The turbulent wind in plant and forest canopies

Anyone who has walked in a tall forest on a windy day will have been struck by how effectively the trees shelter the forest floor from the wind. Tossing crowns and creaking trunks, and the occasional crash of a falling tree, attest to the strength of the wind aloft, but only an occasional weak gust is felt near the ground. An attentive observer will also notice that these gentle gusts seem to precede the most violent blasts aloft. Clearly, the presence of the foliage absorbs the strength of the wind very effectively before it can reach the ground. It is surprising, therefore, that only in the last 2 decades have we arrived at a satisfactory understanding of the way that interaction with the foliage changes the structure of the wind in tall canopies and leads to significant differences from normal boundary-layer turbulence. In this chapter I explore these differences, focusing especially on what the special structure of canopy turbulence implies for disturbance ecology. Gentle gusts at the ground foreshadowing the strong blasts aloft are but one example of the many curious and counterintuitive phenomena we will encounter.