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DRAFT MANUSCRIPT 304 — WIND EFFECTS Page 1/19

Chapter title: Wind effects

Manuscript Number: 304 Author: Werner Eugster

ETH Zürich Institute of Plant Sciences ETH Center LFW C55.2 CH–8092 Zürich, Switzerland E-Mail: [email protected] Fax: +41 44 632 1153

Number of words: 6917

Number of figures: 8 Number of tables: 1

Number of multimedia annexes: 0

Keywords Ballooning in spiders,

bird migration, directional growth response,

insect flight, insect migration,

kleptoparasitism, metabolic stress,

trace gas exchange, turbulence,

vegetation roughness, wind pruning and salt spray,

wind throws.


Article synopsis

There are two categories of wind effects in ecology: (a) the effect of the vegetation surface on the wind, how it lowers wind speed near the ground, shelters niches from strong winds where small animals and plants can establish and live; and (b) the effect that wind and turbulence excert on many aspects of animal behaviour, plant growth and survival, and the overall metabolisms of organisms.

This article focuses on the very general physical relationship between wind (and thus turbulence) and other environmental factors such as thermal heat loss and metabolic rates of organisms, and the exchange of trace gases such as CO2, followed by a summary of the most relevant specific aspects of wind effects in the ecological literature. Topics included are: mean wind speed and turbulence; wind over the vegetated surface; changes in surface roughness; turbulent mixing and trace gas exchange; dispersal of pollen, spores and microorganisms; influence on small animals and seed dispersal; influence on bird and insect migration; wind chill and heat index; metabolic stress by wind; wind throws and wild fires. Examples of wind effects on both animal and plant life are given.

Article body

Introduction There are two categories of wind effects in ecology: (a) the effect of the

vegetation surface on the wind, how it lowers wind speed near the ground, shelters niches from strong winds where small animals and plants can establish and live; and (b) the effect that wind and turbulence excert on many aspects of animal behaviour, plant growth and survival, and the overall metabolisms of organisms.

Studies on forest recovery in North America have pointed to the important role of high winds in temperate forests. Although such catastrophes are rare, they could be instrumental in the creation and maintenance of mosaic patterns and hence the diversity of these woods. Some attempts to reconstruct the history of winds during the past glacial maximum (about 18,000 years ago) indicate that tropical storms generating winds of hurricane force were scarser, less intense and shorter than those of the present day, with important consequences for forest ecology which include the influence on development, structure and composition of the migrating and reassembling forests of the mid- and higher latitudes. However, direct evidence of the effects of wind on forests is hard to come by, and also other aspects of wind effects in a wide variety of ecologically relevant topics, are rather scarcely covered in the scientific literature. Almost all studies reviewed for this article are based on an ecological question that suggest some partially unknown dependence on environmental variables. Mostly temperature, precipitation, and humidity are considered important variables in such investigations, and wind effects are rather considered a possible or likely additional side effect of the overall ecological process under investigations. It is therefore not surprising that there are almost no systematic studies in the scientific literature that cover all aspects of wind in all details.


Some specific aspects, where the ecological importance of the wind is rather obvious are covered in much greater detail in other topics (366: Wind shelter belts; 563: Wind erosion). Thus, we focus here on the very general physical relationship between wind (and thus turbulence) and other environmental factors such as thermal heat loss and metabolic rates of organisms, and the exchange of trace gases such as CO2 shall be addressed before summarizing the most relevant specific aspects of wind effects in the ecological literature.

Mean wind speed and turbulence Wind is a vector variable, but in many scientific applications only the scalar

wind speed is investigated or of interest. Since wind—that is the term for the atmospheric motion over the solid surface of the Earth with respect to the surface itself—is mainly driven by pressure gradients on relatively large scales over the globe, this term is often used as a short-cut for horizontal mean wind speed. Over sufficiently long observation intervals and over large surface areas there is no net loss or gain of air due to vertical motion, thus the vertical component of the wind vector is considered to be zero. Hence, the wind vector is approximated as a two-dimensional entity that can be described by the scalar horizontal wind speed and the wind direction. All standard weather stations use this basic concept for recording wind. This is however not necessarily the best possible simplification for small-scale and short-term investigations, and thus has important implications for ecological processes. As an example, three-dimensional wind gusts in autumn can easily pick up fallen leaves from the ground, despite the fact that the leaves are relatively heavy and the mean vertical and horizontal wind speed over an hour or longer may be rather low. But on the time scale from tenths of a seconds to several minutes turbulence—which includes such wind gusts—can be the most relevant wind effect. The turbulent time scale typically extends up to one hour, whereas longer time scales are associated with mean wind effects. There is not a sharp separation between turbulence and mean wind, although a spectral gap between the two time scales has been postulated by some scientists. In reality, there is a confounding effect with the diurnal course of wind speeds that show different and locality-specific conditions during the day as compared to the night.

