Wind - Agroecology: The Ecology of Sustainable Food Systems - page 89

downslope. Such movement is very localized at first, but
eventually winds moving down single canyons can join in
an entire valley system to create a mountain wind . During
the day, the opposite effect can occur, and a valley wind
forms as heating of the valley floor causes warm air to
rise upslope.
When large air masses are forced over a mountain
range and down onto a plain or valley below, the falling
air mass expands. As a result, it heats up and its relative
humidity falls. This heating and drying process is called
catabatic warming and is responsible for the familiar rain
shadow effect. Winds caused by catabatic warming occur
commonly in the winter along east-facing slopes of the
Sierra Nevada and Rocky Mountain systems when a
cyclonic storm system moves inland and pushes air ahead
of itself, forcing the air over these mountain ranges. As
the air descends down the eastern or lee side of the moun-
tains, it creates warm winds known as Chinooks that can
be very gusty and cause rapid melting of snow on the
surface. Since the ground usually stays frozen during these
rather short-duration winds, plants can suffer considerable
damage from desiccation.
A similar kind of wind occurs occasionally during the
summer on the coastal slopes of southern California and
central Chile. When high-pressure cells form inland, the
falling air associated with these cells is pushed over the
coastal range mountains and down to the coastal plains
below. Called sundowners or Santa Anas, these warm
winds can come up quickly at the end of the day, forcing
temperatures to rise 10 to 15°C and relative humidity to
plummet from near dew point to less than 20%, all in just
a few minutes. This is a time of high fire danger, and crops
can be damaged by the dry, gusting winds. A similar
phenomenon can occur on the Isthmus of Tehuantepec in
southern Mexico, where during the dry-season months
high-pressure systems on the western side of the country
create hot and dry downslope winds on the eastern side.
Called southers or sures, these winds accentuate the
dryness of the dry season months.

humidity, and as long as the stomata are open, water vapor
from inside the leaf flows out. When there is no air move-
ment, the movement of saturated air outward from the
stomata creates a boundary layer of saturated air around
the leaf's surface. Air movement removes this boundary
layer, increases transpiration, and increases overall water
loss from the plant. The rate of desiccation increases pro-
portionately with wind speed until a wind speed of about
10 km/h, where a maximum rate of loss is reached.
Normal water loss from the plant can be readily
replaced by uptake from the roots and subsequent trans-
port to the leaves. But if the rate of desiccation exceeds
replacement, wilting can occur. Excessive wilting can
seriously affect the normal leaf function, especially photo-
synthesis, leading to slower growth of the entire plant and
even death.

D WARFING

There is a direct correlation between the wind and short-
ening of plant stature. The plants in alpine and coastal
dune ecosystems are often short because of relatively con-
stant high wind velocities. Crop plants that grow in areas
with constant wind normally have shorter stature than the
same crops planted in areas free of wind. Short stature is
the result of constant desiccation causing smaller cells and
a more compact plant. Where winds are more variable,
and extensive periods of calm alternate with periods of
high wind, plants tend not to be dwarfed.

D EFORMATION

When winds are both relatively constant and mostly from
the same direction, they can permanently alter the growth
form of plants. Windbreaks that show bent or deformed
plant development are good indicators of a constant pre-
vailing wind. Deformation can take many forms, from a
permanent lean away from the wind, to a flag shape or
a prostrate habit. Windborne ice is especially effective
in contributing to the deformation of vegetation.

P LANT D AMAGE AND U PROOTING

DIRECT EFFECTS OF WIND ON PLANTS

If excessive winds are relatively unusual events, and espe-
cially if they occur during heavy rain or snowfall, wind
can cause damage to standing plants. Leaves can be shred-
ded or removed, leaf surfaces can be abraded, branches
can be broken off the trunk, tops can be removed, and
whole plants can be uprooted. In areas where hurricanes,
cyclones, or tornadoes occur, even mature plants that have
been growing many years can suffer severe damage.
Single tall trees left, following selective logging, are very
prone to wind fall once they lose the protective environ-
ment of surrounding trees in a forest. This kind of damage
demonstrates the importance of windbreaks (discussed
later in this chapter).

The physical effects of wind on organisms can be of con-
siderable ecological importance. This is especially true in
areas prone to more constant wind, such as flat plains,
near the edge of the ocean, or in high mountain areas. In
general, as with all factors of the environment, the mag-
nitude of the wind's effect is dependent on its intensity,
duration, and timing.

D ESICCATION

Each stomatal opening in the leaf of a plant leads to an
air space in which gas exchange occurs at the surrounding
cell wall membranes. This air space is saturated with