Améliorer la gestion des ongulés sauvages en forêt tempérée - Enseignements d’une approche intégrée appliquée au sud de la Belgique

Populations of wild ungulates and their pressure on forest ecosystems have increased sharply in temperate Europe over the last two centuries. By disrupting forest regeneration and timber production, ungulates cause economic losses, alter stand composition and affect ecosystem services provided by forests. These impacts are particularly worrying in the context of the biodiversity and climate change crises, which heighten the need for resilient forest ecosystems and for renewable, low-carbon materials.
In Western Europe, hunting is the main ungulate population regulating strength. However, the lack of consensus between hunters and other stakeholders limits the effectiveness of management. Approaches based on population densities comparison with target densities remain hampered by the cost and imprecision of estimates. A form of adaptive management based on monitoring indicators sensitive to the balance between populations and their habitat has therefore emerged as the most pragmatic.
Setting verifiable and realistic management objectives is essential but remains challenging. The economic and silvicultural consequences of pressure result from complex interactions and manifest themselves with a time lag of several years. Reaching the management goals may require to bring the system beyond sensitivity ranges of the indicators. Some goals also may be unrealistic depending on the environmental and silvicultural context. To improve coordination among stakeholders, it is necessary to reduce these uncertainties. It is therefore essential to gain a better understanding of the relationships between the physical condition of ungulates, population density and forestry impacts.
This research adopted a comparative approach at the regional level to study the relationships between deer population density, the ecological status of their populations and silvicultural impacts along marked environmental gradients. (1) An ecological carrying capacity index was generated using a predictive model of fawn body mass, calibrated against 1,391 body mass data points from mortality reports. Two types of pressure exerted by deer on the Walloon forest were studied in relation to carrying capacity and deer population density. (2) The rate of bark stripping in young coniferous stands in the Ardennes was sampled annually over 13 years, across 2,570 km² of forest and more than 3,000 plots. (3) The disruption of natural regeneration in the acidophilous beech forests of the Ardennes and Lorraine and in the oak-hornbeam forests of Famenne was assessed using a network of 593 ungulate exclosure plots monitored over four growing seasons. The plots were distributed semi-randomly within the forest stands. Pre-established criteria of plot suitability for regeneration were applied to filter the sites. Moreover, local forest managers chose the exact location of the most favourable regeneration conditions nearby the theoretical sampling point or discarded the point when they assessed no favourable conditions were encountered nearby. The models developed identified the main damage drivers and the best ways to limit them.
The exclosure network confirms the decisive role played by ungulates in regional regeneration deficits. They caused growth losses to non-climax species corresponding to their local growth rates measured within the enclosures. This correlation did not result from the adjustment of deer populations to the environment’s carrying capacity since growth within the enclosures was independent of this. Two main avenues have been identified to address the observed regeneration bottlenecks. Increased regulation of all ungulate species may be necessary. The application of silvicultural practices that limit the vulnerability of shade-intolerant, palatable tree species could also mark hinge towards the diversification of beech-oak forests. The discussion challenges the limited attention given to this second approach in the light of the very recent shift from the closed forest to the heterogeneous semi-open vegetation paradigm.
The results of the exclosure network also demonstrate the value of such experimental setup for assessing the feasibility of regeneration objectives and the impact of ungulates on them. Identifying the obstacles to achieving management objectives and exploring ways to adapt silvicultural and forestry practices helps to address a key need for the implementation of adaptive management of the forest-ungulate system. Given that the pressures observed sharply respond to the understory vegetation growth, technical solutions must be fined restrict sample to favourable regeneration conditions without requiring operator expertise.