Laminar Flow and Turbulent Winds Turbulence is generated inside a laminar flow when there is mechanical friction

or thermal convection that perturbs the flow. Figure 1 illustrates this for a laminar flow with a certain wind speed that moves from a smooth onto a rough surface. At a certain distance downwind of the leading edge of this increased roughness, the lamiar flow becomes chaotic, that is, turbulent. However, this turbulence does not completely reach the surface, a minute laminar surface layer always exists, over the surface of any object, including plant leaves (Figure 2). Turbulent exchange of heat, moisture, CO2 and other trace gases is by far more efficient than exchange in laminar flows (see below). Once that the air is turbulent the flow does not easily become laminar again since the transition from turbulent to laminar flow is not as clearly defined at it is in the opposite direction. The decay of turbulence in the air is subject to the physical rules of turbulent kinetic energy dissipation that ends up in the Brownian motion of single molecules and thus dissipates kinetic to thermal energy.

<Figure 1 near here>


<Figure 2 near here> Wind has a kinetic energy,

!

Ek

=m

2V2 (1)

with m the unit mass of the unit air volume, and V the scalar wind speed of the three-dimensional wind vector, that is composed of mean kinetic energy and turbulent kinetic energy. At moderate to high wind speeds the mean kinetic energy is by far greater than the turbulent kinetic energy. Additionally, wind carries a momentum,

!

" = m #V . (2) At high wind speeds, especially during storms, a very high kinetic energy (both

mean and turbulent components increase with increasing wind speed) may result from the wind, which is responsible for the devastating damages by hurricanes and other storm events, that however are also important for producing gaps (wind throws) in forest ecosystems and thus for these ecosystems’ overall life cycle. Turbulent motions that are most relevant at lower wind speeds do not have such a devastating effect when mean wind speed is low. The kinetic energy of the mean wind is what wind mills profit from, and also migratory birds benefit from this component. Near the ground, the vegetation has to absorb both the kinetic energy of the wind and its momentum, but in this case the momentum absorption is by far the more relevant process and as a first approximation only momentum transfer by the vegetation is considered, neglecting the additional effect of energy absorption.

For flying birds it is mostly the kinetic energy of the wind that influences their daily life. It has been shown for Sandwich Terns (Sterna sandvicensis) on the isle of Griend, the Netherlands, that their loss of pray to the competing kleptoparasitising Blackheaded Gulls (Larus ridbundus) significantly increases with wind speed. Thus, wind directly reduced the ability of Sandwich Terns to defend their pray (mostly fish) against attacks of Blackheaded Gulls. This had a negative effect on the amount of food transported to the colony, while kleptoparasitism increased. Therefore, wind speed severely affected energy intake of the chicks and had strong negative effects on chick growth. During the first two weeks post-hatching, kleptoparasitism was relatively low and had only small effects on chick growth, even under unfavourable weather conditions. From then on, the negative effects of kleptoparasitism on growth became considerable.

Wind over the vegetated surface Vegetation is the most important interface between the atmosphere and the solid

ground in terrestrial ecosystems. On the one hand, vegetation adopts to wind conditions and special plant social community compositions are found in the Arctic and Alpine environments where persistant and strong winds influence exposed locations such as hills, mountains, and crests. On the other hand, vegetation makes the Earth’s surface rougher than what would be the case over bare soil (Table 1), and thus strongly influences the wind speed (Figure 3) and direction in atmosphere near the ground. The wind is driven by pressure and temperature differences on large scales, whereas the Earth’s surface does not move and stays put under most occasions. Exceptions are very special conditions during hurricane-force winds, and certain exposed locations with corresponding soil conditions, where bluffs are created by the steady wind movement. Under normal conditions, wind speed at some nonzero height


above the ground must be zero to fullfill the criterion that the vegetation stays in place. Rough vegetation such as forests excert a much higher roughness (on the order of one meter) to the atmospheric wind motion than shortcut grass (on the order of millimeters to centimeters; see Table 1). And based on this roughness the increase in wind speed with height above the vegetation depends strongly on vegetation type and structure. This vertical wind speed profile tends to increase logarithmically with height above the canopy (Figure 3), and the physical process responsible for this is momentum absorption. Tall vegetation such as forests absorb momentum with their roots, but also in the dynamic motion of the stems. Thus, under strong winds it depends on the rooting type of the tree and the wood quality whether a tree can be uprooted or whether the stem breaks at a certain height above the ground.

