The motion of trees in the wind: a data synthesis

Abstract

Abstract. Interactions between wind and trees control energy exchanges between the\natmosphere and forest canopies. This energy exchange can lead to the\nwidespread damage of trees, and wind is a key disturbance agent in many of\nthe world s forests. However, most research on this topic has focused on\nconifer plantations, where risk management is economically important, rather\nthan broadleaf forests, which dominate the forest carbon cycle. This study\nbrings together tree motion time-series data to systematically evaluate the\nfactors influencing tree responses to wind loading, including data from both\nbroadleaf and coniferous trees in forests and open environments. We found that the two most descriptive features of tree motion were (a)\xa0the fundamental frequency, which is a measure of the speed at which a tree\nsways and is strongly related to tree height, and (b)\xa0the slope of the power\nspectrum, which is related to the efficiency of energy transfer from wind to\ntrees. Intriguingly, the slope of the power spectrum was found to remain\nconstant from medium to high wind speeds for all trees in this study. This\nsuggests that, contrary to some predictions, damping or amplification\nmechanisms do not change dramatically at high wind speeds, and therefore wind\ndamage risk is related, relatively simply, to wind speed. Conifers from forests were distinct from broadleaves in terms of their\nresponse to wind loading. Specifically, the fundamental frequency of forest\nconifers was related to their size according to the cantilever beam model\n(i.e. vertically distributed mass), whereas broadleaves were better\napproximated by the simple pendulum model (i.e. dominated by the crown).\nForest conifers also had a steeper slope of the power spectrum. We interpret\nthese finding as being strongly related to tree architecture; i.e. conifers\ngenerally have a simple shape due to their apical dominance, whereas\nbroadleaves exhibit a much wider range of architectures with more dominant\ncrowns.\n