (age and density of colonies, condition of soils, (micro) climate conditions, topographical conditions, etc.) and that the quality of observations can vary from one network to another. The defoliation values recorded in the neighbouring regions make it possible nonetheless to establish orders of magnitude. At the European level then (30 countries), average defoliations observed in 2009 were around 19% for beeches and 24% for oaks. In 2014, the average defoliations observed in Flanders were 19% for beeches and 25% for common oaks (Sioen G. et al, 2015), which were comparable percentages to those observed at the Brussels level. However, 11% of the beeches and 32% of the common oaks showed a level of defoliation of more than 25%; all species combined, this indicator amounts to 21%. In the Walloon Region, the average defoliation observed in 2013 was 33% for beech, 35% for the common oak and 23% for the sessile oak; the proportion of deciduous trees showing a level of defoliation higher than 25% was 36%. 

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 three Regions of a 'structural plan' that determines the major orientations and common management principles for the Sonian Forest, or increasing demand for green recreational areas as a result of the growth of the population, etc.