A Study of Branch Dynamics on an Open-Grown Tree

This study investigates the dynamic properties of branches on an open-grown tree, where most of the mass is in the branches rather than in the trunk. When large branches on open-grown trees sway in winds, they individually and collectively influence how the whole tree sways. If branches are removed, as in pruning, the effect on tree sway has not yet been studied and the literature is almost nonexistent regarding recommendations for pruning open-grown trees to reduce wind damage. Trees come in many shapes and sizes and in urban areas, usually grow in open spaces and develop many branches. In forests, and particularly in closely spaced plantations, trees grow with an upright central trunk and develop significantly less branch mass. Forest conifers have been studied to identify their dynamic properties in winds, but the results may be different for open-grown trees. A 19.7 m tall silver maple (Acer saccharinum) with four codominant branches was tested by pulling and then releasing each branch to determine the dynamic properties. Branches were progressively removed and the tests repeated. The sway response was recorded with strain instruments attached to the trunk and accelerometers attached to each branch. The dynamic properties of frequency and damping were determined for all tests. The tree with all branches attached, in full foliage was difficult to sway because of damping from the branches. Significant changes in oscillating frequency and damping were observed only after most of the branches (greater than 80%) were removed. The results support the concept that branches provide damping, which dissipates energy from the wind as a mechanism to help trees survive.