In Wallonia, the authorities recommend maintaining a distance of 100 m between any forest and the mast of a wind turbine. The aim of this study is to assess the relevance of this guard distance on the basis of acoustic data and data available in the literature. The question of the vertical distance between the rotors and the nearest canopy was also studied.

Analysis of recordings made at 199 listening points in Wallonia shows that bat activity measured on the ground is highly variable and cannot be explained very well by the environmental variables tested. A significant effect of the proportion of forest within 250 m, the distance to the edges, the density of the network of hedges and tree lines and light pollution was nevertheless demonstrated. With regard to distance from forest edges, the average activity for all species combined drops significantly beyond a distance of around 50 m.

Data from 13 campaigns of continuous recordings on measuring masts were collected in order to explore the role of the vertical distance between the microphones and the canopy or the ground, as well as that of the horizontal and diagonal distance to the edge. Despite a very wide dispersion of points, significant effects were found. The main effect was a decrease in activity with height. Activity at blade root height (33-90 m) seems to be little influenced by the distance to the forest edges.

These results from the analysis of Walloon data, combined with a detailed analysis of the scientific literature, have made it possible to draw up a series of recommendations concerning the possible move of rotors closer to forest areas.