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Résumé :
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Sitka spruce, Picea sitchensis (Bong.) Carr. is now the species of tree most extensively planted by the Forestry Commission in Great Britain, occupying in 1963 416,000 acres out of a total of 1,752,000 acres. One fifth of this has now reached the thinning stage. Private owners also plant it widely. Some of the older plantations are being severely damaged by storms, and the prospects are that the problem will become greater as more plantations reach a susceptible size. The type of damage which most commonly occurs is uprooting, often of large numbers of trees, but occasionally large-scale stem- breakage can occur. Day (1949, 1963 and elsewhere) drew attention to the effects of various forms of soil impedance on the root systems, health and stability of crops. The present investigations confirm his observations and provide new quantitative evidence on the effects of soil impedance and new factors on the stability of trees. Investigations of the various factors rendering crops susceptible to wind-throw were initiated in 1960. A technique for pulling trees over was developed (Fraser 1962) which enabled studies to be made of the root systems, and of tree mechanics. Sites were selected around Britain on a range of soil types. A number of old silvicultural experiments were also used, in order to study as many of the factors as possible which might influence root development. These studies are incomplete, especially in that comparisons have not been made with other species of tree, but a considerable amount of useful results have been accumulated. Certain distinct patterns have become apparent which it is hoped will be helpful in making silvicultural and managerial decisions
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