Effect of Pulling Direction on Destructive Tree-Pulling Tests in Hemiboreal Scots Pine Stands in Northern Europe

Static pulling tests are essential for evaluating tree mechanical stability, hence the ability to sustain wind loading. In the tests, the trees are usually pulled in one direction, which can raise concerns about the bias of the estimates due to thigmomorphogenetic adaptations to wind, hence the effect of pulling direction. In this study, the effect of pulling direction on tree mechanical stability estimates (basal bending moment at primary and secondary failures) was assessed based on data collected from trees growing in stands regenerated naturally and by mounding. Scots pine (Pinus sylvestris L.) stands in three sites with slightly differing wind climates in hemiboreal forests in Latvia were studied. Pulling direction (difference between wind and pulling direction) had a significant effect on basal bending moment at fatal failure, indicating pulling direction-related bias that was not affected by stand regeneration. Stand regeneration had a significant effect on susceptibility to primary failure, which is intrinsic permanent wood damage, implying altered strength of storm legacy effects. Primary failure, however, was not affected by pulling direction.