Agriculture Reference
In-Depth Information
The transport and deposition of salt by wind can have
major impact on the zonation of vegetation along the
coast, and requires that only salt-tolerant crops be planted
in areas subject to deposition. In some locations, natural
topographic features along the coast, such as sand dunes,
block wind-blown salt, allowing salt-sensitive crops to be
planted on their leeward side. Avocado trees, for example,
were once planted in such protected locations along the
coast of California from Santa Barbara to San Diego (but
more recently such protected areas have become much
sought after locations for residential home construction).
Windbreaks may also be used to achieve the same effect.
are wind-pollinated, and depend on wind to distribute
pollen from the male structures of plants to the seed-
producing female structures of other plants.
MODIFYING AND HARNESSING WIND IN
AGROECOSYSTEMS
An understanding of the impacts that wind can have on
agroecosystems, as well as the mechanisms of those
impacts, gives farmers the opportunity to develop means
of both mitigating the negative effects, as well as taking
advantage of positive effects. In addition, the energy of
wind can be harnessed for an array of uses in agriculture.
T RANSPORT OF D ISEASE AND P EST O RGANISMS
M EASURING W IND
Wind serves as a means of transport for a range of organ-
isms that are pests or diseases in agroecosystems. Bacteria
and fungi depend on wind to transport spores from
infected plants to new hosts, and many insect pest species
take advantage of the wind to move long distances in the
environment. Several aphids, for example, have a winged
stage for dispersal and a wingless stage for development
of sedentary pest populations on host plants. The wings
of these aphids do not serve for much more than holding
the insects aloft while the wind carries them where it may.
Of course, if the landing site is an uninfested host plant,
a pest problem can develop.
The females of many insect pests, such as the apple
codling moth, release a sex pheromone and then depend
on wind dispersal of the chemical in order to attract males
for mating. The seeds of a large number of unwanted
plants or weeds in agroecosystems are dispersed by wind
as well. Since small propagules and even small organisms
can be lifted hundreds of meters into the air on wind
currents and then transported several hundred kilometers
away, it is very difficult for farmers to escape the constant
“rain” of potential problems. We will deal with the agro-
ecological management of such dispersal problems in
Chapter 15.
Wind is usually measured with a device known as ane-
mometer. Cup anemometers consist of three or four hor-
izontally rotating arms with small cups on the ends fixed
to a vertical shaft that activates a dial or recorder as it
turns. Such a device will record wind from any horizontal
direction, and based on the total revolutions measured,
average wind velocity over time can be determined. A fan
anemometer can record lower wind speeds more accu-
rately, but has to be pointed in the direction of the wind.
Thermal anemometers, which operate on the basis of the
relation between ventilation and heat transfer, are used for
very low wind speeds that are not recorded well with fan
or cup systems. Other types of equipment exist to record
wind gusts and wind direction.
Measuring average wind speed and direction is only
one part of gaining an understanding of patterns of air
movement in an agroecosystem. It is also important to
know how local wind patterns are reduced to microcli-
matic patterns as wind encounters barriers. The barriers
can be individual plants, natural topographic variation, or
intentionally placed barriers of some kind. Use of such
barriers will depend on how they effect the wind we are
trying to modify or take advantage of.
B ENEFICIAL E FFECTS OF W IND
T ECHNIQUES FOR M ODIFYING W IND P ATTERNS AND
M ITIGATING W IND E FFECTS
Some of the most important beneficial effects of wind take
place at the microclimatic level. Internal to the agroeco-
system, especially in the canopies of cropping systems,
air movement is essential for mixing the atmosphere.
Good air circulation maintains optimal gradients of CO 2 ,
disperses excess humidity, and can even increase active
gas exchange. Adequately mixed air lowers humidity
levels at the leaf surface, thereby reducing the potential
for many diseases. In warm climates, wind also has the
important effect of enhancing convective and evaporative
cooling in the direct sun.
Wind is also required for the production of grain
crops such as corn, oats, and wheat. These crop plants
There are many ways to manage the wind environment in
cropping systems. Some are as simple as orienting the
planting of rows of a crop in such a way as to funnel a
prevailing wind through the crop; others are more dra-
matic, such as planting windbreaks or shelterbelts, or
using intercropping systems that combine wind-sensitive
crops with more tolerant ones.
Windbreaks
Windbreaks (also known as shelterbelts and hedgerows)
are structures — usually made up of trees — that modify
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