Title:
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Stability of continuous cover forests
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Two field campaigns were carried out in which the wind and tree interaction was investigated. Wind profiles, turbulence, and turning moment at the tree base were measured with a high temporal resolution for a group of nine trees in both surveys. The first experiment took place in a mature even-aged Sitka spruce forest stand, which appeared to be more stable than model calculations anticipated. Differences in wind loading between the individual trees were calculated and related to tree properties. Absolute turning moments were positively correlated with tree properties such as diameter at breast height, tree height, and stem weight. The estimated turning moment for tree failure for the strongest tree in the sample is more than five times higher than the value for the weakest one. However, due to their dominance and their exposed position in the stand the biggest trees also experience higher wind drag. The results suggest that the balance between individual tree resistive moment and applied moments is such that the critical wind speeds for damage are very similar. This implies that the tress are adapted to their local wind climate. Post damage surveys from continental Europe suggest that irregular forests are more stable then regular ones. This hypothesis was tested in the second field campaign in which the wind and tree interaction were compared in two contrasting stands. One group of trees was located where an understorey was present, while two other trees were at a location with no understorey at all. The analysis suggests that the trees with an understorey benefit from the understorey in terms of wind loading and wind damage risk.
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