Windthrow

Abstract

The factors that influence storm damage in forests are summarized and the four kinds of storm damage in forests are presented. Two equations for the estimation of windloads are explained including the problems that their use involves. By use of wind-induced bending moments of trees the occurrence of windbreakage or windthrow is discussed. The response of trees to windloads is mostly in form of damped bending sways. Based on artificial force effects, their shapes are shown to be dependent on soil conditions. If trees are continually exposed to dynamic windloads the shape of their sways is irregular at first sight. During strong winds and storms, the most important load on trees is generally stochastic and can be analysed by the spectral method. Based on this method, results from experimental investigations on wind-induced sways of tall spruce trees within a stand are shown, including power spectra of Reynold’s stress as measure of windload, stem accelerations as measures of tree response, and magnitudes of mechanical transfer functions. The main result is that conifers such as spruce trees can be compared to a narrow bandpass filter, i.e. they can take in energy from the turbulent wind field only at a certain frequency range.