Impacts of Climate Change on Agriculture - Climate Resilient Agriculture for Ensuring Food Security - page 55

continue to increase beyond a specifi c threshold,
a crop's productive summer growing season
could become shorter, thus reducing the yield
(Cure and Acock 1986 ).
Crops such as rice, potatoes, corn, wheat, and
soybeans have optimal microclimate tempera-
tures and an optimal growing season. Recognizing
these optimal levels will enable farmers to alter
their mix of crops in response to their region's
changing temperatures. However, turning to dif-
ferent crops will not guarantee that a farmer will
produce the same amount of food or enjoy the
same profi ts.

not begin until temperature exceeds a threshold;
then the rate of development increases broadly
linearly with temperature to an optimum, above
which it decreases broadly linearly (Squire and
Unsworth 1988 ).
However, the effect of this development on
plant biomass depends on whether the growth
habit of the plant is determinate (i.e., it has a dis-
crete life cycle which ends when the grain is
mature, such as in cereals) or whether it is inde-
terminate (i.e., it continues to grow and yield
throughout the season, such as in grasses and
root crops). Temperature increase shortens the
reproductive phase of determinate crops,
decreasing the time during which the canopy
exists and thus the period during which it inter-
cepts light and produces biomass (Fig. 4.4b ).
The canopy of indeterminate crops, however,
continues to intercept light until it is reduced by
other events such as frost or pests, and the dura-
tion of the canopy increases when increased
temperatures extend the season over which crops
can grow (e.g., by delaying the fi rst frosts of
autumn) (Fig. 4.4c ). An increase in temperature
above the base but not exceeding optimum tem-
peratures should therefore generally lead to
lower yields in cereals and higher yields of root
crops and grassland, though higher temperatures
may also lead to higher rates of evaporation and
therefore reduced moisture availability that can
also be expected to affect yields.

4.3.3 Effects on Growth Rates

In high mid-latitude regions (above 45°), at high
latitudes (above 60°), and at high altitudes, tem-
perature is frequently the dominant climatic con-
trol on crop and animal growth. It determines the
potential length of the growing and grazing
seasons and generally has a strong effect on the
timing of developmental processes and on rates
of expansion of plant leaves. The latter, in turn,
affects the time at which a crop canopy can begin
to intercept solar radiation and thus the effi ciency
with which solar radiation is used to make plant
biomass (Monteith 1981 ).
In general, plant response to temperature fol-
lows as indicated in Fig. 4.4 . Development does

Fig. 4.4 Temperature and development of canopy expan-
sion. ( a ) Idealized relation between developmental rate and
temperature. Development does not begin until temperature
exceeds a threshold ( T b , the base temperature); then devel-
opmental rate increases linearly with temperature to an

optimum ( T o ), above which it decreases linearly. ( b ) and ( c )
Effect of temperature on the relation between time and frac-
tional interception of solar radiation by a canopy, for a
determinate ( b ), and indeterminate ( c ) species (----- cooler;
—— warmer) (Squire and Unsworth 1988 )