E7 Wind

Topic

The wind exerts a drag force on the plant's crown that increases with wind speed and the extent of the crown area exposed to the airflow; this drag force is the first of several aerodynamic forces the wind applies to the plant.

The wind speed acting on any given part of the crown is not uniform across its height but follows a logarithmic velocity profile within the canopy: near the ground—where the canopy itself slows the wind—speeds are lower, whereas higher up within the canopy, wind speed increases logarithmically, meaning the upper sections of the crown experience a proportionally greater drag force than the lower sections.

Because the drag force acting at any given height within the crown is applied at a distance from the tree's base, it generates an overturning moment; the greater the drag force—and the higher up in the crown it is applied—the greater the overturning moment the tree must withstand at its base to avoid toppling.

To reduce the drag force and the resulting overturning moment, the plant's flexible crown responds through aerodynamic reconfiguration: its branches and leaves fold and align with the wind direction, reducing the crown area exposed to the airflow. This results in a lower drag force, thereby closing the feedback loop between the flexible crown's aerodynamic reconfiguration and the wind that triggered it.

When the drag force and overturning moment generated by the wind exceed the plant's structural limits, the wind acts as an agent of mechanical damage. In addition to exerting mechanical forces, wind facilitates gas exchange between the leaf and the atmosphere, as air movement reduces the resistance to exchange imposed by the boundary layer of stagnant air surrounding the leaf; the stronger the wind, the thinner this stagnant air layer becomes, and with less resistance, it is easier for the leaf to exchange carbon dioxide, water vapor, and heat with the air.

Wind also acts as a dispersal vector for pollen and seeds: the same airflow that exerts drag force on the tree crown transports the plant's pollen and seeds away from it, enabling reproduction and colonization of new locations.

Finally, wind serves as a developmental signal for the plant: the repeated mechanical friction the wind applies to the stem and branches triggers thigmomorphogenesis—the process by which the plant increases its basal diameter and reinforces its woody tissue in response to wind-induced swaying. Thus, wind acts simultaneously as an agent of mechanical damage, a facilitator of gas exchange, a dispersal vector for pollen and seeds, and a developmental signal; the drag force exerted by the wind on the crown—depending on its intensity and the plant's response—determines which of these four roles ultimately predominates.

ID:142