Norway spruce (Picea abies (L.) Karst.) is a major production tree species for the European wood industry. However, it is highly sensitive to bark stripping (BS) by red deer (Cervus elaphus L.), which causes large timber losses. Because the red deer population has increased over the last decades, a better understanding of the underlying causes driving BS is urgently needed. BS outbreaks are multifactorial: winter food shortage, local and regional deer abundance and stand properties (thermal and visual cover, and scarcity of herbaceous layers) have been found to drive BS rates. Although the influence of each of these factors is well studied, there is still no consensus on their relative contributions to BS or what mitigation strategy is best. In this study, we made use of a long-term, large-scale BS inventory (13 years over 2570 km 2) to fill these gaps. We modeled winter and summer BS rates in response to the most explanatory variables, selected from a large set of environmental variables. Our results highlight the prevalence of deer abundance combined with high sensitivity of coniferous thickets and pronounced habitat selection by deer for these stands. Our findings show that the control of red deer populations is a key measure for reducing BS in Norway spruce stands.

Large ungulate populations are known to cause economic damage to agriculture and forestry. Bark damage is particularly detrimental to the timber production of certain species, including Picea abies (L.) Karst. (Norway spruce): after bark is wounded, rot often spreads in the trunk base, damaging the most valuable trunk section. Numerous studies have provided valuable information on various aspects of this process but the financial consequences over a full timber production cycle remained poorly quantified and uncertain. To fill this gap, we coupled a forest dynamics model (GYMNOS) with models of damage occurrence and decay spread. We simulated the effect of ranging levels of bark-stripping damage on financial losses. The simulations were repeated for sites of ranging fertility and with different protection measures (fences or individual protections), in Southern Belgium. The net present values of these different simulations were estimated and compared to estimate the cost of the damage and the cost-effectiveness of the damage protections. Protecting plantations against bark-stripping damage with fences was found unlikely to be worthwhile. By contrast, individual protections placed on crop trees could be helpful, particularly in the most fertile stands. Loss of revenue depended greatly on the factors tested: we estimated that the average damage cost could be about 53e/ha/year, reducing timber yield by 19%. A model was built to predict the damage cost for different values of the discount rate, site index and bark-stripping rate. This model could help develop more effective management of Norway spruce plantations and deer populations.

Large ungulate populations are known to cause economic damage to agriculture and forestry. Bark damage is particularly detrimental to the timber production of certain species, including Picea abies (L.) Karst (Norway spruce): after bark is wounded, rot often spreads in the trunk base, damaging the most valuable log section. Numerous studies have provided valuable information on various aspects of this process, for example identifying factors governing tree sensitivity to bark-stripping or estimating the rate of decay spread in the trunk. The financial consequences over a full timber production cycle have nevertheless remained poorly quantified and uncertain. Coupling a forest dynamics model (GYMNOS) with models of damage occurrence and decay spread, we simulated the effect of ranging levels of bark-stripping damage on financial losses. The simulations were repeated for sites of ranging fertility and with different protection measures (fences or individual protections). The net present values of these different simulations were estimated and compared to estimate the cost of the damage and the cost-effectiveness of the damage protections. Protecting plantations against bark-stripping damage with fences was found unlikely to be worthwhile. By contrast, individual protections placed on crop trees could be helpful, particularly in the most fertile stands. Loss of revenue depended greatly on the factors tested: we estimated that the average damage cost could be about 53 €/ha/year, reducing timber yield by 19% in the study area, in Southern Belgium.

No abstract