<Table 1 near here>


Wind Pruning and Salt Spray To unroot a tree normally requires strong gusts in heavy mean winds, as for

example during storms. If winds are strong and steady, but not very gusty, then the energy may not be sufficient to unroot a tree and thus the effect of wind pruning may shape trees and shrubs (Figure 4). Along the seacoast of British Guiana, along the subtropical shores of the island and Trinidad and southern California, and the subarctic shores of Hudson Bay and Labrador the wind is reported to result in pruned trees. It appears that the steady subtropical winds have a similar pruning power as the icy blasts of the subarctic. However, this is not necessarily an effect of the wind alone. It has been argued that the proximity to the sea leads to a high load of salt spray in the wind, and that the toxicity of that salt may be the true ecological factor of wind pruning. Salt spray deposition on young shoots seem to actually kill many of them, thus causing the pruning. This observation is however only weakly based on pH readings along a transect from the shore of the Belcher Islands off the Hudson Bay coast, and it is also noted that the drying effect of the wind, possibly in combination with salt spray and other factors (ice and sand particles) may be as important.


Directional Growth Response Besides pruning steady winds from a persistent wind direction can lead to

directional growth response of trees as it is widely observed in coastal areas, in deep mountain valleys with a well-developed valley wind system (Figure 5), or on wind exposed crests, rims, and hills. Since wind speeds generally increase with altitude from the lowland to the mountains, it has even been argued that high-altitude plants in wind-swept mountains may be less affected by global warming, and that the spread of lowland plant species into uplands as predicted by some global warming scenarios may be strongly restricted in higher altitudes due to the lack of adaptation of lowland plants to such steady and comparatively strong winds.



Changes in surface roughness: the edge effect Sharp edges of vegetation—which are less abundant and less pronounced in

natural ecosystems than in anthropogenically disturbed and shaped ecosystems such as agroecosystems and managed forests—are subject to wind effects that depend strongly on the distance to the change in roughness. When the wind first blows over a smooth (e.g. grass) surface and then abruptly has to change to a rough (e.g. agricultural crop or forest) surface, then momentum is created within a relatively short distance as wind passes over this roughness change (Figure 6). This additional momentum has to be absorbed by the vegetation downwind to obtain a new equilibrium with the rougher surface. This leads to the phenomenon that in a wheat field for example there may be a few rows of plants directly at the roughness change that seem quite unaffected even by strong winds, while only one meter downwind one or several rows may be completely flattend by this additional momentum. In the case of forests, wind throw often excludes the trees at the forest edge, partially due to the same phenomenon. But trees also can adapt to constant wind pressures by building special cells to counteract this pressure. This is best known for trees in mountain vallies and along seashores with persistant and sufficiently strong winds in specific directions. In mountain valleys these are the up-valley (daytime) and down-valley (nighttime) wind directions. Which one is stronger depends on the complex combination of orientation of the valley, length, topographic differences in the surroundings, and more. But by studying trees which are leaning in the direction of the dominant strong winds (Figure 5) it is easy to determine the locally dominant wind system. Near costs it is the diurnal sea breeze that dominates wind pressure on trees, while the nocturnal land breeze is in most cases much weaker.

<Figure 6 near here> On smaller scales, linear landscape elements such as hedgerows, tree lines and

tree lanes are ecologically important surface roughness elements, especially in otherwise rather smooth agricultural landscapes. In the Netherlands, for example, it has been shown that among other possible functions (orientation clues, foraging habitat) such linear elements provide shelter from wind and/or predators for the two bat species Pipistrellus pipistrellus and Eptesicus serotinus.

Turbulent mixing and trace gas exchange Turbulent exchange is roughly three to four orders of magnitude more efficient

than diffusive mixing in a laminar airflow. For trace gas exchange between the atmosphere and the plants the tiny laminar layer surrounding each leaf (Figure 2) is thus non-negligible. Given this huge difference in effectiveness of turbulent versus diffusive transport a laminar boundary layer of 0.1–1 mm provides a similar resistance against the free exchange of CO2 between the atmosphere and the plant stomates as does 1 m of turbulent air. In Figure 2 it is clearly seen that the laminar layer separates the turbulent atmosphere—where CO2 is available in vast quantities—from the stomatal opening and the substomatal cavity, the buffer from where CO2 is used for photosynthesis. Any changes in turbulence, wind speed, and wind direction will also affect the thickness of this laminar boundary layer and thus have an effect on the exchange of trace gases, heat, and momentum between plants and the atmosphere. Figure 7 shows that depending on plant leaf shape the laminar boundary layer and thus the wind speed profile at varying distances from the leaf surface show a relatively large microscale variation.


