LES ENCLOS-EXCLOS : UNE TECHNIQUE ÉPROUVÉE POUR L’ÉVALUATION ET LE MONITORING DE L’ÉQUILIBRE FORÊT-GRANDE FAUNE

French. Afin d’objectiver l’équilibre entre la flore et la grand faune d’un territoire, l’installation d’un réseau d’enclos-exclos présente l’avantage d’être à la fois scientifiquement robuste et visuellement parlant. Il a été testé sur le cantonnement de Florenville, sur deux territoires à la pression de gibier contrastée.

English. During the last decades, populations of large ungulate herbivores have increased and their influence on forest vegetation have been clearly highlighted. Therefore, monitoring game pressure have become unavoidable for sustainable forest management. Such monitoring require a rigorous approach in order to evaluate objectively the forest-game balance. Under these conditions, the use of exclosure experiment has appeared an interesting solution. They enable to observe the ecosystem evolution due to forest-game imbalance as well as to detect any early deterioration of a situation that was initially considered acceptable. The enclosure-exclosure devices compare, on the one hand, the real environment (exclosure) fully accessible by wildlife and, secondly, a “control” fenced habitat (enclosures) inaccessible to all populations of large ungulates (e.g. wild boar, red deer, roe deer and mouflon). In 2006, enclosure-exclosure devices (4 x 4 m) were installed within forest gaps in order to monitore the understory dynamic in Wallonia. Observations were collected until 2012 and we computed a set of indicators characterizing the ecological changes due to large herbivores pressure on forest ecosystems. Such devices played a key educational role as it provide a visual comparison of two contrasted situations. Additionally, quantitative information were collected to perform further analysis (floristic survey, as well as height, number of stems and the cover of vegetation). We identified indicators of the ecological changes that responded within two years whereas other indicators required at least 4 monitoring years in order to quantify correctly herbivore pressure. Short-term indicators allow a rapid detection, but they have the disadvantage of being very sensitive to exogenous factors (climate changes, site conditions). At the opposite, medium-term indicators, such as regeneration growth, require a longer monitoring period, but they are more robust (mitigation interannual variations of environmental factors) than the short-term indicators. We evaluated the relative efficiency of various indicators using 17 enclosures distributed in two zones with contrasted deer densities. Some of the tested indicators allowed detecting significant differences between the two zones (Student's t test). In particular, among the short-term indicators we obtained significant result with the ground vegetation richness, regeneration richness, seedling density and height of Rubus fruticosus L. and Rubus idaeus L.. Among the medium-term indicators, the most relevant indicators were the average height of beech and birch, ground vegetation richness, seedling density and total cover of vegetation.