A prism-based system for monitoring the swaying of trees under wind loading

Abstract

This paper describes a prism-based system for accurate measurement of the stem movement and frequency response of trees under dynamic wind loading. Tree movement was measured along with wind speed at the edge of a stand of Scots pine ( Pinus sylvestris L.) close to 11 m in height. A 16 m mast was erected just upwind of the edge and instrumented to measure wind profile and wind direction. One tree located close to the upwind mast was instrumented with a target prism and accelerometers at the mid-canopy level in order to study tree swaying and frequency response. The suitability of this system for measuring the frequency response of trees under field conditions was tested by comparing the mechanical transfer functions for the tree obtained in this way with those from the accelerometers. Good agreement was found at frequencies close to and above the tree's natural frequency but differences emerged at low frequencies, suggesting that the accelerometers overestimate the low frequency response. The accuracy and repeatability of stem displacement measurements made using the prism-based system were found in laboratory tests to be very high. The new system can be adjusted quickly to make measurements at different heights on the same tree or on other trees by simply adding additional target prisms.