Temperature - Agroecology: The Ecology of Sustainable Food Systems - page 59

5 Temperature

The effect of temperature on the growth and develop-
ment of plants and animals is known and easily demon-
strated. Each organism has certain limits of tolerance
for high and low temperatures, determined by its partic-
ular adaptations for temperature extremes. Each organ-
ism also has an optimum temperature range, which can
vary depending on the stages of development. Because
of their different reactions to temperature, papayas are
not planted in the cool coastal temperate environment
of the Monterey Bay of California, and apples would
not do well if planted in the humid tropical lowlands of
Tabasco, Mexico.
Thus the temperature range and degree of tempera-
ture fluctuation in an area can set limits on the crop
species and cultivars that a farmer can grow, and can
cause variations in quality and average yield for the crops
that are grown. It is necessary to consider the temperature
factor in selecting crops that are appropriate to the range
of temperature conditions that might occur from day to
day, between day and night, and from season to season.
Aboveground temperatures are as important as those
below ground.
When we measure the temperature of the air, soil, or
water, we are measuring the heat flow. In order to fully
understand temperature as a factor, it is useful to think of
this heat flow as part of the energy budget of the ecosys-
tem, the basis of which is solar energy.

reflected or absorbed. The absorption process at the sur-
face, by which short-wave light energy is converted into
long-wave heat energy, is known as insolation . Heat
formed by insolation can be stored in the surface, or
reradiated back into the atmosphere. Some of the heat
reradiated into the atmosphere can also be reflected back
to the surface.
As a result of these processes, heat energy is trapped
at and near the earth's surface, and the temperature there
remains relatively high compared to the extreme cold of
the outer space. Overall, this warming process is termed
“the greenhouse effect.”
Temperatures at the earth's surface vary from place
to place, from night to day, and from summer to winter;
nevertheless, an overall equilibrium is maintained
between the heat energy gained by the earth and its
atmosphere, and the heat energy lost. This balance
between heating and cooling is represented in the fol-
lowing equation:

S (1- α ) + L d - L u ± H air ± H evap ± H soil = 0

where
S = solar gain
α = the albedo of the earth's surface (with a value

between 0 and 1)
L d = the flux of long-wave heat energy to the surface
L u = the flux of long-wave heat energy away from the
surface
H = the gain or loss of heat energy from air, soil, and
water (evap).

THE SUN AS THE SOURCE OF HEAT ENERGY
ON EARTH

The energy flowing from the sun is predominantly short-
wave radiation, usually thought of as light energy made
up of both visible and invisible spectra. Recall that the
fate of this energy once it reaches the atmosphere of the
earth was discussed in the previous chapter and dia-
grammed in Figure 4.1. Incoming solar radiation is either
reflected, dispersed, or absorbed by the atmosphere and
its contents. Reflected and dispersed energy is little
changed, but absorbed energy is converted to a long-wave
form of energy manifested as heat. Similarly, the short-
wave energy that reaches the earth's surface is either

This equilibrium is currently undergoing a shift in
response to human-induced changes in the atmosphere.
These changes include a rise in carbon dioxide levels
from the combustion of fossil fuels. As more carbon
dioxide and other “greenhouse gases” are added to the
atmosphere, more heat is trapped between the atmo-
sphere and the surface. Many scientists are concerned
about the possible impacts on agriculture by a global rise
in temperature.

59