Measurements of the natural frequencies of trees

Recent theoretical work has suggested that tree natural frequency is an important indication of tree stability in windy conditions. This paper reports natural frequency measurements on a large number of urban trees using a remote laser-based method. The majority of the measurements were made on healthy lime trees, and showed a consistent variation of natural frequency with tree size, with out-of-leaf, winter frequencies being substantially greater than in-leaf, summer frequencies. These measurements also revealed that different tree geometries can produce significant changes in natural frequency. A number of measurements were made on diseased lime trees (crown dieback, rot, etc.) and these displayed natural frequencies that were very different from those for the healthy trees.