<Figure 7 near here> On much larger scales, as wind blows over oceans and open water, it induces

mixing of the surface layer, thereby enhancing the exchange of gases across the water surface, which is important for the oxygen content in the water and uptake or release of CO2 and CH4 to and from water bodies. Similar mixing occurs in the air above the surface which reaerates the plant canopy and essentially is responsible for resupplying photosynthetically active plants with CO2 from the atmosphere, while at the same time O2 produced by plants is carried away and mixed into the surface layer of the atmosphere.

Evaporation and Transpiration A widely investigated topic of wind effects on ecosystems not covered in this

article is found in the hydrological and biophysical literature on evaporation of water from ecosystems and transpiration from plants, either as a component of the hydrological cycle (the viewpoint taken by ecohydrologists), or in combination with CO2 exchange (the ecophysiological viewpoint).

Dispersal of pollen, spores, and microorganisms The explosive pollen release from many wind-pollinated plants, particularly tree

species with copious pollen production, is triggered by moderately gusty winds. Similarly, spores from the Swiss fern Asplenium ruta-muraria are released by wind-induced shaking of the leaves (ballanemochory) or by the physical energy of impacting raindrops. In some palms (Chamaedorea pinnatifrons (Jacq.) Oerst. and Wendlandiella sp.) in Peruvian Amazonia the release of the pollen is triggered by movements of insects inside the flowers, and the term “insect induced wind pollination” has been suggested since these insects do not normally also visit the female flowers of these palms.

On the ground, a wind gust can pick up small dust particles, sedimented pollen and microorganisms such as bacteria and mites from the surface or host organism. Once in the air, moderately turbulent winds are already sufficient to keep such small biotic and abiotic objects aloft. The general concept of updraft of a voluminous body in the atmosphere is described by Stoke’s law of sedimentation, where the terminal falling velocity Vt of an object is

!

Vt =2mg

cw"A (1)

with m the mass (kg m−3), g the gravitational acceleration (≈9.81 m s−2), cw the

friction coefficient (≈ 1 for circular bodies, <1 for aerodynamically formed bodies), ρ the density of the air (≈ 1.2 kg m−3 at sea level), and A the projected surface of the body (m2). Figure 8 shows this terminal falling velocity for small organisms of 1 µm to 2.5 mm and how typical vertical wind speeds in the air can counteract the falling of such objects, once they are dispersed in the air. In this respect wind has almost exactly the same effect as the water flow in rivers: under high turbulence and horizontal speeds animals may find sheltered spots where they are not picked up by the motion of water or the air, but once they loose adhesive contact their body size and weight


may be too small to grasp ground again, and they become suspended in the fluid until they happen to end up in a calmer area where their settling velocity is greater than the wind (or water) motion, which allows them to reach the ground surface again. Figure 8 shows that bodies with a diameter smaller than ≈10 µm are normally too small to return to the ground in the turbulent atmosphere. Thus, for such small bodies, impaction becomes the most relevant process how they can be elimiated from the atmosphere, that is, when they physically hit the surface of a tree or another plant. The sticky stigma of a flower’s pistil further helps to capture pollen even when impaction is weak.


Wind pollination is considered inefficient compared with insect pollination. This finding has led to the hypothesis that the rise to dominance of the angiosperms over gymnosperms at evolutionary time scales is due to reproductive innovations, especially those involving coevolution with biotic gene dispersers. This has most likely contributed to the present-day situation that conifers are biogeographically restricted to stressful environments where gymnosperms may suffer a comparative disadvantage if pollinators face persistently high wind speeds.

Influence on small animals and seed dispersal Small insects need to adopt to wind speeds. Studies carried out in a wind tunnel

indicate that weak winds >0.2 m s−1 already have an effect on the flight and landing behaviour of the bug Prostephanus truncatus (Horn). In the open landscape, moskitoes (Anopheles marajoara in Brazil) can only freely navigate in air with wind speeds below about 0.85 m s−1 (3 km h−1). From the aphid parasitoid Aphidius nigripes it is reported that males generally did not reach females at wind speeds of 1.0 m s−1, as the majority of individuals taking flight in the pheromone plume (81.8%) were unable to sustain upwind flight. The general picture is that as wind speed increases these small animals increasingly loose control over their flight trajectory and may no longer target their pray or mate as desired. Swallows, for example, are known to fly close to the ground before thundershowers, where the prefrontal increase in wind speed restricts the activity of small flying insects to the few lowest meters close to the ground. In summer, when moskito abundance is enormous in the Arctic, reindeer and caribou select windy locations for resting and rumination, preferrably close to the sea shore, on gravel pads in large rivers with a well-developed diurnal valley wind system, or on snow fields with thermotopographic wind resulting from the contrasting surface temperatures between snow and vegetation or rocky surfaces.

