Quantifying bark-stripping damages to address the relationship between external and internal wood defects

The detachment of tree bark (i.e. bark-stripping) by wild ungulate species can be observed in many European forests and it can lead to significant damages to the wood. In our study, bark-stripping of different intensity and age was recorded for 110 European beech (Fagus sylvatica L.) trees growing in the Harz Mountains, Germany. Our goal was to compare the performance of a conventional and a structure-from-motion approach to quantify the external damage and to evaluate whether bark-stripping damages on the outside may accurately reflect internal wood defects. Also, we wanted to know whether potential relationships between inner and outer damages are affected by the condition of the wound. Surprisingly, despite large difference between the two approaches (mean absolute error: 46%) the wood surface on the sawn boards (internal damage) could not be predicted more accurately using structure-from-motion, for the assessment of the external damage when compared to the conventional measurements based on damage height and damage width (rhombus formula). The internally damaged area was smaller than the externally damaged area across all wound categories. In contrast though, and most notably, the length of the stem section negatively affected by the damage was always greater inside than outside. We conclude that sophisticated measurements of the outside wound area do not allow for a better estimation of the internal wound area. Foresters may continue to rely on simple measures of damage extension on the outside of the tree to estimate internal damages. They should however consider the wound status as an additional information for a better prediction of internal damages.