© 1986 by Institute of Chartered Foresters
Components of Tree Stability in Sitka Spruce on Peaty Gley Soil
Forestry Commission, Northern Research Station Roslin, Midlothian, EH25 9SY, UK
The stability of 20 m tall trees was investigated by pulling them over with a winch. The turning moment at the stem base was resolved into two components, one due to the applied force, and the other to the horizontally displaced weight of the stem and crown. Vertical displacement of the root-soil system was measured and observations made of the progressive failure of the soil around and underneath the root-soil plate.
When trees were pulled from the crown region, the soil failed when the crown had deflected c. 4 rn horizontally and when the applied force wasonly about 70% of that required for uprooting. By the time that the maximum turning moment at the stem base due to the applied force had been reached, many roots had broken, crown deflection was c. 8 m and the deflected weight of the stem and crown made a substantial contribution to the uprooting forces.
By repeatedly pulling trees during a sequence of cutting or breaking the roots and soil, the total resistive turning moment afforded by the anchorage was resolved into the following components: i. soil resistance (the resistance to uprooting afforded by the soil underneath and at the sides of the root system); ii. the resistance of roots placed under tension on the windward perimeter; iii. the weight of the root-soil plate; and iv. resistance to bending at the hinge on the leeside.
The importance of these components varied between trees and changed during the course of uprooting. Soil resistance was the largest component in the early stages, but when the turning moment due to the applied force was maximal, the components of anchorage were in the order windward roots < weight < hinge < soil resistance. The major effect of the windward roots on anchorage in these shallow root systems highlights the importance of features which interfere with their lateral development, such as the furrows produced by spaced ploughing.
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