Responses of an individual deciduous broadleaved tree to wind excitation

Although responses of aerial tree parts to wind excitation are of crucial importance for deciduous broadleaved tree stability against wind loading, only a few field studies have been conducted that explore wind-tree part interactions. Therefore, responses of aerial parts of an individual Norway maple tree (Acer platanoides) to wind excitation were measured in a field study near the city of Freiburg (Southwest Germany) between July 2009 when the tree was in leaf and March 2010 when the tree was out of leaf. Using Fourier analysis, for most of the sampled tree parts six frequencies associated with the first three vibration modes in two motion directions were identified. In agreement with previous studies, vibration frequencies were lower when the tree was in leaf compared to the leafless period. The results indicate the importance of the foliage for the seasonal variation in aerial tree part motion damping. For the analyzed wind speed range, it is shown that the importance of higher order vibration modes localized on the sampled tree parts for total tree part response to wind excitation decreases with increasing wind speed. Results from wavelet analysis demonstrate that tree part response strongly covaried in the time frequency-space only with wind components having frequencies much lower than the frequencies associated with the dominant tree part vibration modes.