Stability of continuous cover forests

World-wide, wind damage causes major economical losses to commercial forestry, in particular in windy climates such as in the British Isles. Wind risk models are valuable tools for forest management planing to predict and minimise the risk of wind damage. British foresters use the ForestGALES model which predicts the critical wind speed and risk for forest stands in Britain. Due to a number of factors the number of British forests that are managed under low impact silviculture systems is increasing. The associated stand structure is more irregular than those under standard management and the ForestGALES model is at the moment not able to predict the critical wind speed for these structured stands. The aim of this PhD project was to collect field data to help in the development of the ForestGALES model for application in irregularly structured forest stands. Two field campaigns were carried out in which wind and tree interactions were investigated. Wind profiles, turbulence, and turning moment at the tree base were measured with high temporal resolution for a group of nine trees in both field studies. 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 was 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 bigger trees also experienced higher wind drag. The results suggest that the balance between individual tree resistive moment and applied moments is such

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