Seven fully excavated 16 year-old root systems of Sitka spruce were analysed. All roots in excess of I cm diameter at their origin on the stumps were analysed, data being collected until root diameter declined to less than 0.5 cm. Root morphology and distribution was identified as a balance between systematic biological mechanisms and their disruption by environmental factors, particularly changes of soil density and soil surface contours. The biological mechanisms have been modelled and the model is capable of simulating root systems in response to a few simple input variables e.g. the number of roots originating at stumps, stem ratius, total number of roots of all orders etc. Additonally equations have been developed to estimate the distribution of root diameter, and root weight at all distances from tree stems and a similar equation permits the estimation of tree diameter at chosen heights. These latter estimates being utilized to calculate the turning moment of stems when ...