Some spider species benefit from this effect by letting themselves drift away—termed ballooning in the scientific literature—to explore new habitats using a short thread that increases their updraft and thus drifting distance at higher wind speeds. Although there are contradictory views between authors about the importance of various environmental factors, it is widely agreed upon the upper wind speed limit of 3 m s−1 for ballooning. Most work has been focusing mostly on the meteorological conditions at the time of take-off, whereas literature on the underlying motivation of the spiders and instigation of pre-ballooning behaviour (climbing to a prominent point and silk release) is very limited and largely considered supposition by some experts. Since spiders are important polyphagous predators on arable farmland, the high mobility of ballooning species means that they are often the first to arrive in a crop


newly infested with pests, and have a role in controlling the outbreak until more specific predators arrive.

In a similar way as ballooning spiders take advantage of the horizontal translocation by wind plants profit from the wind to disperse their seeds. The parachute type seeds of Asteraceae and the winged seeds of Acer, Fraxinus, Ulmus and many coniferous trees are good examples of how plants benefit from available wind to spread out faster than would be possible without the help of the wind. For seeds that do not have wings, hairs or parachute type annexes, the Stoke’s settling velocity (Eqn 1, Figure 8) applies and explains why in general small seeds are wind dispersed because of their long residence time in the atmosphere (the residence time is inversely proportional to the terminal fall velocity) than large seeds, that lead to small dispersial kernels downwind of the seeder plant unless the seeds are dispersed by animals. A special case exists for vegetation near open water bodies, where large buoyant seeds can float on the water and be dispersed by its currents, such that even large and heavy seeds can be transported over long distance that would not be possible by the wind alone.

Nature has brought about a wealth of shapes and forms of seeds that do not correspond with the simplest version of a spherical with cw=1. For some plants, specific wind tunnel studies have been carried out to determine the true dispersal capacity of seeds. For six Canadian perennial grassland species with different seed aerodynamic attributes it was investigated how dispersal distances vary with varying wind speeds and release heights. Dispersal distances of long-range dispersed seeds (99th percentile values) increased exponentially with wind speed. At wind speeds of 14 m s−1, predicted maximum distances were 10–15 m for small and relatively heavy spherical seeds and 20–30 m for large and relatively light cylindrical or disk-like seeds. In the study area, wind gusts >10 m s−1 at plant height occur at least annually, and plants of the selected species live up to several decades. This suggests a great potential for long-range dispersal during the lifetime of a plant. It is argued that plants may gain wider dispersal of seeds by increasing the release height (e.g., taller infructescences) and by requiring stronger winds to release seeds (e.g., dispersal in autumn and winter).

Transport distance is one aspect, the other is the release of seeds from the flower heads by wind speed. In a wind tunnel study with flower heads of two thistle species, Carduus nutans and Carduus acanthoides, with ripe seeds, the effect of laminar versus turbulent flows of increasing velocity was investigated. Seed release increased with wind speeds of both laminar and turbulent flows. However, far more seeds were released, at significantly lower wind speeds, during turbulent flows. In other cases, the seeds are primarily dispersed by the wind, followed by secondary dispersal by rodents living on the ground which collect the seeds and cache them in the soil. Treatment by rodents, primarily yellow pine chipmunks (Tamias amoenus), of four species of pine seeds, lodgepole pine (Pinus contorta, 8.7 mg seed weight), ponderosa pine (Pinus ponderosa, 55 mg), Jeffrey pine (Pinus jeffreyi, 157 mg), and sugar pine (Pinus lambertiana, 213 mg), that vary in size and weight was studied in the Carson Range of western Nevada. For the species examined, seed size appeared to have had little effect on several other attributes, including mean dispersal distance, substrate choice, and microhabitat choice. It was found that although a larger seed size and weight decreases primary wind dispersibility of pine seeds, the secondary dispersal by scatter-hoarding rodents compensates for poor wind dispersal so that total dispersibility of large-seeded pines is not compromised.


