Bark stripping rate as an indicator to monitor red deer populations

Wild ungulates, including red deer, have become abundant in Wallonia (Belgium) as well as in many temperate European forests. Their ecological and economical impacts, notably bark-stripping, are widely documented. Instead of inaccurate and time consuming census of deer populations, forest managers argue for sets of validated indicators to monitor their impact on the environment, including the damages on timber production. Number of bark stripped trees may vary considerably from year to year and between sites and the reasons of these variations are not yet fully understood. This study aimed to identify and assess the effects of determinant factors on damage probability. A systematic sampling inventory was realised from 2004 to 2016 on sensitive stands (8–36 years old stands of Picea abies (L.) H. Karst). More than 52,000 trees and 3,500 sampling plots where observed each year in 82 kha of forest divided into 64 blocks facing contrasted red deer densities. Annual climate, red deer population densities and landscape indicators were computed at block level based on pre-existing data sources. Yearly barkstripping probability was estimated using a generalized linear mixed model. Population densities, number of froze days, of heavy precipitations and snow cover proportion showed significant and positive effect on barkstripping. The availability of close agricultural lands was the only landsape variable having significant effect but the error structure had to be handle by application of a random effect. Standardisation of climate effect on bark peeling rate produced a more stable indicator over time than raw bark-stripping rate. That indice was strongly correlated (R² = 0.67) with abundance indices derived from spotlight counts in a forest block where red deer numbers have been significantly decreased during the study period. Our findings suggested that bark stripping rate can provide useful informations not only on economic damages but also on changes in deer populations.