Field measurement of wind characteristics and induced tree response during strong storms

Typhoons have caused considerable damage to individual tree and forest ecosystems.To reduce windinduced tree damage and better predict the risk of damage,improving our understanding of wind-tree interactions during strong wind conditions is important.To this purpose,wind characteristics and movements of an individual Betula platyphylla Suk.in a forest stand were monitored during three typhoons(Bavi,Maysak and Haishen).Results revealed that the average wind twists with increasing height,with a large twist gradient within the canopy and a small twist gradient outside the canopy.The maximum wind twist angle was approximately 110°.The disturbance of trees increases the turbulence intensity of the wind field in the canopy.The maximum power of the wind spectra and the turbulence anisotropy of the three turbulence components decrease with increasing height.B.platyphylla did not resonate with the wind in any of the typhoons but responded strongly to gusts near its free vibration peak frequencies.The peak frequency of the mechanical transfer function of B.platyphylla is essentially the same as the peak frequency of the response power spectra.The mechanical transfer func-tion of the wind-induced response of the tree is almost the same as the transfer function of the damped harmonic oscil-lator which has similar characteristics to coniferous trees.