Mode coupling and signal energy distribution in an open-grown European beech tree

The wind-induced response of the branches, the stem, and the roots of an open-grown European beech (Fagus sylvatica L.) was investigated to determine how the airflow’s kinetic energy transferred to the tree is dissipated. The instrumentation in the above-ground tree parts consisted of 69 Tree Response Sensors. The response of the primary roots to the wind load was recorded with six Tree Strain Sensors. The measurements were used to record the tree’s main vibration modes and quantify the energy dissipation from the measurement signals. The signal energy is reduced linearly by 90% from the crown periphery to the stem base. The base of the stem responds to higher wind loads than the branches and the upper part of the stem. The first-order mode frequency band is the narrowest in the roots. To dissipate the kinetic energy transferred to it as efficiently as possible, all tree parts vibrate in overlapping frequency bands in fundamental mode, resulting in mode coupling from the crown periphery to the roots. Mode coupling enables the elastic energy to be transferred as efficiently as possible via the roots to the ground to withstand wind loads and return to the resting position as quickly as possible.