Temperature - Agroecology: The Ecology of Sustainable Food Systems - page 65

dormant during the cold period, bulbous plants that die
back to the belowground plant parts, and annuals that
complete their life cycle and produce seeds, are all exam-
ples of plants avoiding cold.

occur over the long term in a certain geographic area.
Climatology, or the study of climatic patterns, can tell
us what the average temperatures for any particular part
of the earth might be, and the degree of variation from
the average that can be expected. There is little chance
in the near future that humans will be able to intention-
ally modify climate on any kind of large scale. This
is especially true for temperature. The large-scale
aspects of climate, such as cold fronts, windstorms, and
rainfall patterns, are best dealt with by selecting crops
adapted to the range of climatic conditions that are
expected.
But at the level of the individual crop organism or
crop field, there is an aspect of climate that can be
managed — the microclimate . Microclimate is the local-
ized conditions of temperature, humidity, and atmo-
sphere in the immediate vicinity of an organism.
According to some definitions, the microclimate is made
up of the conditions in a zone, four times the height of
the organism being considered. Although microclimate
includes factors other than temperature, farmers are
most likely to be concerned with temperature when
modifying microclimate or taking advantage of micro-
climatic variations.

T HERMOPERIOD IN P LANTS

Some plants need daily variation in temperature for opti-
mal growth or development. In a classic paper in ecophys-
iology (Went, 1944), it was demonstrated that tomato
plants grown with equal day and night temperatures did
not develop as well as tomato plants grown with normal
day temperatures and lower night temperatures. This
response occurs when the optimal temperature for growth
— which takes place mostly at night — is substantially
different from the optimal temperature for photosynthesis
— which takes place during the day.
Diurnal variation in temperature is encountered by
plants in many natural ecosystems and open-field agroeco-
systems, but in very controlled agroecosystems such as
greenhouses, the diurnal temperature variation is much
less pronounced. In other situations, plants from climates
with cool nights do not do as well in regions with relatively
constant day and night temperatures, such as the humid
tropics or in temperate continental regions during the
summertime.

M ICROCLIMATIC P ROFILE

V ERNALIZATION

Within a cropping system, the conditions of temperature,
moisture, light, wind, and atmospheric quality vary with
specific location. Conditions just above the canopy of the
cropping system can be very different from those in the
interior, at the soil surface, and below the soil into the
root zone. The specific microclimatic conditions along
a vertical transect within a cropping system form what
is called the microclimatic profile of the system. Both
the structure of the system and the activities of the com-
ponent parts have impact on the microclimatic profile.
The profile also changes as the component plant species
develop.
Tabl e 5. 1 shows the microclimatic profile of a corn,
bean, and squash intercropping system in a schematic
form, with each factor measured in relative terms through
five layers of the canopy. In such a system, the microcli-
matic profile is very different at each stage of develop-
ment, from early germination to full growth.
The belowground microclimate profile is also impor-
tant; it extends from the soil surface to a small distance
below the deepest roots of the crop plants. Under certain
conditions, the soil and atmospheric microclimates of a
crop may be so different as to cause problems for the crop.
For example, warm wind currents when the soil is very
cold can cause desiccation of the aboveground part of the
plant since the roots are unable to absorb water fast enough
to offset water loss.

Some plants need to undergo a period of cold, called
vernalization , before certain developmental processes can
take place. For example, in the California grasslands,
many native herbaceous species will not germinate until
after a cold spell of several days duration, even though
rainfall may have already occurred. Since the timing of
the first rain of the season in this area is highly variable
and early rain is usually followed by a very dry spell
before more consistent precipitation begins, if germination
were to occur with the initial rainfall, most of the new
seedlings would probably not survive. There is thus a
selective advantage to delaying germination until after
vernalization has occurred.
Many agricultural and horticultural plants respond to
vernalization. Lily bulbs, for example, are treated with
cold at the appropriate time before planting so that they
can be blooming for Easter in north temperate areas. In
other cases, seeds of crops are treated with cold before
planting in order to ensure more uniform germination.

MICROCLIMATE AND AGRICULTURE

Temperature has thus been discussed as a factor of cli-
mate. Climate is made up of the fairly predictable, but
highly variable, patterns in atmospheric conditions that