Influence on bird and insect migration At a much larger scale many long-distance migratory bird species have adopted

flight tracks that best profit from large-scale wind fields on the Earth, which saves energy and thus increases the survival rate. On the other hand side, migratory birds that are facing strong headwinds, may suffer severe losses if such an occurrence combines with low temperatures, scarce food resources or the like. In general, the nocturnal and diurnal wind directions and speeds are not necessarily the same. Near the ground the so-called low-level jet, a relatively strong wind with its maximum speed at only 100–300 m above the ground surface is active at night, whereas the atmosphere may be calm during the day, and wind speeds are higher aloft.

In a study on the migration patterns and environmental effects on stopover of monarch butterflies at Peninsula Point, Michigan, it was found that wind direction had a significant influence on the number of monarchs recorded on each count over a seven-year period, with higher counts during north winds.

On smaller scales, it has been shown for two dragon fly species, Pantala hymenaea and Pantala flavescens, in natural flight over a lake at ambient wind speed and direction, that they are able to compensate at least partially for crosswind drift, which shows evidence for use of a ground reference to correct for drift when flying over water, and their ability to cope with much higher wind speeds (5.0 m s−1) than small insects are able to.

No wind effect? Although there are many studies that found ecological effects of wind on

animals, plants, and ecosystem processes, it should be remembered that there are other studies that were unable to find such effects. For example, the bat Pipistrellus pipistrellus in Oxfordshire did not show an apparent response to wind nor rain in the time spent outside the roost. This is remarkable since it feeds on insects and one might expect a behaviour similar to the one known from swallows. Since it is generally difficult to publish negative results in the scientific literature, and moreover wind as a three-dimensional vector variable makes it particularly challenging to derive the relevant information from simple measurements (e.g. if mean wind speed was measured when actually turbulent kinetic energy would have been the variable with higher predictive power) the lack of clear statements on where wind does not have an effect should not come as a surprise.

In forest ecology it has been postulated that there are two main factors why biotic effects of wind have not been well studied: (1) the difficulty of measuring wind in the field and separating its effects from the confounding variables of temperature and humidity; and (2) the expense involved in carrying out wind tunnel experiments in the laboratory.

Wind chill and heat index The bioclimatic temperature sensed by an organism can differ considerably

from the absolute physical temperature that is measured by conventional instruments. For humans, elaborate concepts to compute a wind chill temperature have been established to account especially for the effect of wind. The concept bases on the knowledge that increasing mean wind speeds increase also turbulent, and thus the heat transport away from an organism that has a warmer skin temperature than the


atmosphere. Although it is widely known that the ambient moisture or humidity in the air has an additional influence, for the sake of simplicity most approaches only consider wind speed as a specific factor when considering wind chill.

Controlled experiments with humans were carried out to determine the functional relationship between wind chill and percieved temperature. This was not possible with primates, where thermoregulation is known to be an important ecological constraint. Shade temperatures, solar radiation, humidity and wind speed all serve to alter an animal's ‘perceived’ temperature. In a recent review, three thermal indices currently available were compared. Black bulb temperatures can account for the effect of solar radiation, with wind chill equivalent temperatures and the heat index providing quantifiable estimates of the relative impact of wind speed and humidity, respectively. The authors presented three potential indices of the ‘perceived environmental temperature’ that account for the combined impact of solar radiation, humidity and wind speed on temperature, and performed a preliminary test of all of the climatic indices against behavioural data from a field study of chacma baboons (Papio cynocephalus ursinus) at De Hoop Nature Reserve, South Africa. It was found that the complexity of the interactions among environmental factors that influence thermoregulation in primates will require the development of biophysical models of the thermal characteristics of the species and its environment. Until such models are developed, however, it is concluded that wind chill and heat indices should permit a more detailed examination of the thermal environment, allowing thermoregulation to be given greater precedence in future studies of primate behaviour.

Another widely established approach is not to try to compute a bioclimatic temperature or index, but relate the metabolic energy consumption of an animal to environmental factors.

Metabolic stress by wind Small animals can profit from the presence of a laminar sublayer (Figure 1)

even under highly turbulent conditions. Due to the much lower heat exchange in that laminar layer they may avoid metabolic stress under high winds. This is almost impossible for larger animals, such as breeding arctic shorebirds. It was found that tarsus length in all shorebirds breeding in the Canadian arctic shows an evolutionary response to average metabolic stress encountered across the breeding range, such that birds nesting in metabolically stressful environments have relatively shorter legs. Longer-legged birds living in colder environments will experience greater metabolic costs because their torsos are elevated farther away from the ground's wind-dampening boundary layer. It was suggested that the widely known Allen’s rule that relates the metabolic rate of an organism to its volume should be extended: body-supporting appendages of homeotherms may be shorter in colder environments so as to take advantage of a boundary layer effect, thereby reducing metabolic costs.

