Summary

 

Author

Steven Vogel

Overview

Wind affects a wide range of plant functions, with different functions responding to different speed ranges and to speed changes on different time scales. The diversity of such phenomena challenge our choice of measurement devices, the latter compromised by reliance on the availability of commercial instruments. For some purposes, it remains useful for the investigator to construct task-specific instruments.

General background

Every book on plant physiological ecology includes some section on the effects of wind. It is usually short and relatively unspecific, at least by contrast with the coverage of solar radiation, gas exchange, and temperature – although without explicitly minimizing the importance of wind. We find it all too tempting to focus on those variables that are conceptually and operationally straightforward, ones that can be measured and reported without complexity.

Wind affects, among other matters, plant distribution, morphology, productivity, water loss from both soil and plants, leaf temperature, and mechanical stability and integrity. Unfortunately, no single scale can display the relationship between the magnitude of wind and the magnitude of its effects. Depending on the function under consideration, one might need to consider relatively long-term averages speeds, direction and periodicity of gusting, maxima for various long time scales, minima and their duration over short periods of time. In addition, the interaction of wind and other obviously relevant factors can be complexly situation-dependent. One can easily cite illustrative examples of the diversity and complexity of effects.

• On a clear night even modest wind will increase the temperature of a leaf radiating to the sky; on a clear day the same wind will decrease the temperature of a leaf intercepting direct sunlight.
• Extremely high, mechanically destructive winds may occur only occasionally; the relationship between the frequency of occurrence of such winds and the lethality of their effects may depend on the age at which a particular species produces its first propagules.
• While water loss usually increases with wind speed, it depends at the same time on humidity and a host of other factors, both internal and external. Thus no simple relationship can be asserted, even for a given plant in a given place at a given time.

The figure below illustrates the interactions of illumination and wind. It reports measurements of the temperature of a the middle of a leaf of white oak (Quercus alba) about a meter above the ground, exposed to sunlight but surrounded by trees on a summer afternoon (readings taken every ten seconds with a portable infrared thermometer). Clouds intermittently occluded the sun, causing the major excursions. The ripples particularly evident when the temperature rose correspond to episodes of moving air, imperceptible to the investigator, noted as slight movements of the leaf.

 

Plant responses on different time scales

Plants react to wind in a variety of ways; these vary in both the time scales of the winds to which they are exposed and the characteristic times of their responses. For the kind of very quick changes of the order of ten seconds and very low speeds shown in the figure, we lack specific evidence of active responses even though enzymatically catalyzed biochemical reactions near the upper functional limit of proteins are notoriously temperature sensitive. But lack of evidence should not be taken as evidence of nothing, especially given the long tradition of holding temperature constant when investigating enzymatic reactions. Indeed, one wonders whether plants, especially their leaves, might provide extreme examples of short-term metabolic compensation mechanisms that we homeothermic mammals, biologically unusual, find unnecessary.

On a somewhat longer scale of minutes and hours, stomatal responses appear to predominate and have been relatively well studied. Stomata react to various interacting combinations of illumination, temperature, water availability, and, to a lesser extent, changes in atmospheric composition. Other responses have received less attention.

Over still longer periods, periods over which significant growth can occur, plants adjust their structure as they grow in response to wind. Spiral grain in high-altitude trees may be the best known example, but recognition that they respond to mechanical perturbation, the phenomenon termed “thigmomorphogenesis,” has stimulated investigation of responses to the mechanical effect of wind, that is, responses to drag. In general, plants exposed to either deliberate shaking under controlled conditions or to greater winds in nature produce thicker branches and stems, shorter internodes, and so forth; in short they build more force-tolerant structures. It appears to be unknown whether they show any growth responses to the higher temperatures associated with very low speed winds, such as decreasing leaf diameter or blade area in the following year’s leaves after tree transplantation.

To a considerable extent, our lack of familiarity with the effects of wind, although they are generally admitted to be substantial, is a consequence of the greater difficulty of measuring wind in comparison to, say, temperature or light intensity. Part of the problem comes simply from its irregular and moment-to-moment variability. As much must come from instrumental limitations. An anemometer may be based on any variable affected by wind, variables such as pressure, force, forced convection, or material ablation; each of these bases has advantages and limitations. In general, using pressure (manometers) works very poorly for low wind speeds, so the simple Pitot tubes beloved of engineers prove unsatisfactory. Using force, either as strain gauges, whirling cups, or spinning windmill devices works better, although few are trustworthy below about one meter per second, and few have sufficiently short temporal response to handle the changes encountered at such low speeds. Convective devices, hot wires and hot-bead thermistors in particular, tend to be delicate and often come with awkward sensitivity to changes in temperature as well as wind.

Clouding the picture further is the issue of relevant aspect of wind, noted above. If the potential for reaching damaging leaf temperatures is at issue, then minima may be especially important. If wind-throw poses the hazard under investigation, then maxima must be considered. In general, extremes challenge our instrumentation more than averages, so we give less attention to them.

I suggest that despite the sophistication of the anemometers now available, a place remains for productive use of unconventional measurement techniques, ones designed with an immediate eye for utility in work with plants in the way that Scholander bombs are solely of use in the area. In a separate section I will describe several such unconventional instruments, ones that can be assembled without great expense or expertise, and intended as examples of what can be done with the flotsam and jetsam of our industrial age.

For example:

Hot-wire low-speed anemometer based on a light bulb filament

Drag-based flat-plate low speed anemometer

Literature References

Coutts MP, Grace J, eds. 1995. Wind and Trees. Cambridge, UK: Cambridge University Press.

de Langre E. 2008. Effects of wind on plants. Annual Review of Fluid Mechanics 40: 141-168.

Grace J. 1977. Plant Response to Wind. London: Academic Press.

Nobel P. 2005. Physicochemical and Enviromental Plant Physiology, 3rd ed. Burlington, MA: Academic Press.

Vogel, S. 2009. Leaves in the lowest and highest winds: temperature, force and shape. New Phytologist 183: 13-26.