DRAFT MANUSCRIPT 304 — WIND EFFECTS
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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.
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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.
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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>
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<Figure 2 near here>
Wind has a kinetic energy,
Ek =
m 2
V
2
(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
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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>
<Figure 3 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.
<Figure 4 near here>
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.
<Figure 5 near here>
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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.
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<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 windinduced 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
c w "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
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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.
<Figure 8 near here>
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
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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.
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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
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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 winddampening 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: bodysupporting 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.
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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.
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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 Blackheaded 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.
van Gardingen P. and Grace J. (1991) Plants and wind. Advances in Botanical
Research 18, 189–253.
Vonlanthen C. M., Kammer P. M., Eugster W., Bühler A. and Veit H. (2006). Alpine
vascular plant species richness: the importance of daily maximum temperature
and pH. Plant Ecology 184, 1–9.
Weyman, G. S. (1993). A review of the possible causative factors and significance of
ballooning in spiders. Ethology, Ecology and Evolution 5, 279–291.
Woodward, F. I. (1993). The lowland-to-upland transition modeling plant-responses
to environmental change. Ecological Applications 3, 404–408.
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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
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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
trees
10–15
0.4
savannah
8–9.5
0.4–0.9
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
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
bare
—
0.001–0.01
Woodland
Crops
Grass
Soil
Multimedia annexes and captions
None.
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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, substomatal cavity; sc, subsidiary cell. From Grace (1977).
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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.
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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.
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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.