Another study that investigated the effects of water levels and weather on wintering herons and egrets found that larger and longer-legged species tended to be found in deeper water, although species frequently were found together in shallow water. Severe weather with high winds caused the birds to suspend foraging and remain sheltered from the wind. Consequently, a higher percentage of smaller heron and egret species did not survive severe storms since searching shelter from wind meant fasting. A three-day storm period was simulated to lead to >10% decline in body mass of the smaller herons and egrets.


Wind throws and wild fires Extreme events with high wind speeds are important in the life cycle of many

ecosystems, especially forests. Hurricanes in the tropics, tornadoes and other windstorms further north and south reshape forest ecosystems via windthrows that eliminate the weakest and thus most often the oldest individuals in the forest canopy. For example, in New England forests, leaning is the most prevalent damage to young stands, whereas breakage and uprooting dominated in older stands. Breaking was slightly more important in older conifer than hardwood stands, comprising 6–14% of the stems and generally occurring 1–5 m from the ground, but numbers vary not only widely between species and stand composition but also among storm events in the same stand.

Since heavy storms are often accompanied with severe lightning strikes the wind effect can easily be a combination of wind and fire. When a wildfire starts then the wind conditions will strongly determine how quickly the fire advances with the wind, and what damage is done to the ecosystem. In some cases, such as the Bishop pines, the fire even is necessary to free the seeds in the cones and initiate the life cycle of this forest type. In the gaps the new vegetation can resprout, and since more light and precipitation reaches the ground this provides niches and living space for early successional plants. As a consequence also the fauna may be affected. Organisms with a life size that is much smaller than gaps in forests may only find a suitable ecological niche in the gaps that shift their location over the years. In subalpine forests of the Swiss alps it was found that the gaps created by windthrows add considerably to the species diversity of macrofungi. Larger animals such as black bears in southeast Alaska were found to react in just an opposite way: 58% of the den sites were found in forests that were most protected from catastrophic storm effects, and only 6% in forests most exposed to storm damage. These results suggest that the effect of catastrophic windstorm disturbance on overwinter habitat for black bears is the key factor influencing the site selection for black bear dens.

Further Reading

Cartar, R. V. and Morrison, R. I. G. (2005). Metabolic correlates of leg length in breeding arctic shorebirds: the cost of getting high. Journal of Biogeography 32, 377–382.

de Gayner, E. J., Kramer, M. G., Doerr, J. G. and Robertsen, M. J. (2005). Windstorm disturbance effects on forest structure and black bear dens in southeast Alaska. Ecological Applications 15, 1306–1316.

Doutt, J. K. (1941) Wind pruning and salt spray as factors in ecology. Ecology 22, 195–196.

Ellenberg, H. and G. K. Strutt (1988). Vegetation Ecology of Central Europe. Cambridge: University Press.

Ennos, A. R. (1997). Wind as an ecological factor. Trends in Ecology and Evolution 12, 108–111.

Foster D. R. (1988). Species and stand response to catastrophic wind in central New England, U.S.A. Journal of Ecology 76, 135–151.


Geiger, R., Aron, R. H. and Todhunter, P. (1995). The Climate Near the Ground. Braunschweig: Vieweg.

Grace, J. (1977) Plant responses to wind. London: Academic Press. Hill, R. A., Weingrill, T., Barrett, L. and Henzi, S. P. (2004). Indices of

environmental temperatures for primates in open habitats. Primates 45, 7–13. Moore, P. D. (1988). Forest ecology: Blow, blow thou winter wind. Nature 336, 313–

313. Myers, R. K. and van Lear, D. H. (1998). Hurricane-fire interactions in coastal forests

of the south: a review and hypothesis. Forest Ecology and Management 103, 265–276.

Senn-Irlet, B. and Bieri, G. (1999). Sporocarp succession of soil-inhabiting macrofungi in an autochthonous subalpine Norway spruce forest of Switzerland. Forest Ecology and Management 124, 169–175.

Stienen, E. W. M., Brenninkmeijer, A. and Geschiere, C. E. (2001). Living with gulls: The consequences for Sandwich Terns of breeding in association with Black-headed Gulls. Waterbirds 24, 68–82.

van Dorp, D., van den Hoek, W. P. M. and Daleboudt, C. (1996). Seed dispersal capacity of six perennial grassland species measured in a wind tunnel at varying wind speed and height. Canadian Journal of Botany–Revue Canadienne de Botanique 74, 1956–1963.

