The effect of soil water-table depth on root-plate development and stability of Sitka spruce

Stability was tested on 46-year-old Sitka spruce trees (over 20 m tall) growing on gleyed soils. Trees with a range of root depths on peaty gley and surface-water gley soils were selected by studying their water regime. Trees were pulled over horizontally with a winch and the vertical displacement of the soil-root plate was measured as the load was applied. The resistive turning moment at the point of soil failure and the maximum resistive turning moment were measured. Thin plates, which developed over a shallow water table, had a greater surface area than thick plates that developed over deeper water tables. However, they also had a smaller soil mass and tended to shear in soil material of lower shear strength and were particularly flexible. Thicker plates sheared in stronger soil and were more rigid. Soil-root plate rigidity was shown to be a major factor affecting the contribution of soil resistance to overturning. Regression analysis revealed that the more rigid the plate the greater the applied turning moment at soil failure. Trees that develop rigid soil-root plates through adaptive growth of their structural roots will therefore also have improved stability. The results indicate that intensive drainage of peaty gleys will increase rooting depth and the resistance to overturning.