No resonant response of Scots pine trees to wind excitation

The response of four Scots pine trees to wind excitation was measured and analyzed. We investigated whether wind-induced tree sway in the fundamental mode is affected and enhanced by resonance effects between cyclic wind loading and oscillatory tree response. By combining results from wavelet analysis and analysis of phase amplitude coupling, we demonstrate that the response in the fundamental mode is an indirect reaction of the trees to displacement from their rest position mainly caused by large organized turbulent structures in the canopy airflow, such as sweeps and ejections. In particular, sweeps caused large tree displacement. It is suggested that sway in the fundamental mode mainly results indirectly from the trees' elastic energy that is stored in the stem and roots because of wind-induced compression and tension, and the damping that returns them to their rest position. Since no direct excitation of sway in the fundamental mode was found, it is concluded that there is no resonant response of the studied Scots pine trees to wind excitation.