Modelling the anchorage of shallow-rooted trees

We present here a mechanistic model (the 'Resistance' model) that uses easily measurable or predictable input parameters describing tree, soil and root-soil plate characteristics to predict the anchorage strength of Sitka spruce (Picea sitchensis (Bong.) Carr.) trees. Results show that for a given soil group and rooting depth, the anchorage moment can be modelled as being proportional to the square of the root-soil plate spread. Models are also developed of root-soil plate spread in relation to soil group and tree characteristics and are applied with the Resistance model to provide predictions of anchorage based on the diameter at breast height. To examine the relative sensitivity of trees on different soil groups and with different rooting depths, the models are applied here to an example where the leeward 'hinge' radius is constrained to a maximum of 1 m. The results are discussed in relation to forest wind risk modelling and stand management.