Soil shear strength measurements and their relevance to windthrow in Sitka spruce

Abstract

Abstract. The measurement of the shear strength of the root plate/soil interface beneath mature trees is described. Results from this method are given for a total of 77 sample trees from crops of Sitka spruce aged between 24 and 35 years growing on a peaty gley, a deep peat and a brown earth. Shear strength was much higher for the brown earth than for either the peaty gley or the deep peat, mainly because there was a less clear‐cut boundary to the rooting depth compared with the rooting boundary caused by the high water table on the peat sites. There was a slightly higher shear strength for the peaty gley than for the deep peat, but no apparent difference in shear strength between the peat and the underlying mineral soil in the peaty gley. The shear strength measurements were consistent with observed differences in tree stability and the method of measuring shear strength aids the assessment of susceptibility of trees to windthrow.