Position, size, and spatial patterns of bark stripping wounds inflicted by red deer (Cervus elavus L.) on Norway spruce using generalized additive models in Austria

KEY MESSAGE: Bark stripping wounds by red deer (Cervus elavus L.) were assessed on 9026 Norway spruce trees. Wound variables (length, width, area, relative width, height above ground, and angle) were analysed using generalized additive models with spatial soap film smoothers. Wounds located at the uphill side of trees were larger in summer than winter, and wound size depended on the diameter at breast height (DBH) and was spatially clustered. CONTEXT: In Austria, red deer (Cervus elaphus L.) is the main species causing bark stripping wounds. In winter, they often gnaw at the bark because of food scarcity; in summer, large pieces of bark are detached to help digestion, water, and nutrient uptake or as social behaviour. AIMS: The aim of this study was to analyse wound size (length, width, area, relative width (i.e., width divided by stem circumference)) and wound position (height above ground, angle (i.e., deviation between wound azimuth from slope line)) for winter and summer bark stripping wounds by red deer depending on stand attributes and to describe the spatial patterns of wound size within stands. METHODS: A total of 3832 wounds on 9026 trees in nine experimental stands of Norway spruce (Picea abies (L.) Karst.) located at 47° 19’ N and 14° 46’ E at an elevation of 1009–1622 m were analysed. A linear regression model was fit for wound length over wound width for each season. For all wound variables (wound length, width, area, relative width, position, height above ground, and angle) generalized additive models (GAM) with soap film smoothers, which predict spatial patterns, were fitted. RESULTS: Of all wounds, 79.5 % were inflicted in winter and 20.5 % in summer. Wound length (31.9 cm ± 31.2 SD), width (11.7 cm ± 6.0 SD), area (446.5 cm²± 558.1 SD), and relative wound width (0.177 cm ± 0.098 SD) were modelled depending on summer or winter bark peeling, DBH, and tree coordinates. For wound height above ground (119.4 cm± 26.8 SD) and angle (− 1.9 ± 97.3 SD), no meaningful GAM could be calculated. Seasonal differences between wound length and area were more pronounced than for wound width; differences in height above ground were minimal, but significant. Analyses further showed that wounds were mainly located at the uphill side of the trees. CONCLUSION: The spatial clustering of wound sizes might reduce the efficiency of thinning to remove heavily damaged trees in bark-peeled stands and might increase the number of sample points required to assess deer impact in forest inventories. Also, the uphill location of damages is an important information in inventories.