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Suggested cross-references to other articles

11: Behavioral ecology—Dispersal 644: Population dynamics—Dispersal / migration

366: Ecosystems—Wind shelter belts 563: General ecology—Wind erosion

724: Global Biogeochemical Cycling—Carbon Cycle 1: Short-Term Dynamics 496: General ecology—Fire

33: Behavioral ecology—Thermoregulation 44: Ecological Engineering—Ecohydrology

289: Ecological processes—Reaeration 285: Ecological processes—Photosynthesis

738: Global Biogeochemical Cycling—Greenhouse Effect and Greenhouse Gases


Tables, table headings and footnotes

Table 1: Relations between canopy heights (m) and aerodynamic roughness length (m) for different vegetation types. After Garratt J. R. (1992) The atmospheric boundary layer. Cambridge University Press.

Vegetation Type Canopy height (m) Roughness length (m)

Forest tropical 32–35 2.2–4.8

coniferous 10.4–27.5 0.28–3.9 pine 12.4–15.8 0.32–0.92

Woodland trees 10–15 0.4 savannah 8–9.5 0.4–0.9

Crops vines 0.9–1.4 0.023–0.12 beans 1.18 0.077

corn 0.8 0.064 wheat 0.25/0.4/1.0 0.005/0.015/0.05

wheat stubble 0.18 0.025 Grass thick/thin 0.1/0.5 0.023/0.05

sparse 0.025/0.015/0.45/0.65 0.0012/0.002/0.018/0.039 Soil bare — 0.001–0.01

Multimedia annexes and captions None.


Figures and their captions

Fig. 1: Transition from laminar to turbulent boundary layer as wind blows over

a vegetation surface changing from smooth to rough. From Grace (1977).

Fig. 2: Diffusion pathways at a leaf surface on a windy day. C, cutin; ec,

epidermal cell; ew epicticular wax; gc, guard cell; mc, mesophyll cell; p, pore; s, sub-stomatal cavity; sc, subsidiary cell. From Grace (1977).


Fig. 3: Wind profiles (a) in a pine forest canopy of 16 m height (h), and (b) in a

maize canopy of 2.1 m height. Both profiles show the mean horizontal wind speed, normalized for the wind speed at the top of the canopy (z = h). In case of the forest the specific wind profile inside the trunk space (z/h < 0.5) a secondary maximum of the wind speed can be seen. For maize profiles for light winds of 0.88 m s−1 (▲) and strong winds of 2.66 m s−1 (△) at the top of the canopy are shown. From Raupach M. R. and Thom A. S. (1981). Turbulence in and above plant canopies. Annnual Review of Fluid Mechanics 13, 97–129.

Fig. 4: Wind pruning effect on Metrasideros polymorpha trees of a cloud forest

on the Big Island of Hawaii (left). Under the strong onshore winds leafs are detached from branches until only clusters of leaves at the outer margin of the tree volume remain (right). Photographs by Werner Eugster.


Fig. 5: Trees growing under conditions with persistently high wind speeds from

a specific direction retain their asymmetric shape even when there is no wind. In this example from near Zweisimmen, Switzerland, the daytime up-valley wind (from right to left) shaped the characteristic habitus of these trees. Photograph by Werner Eugster.

Fig. 6: Effect of a change from smooth to rough terrain. Fetch—that is the

distance of uniform surface in the upwind direction—over the rough terrain was: ●, 0.32 m; ×, 1.18 m; ○, 2.32 m; , 6.42 m; +, 16.42 m. From Grace (1977) after Bradley (1968), Quart. J. Roy. Meteorol. Soc. 94, 361–379.


Fig. 7: The boundary layer over a Populus leaf. Profiles of (a) mean wind speed

and (b) turbulence, shown in transverse sections in a laminar free stream. From Grace (1977) after Grace and Wilson (1976) Journal of experimental Botany 27, 231–241.

Fig. 8: The terminal fall velocity of ball-shaped objects (pollen, seeds, bacteria,

microorganisms) compared to typical updrafts in the turbulent atmosphere (up to ≈0.2 m s−1) and typical mean updrafts in convective clouds (thunderstorms). The thick line applies to objects that have a similar density as water. If updrafts are stronger than the terminal fall velocity, then an organism or particle in the atmosphere remains suspended in the atmosphere and will only deposit on obstacles such as trees due to impaction. Larger organisms that are subject to a high terminal fall velocity need active means to keep themselves aloft.

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