The holiday season has been marred for many by power cuts and travel delays, often caused by fallen trees. The pattern of tree fall is literally chaotic: a tree may weather storms for a century before coming down in a moderate breeze.
Past wind damage research has focused on forestry management, but recent US studies of the impact of hurricanes on "urban forests" have revealed a complex picture.
Trees succumb to the wind in different ways. The trunk may break, the tree may topple over, or it might be uprooted, depending on the depth of the soil and the amount of rainfall. Trees in groups survive better than those standing alone, but only if they are adequately spaced, and large trees require at least 10 metres.
Older trees, and those damaged by insects or disease, are more susceptible than young ones. These may be felled as a precaution, but damage to roots or inside the trunk can be invisible until the tree comes down. Pruning and lopping intended to strengthen a tree may make it more vulnerable to wind.
The wind itself is highly variable. For example, localised gusts completely uprooted four leylandii at one property near Harlech last week, while neighbours' trees were completely unharmed.
This unpredictability makes management difficult, short of cutting down every tree within falling distance of a road, railway or power line. Even then, the removal of supporting tree roots increases the risk of mudslides and landslips.
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