Impacts of Climate Change on Agriculture - Climate Resilient Agriculture for Ensuring Food Security - page 49

depend on the relative response of crops and
weeds to climatic changes as well.
The different response of C3 and C4 crops
may encourage changes in areas sown. It may, for
example, accelerate the recent trend in India
towards wheat, rice, and barley and away from
maize and millets, a trend that has largely been
driven by the promise of greater increases in
yield. It may tend to reverse the current trend in
temperate areas away from perennial rye grass (a
C3 crop) towards silage maize (C4) as the major
forage crop, and in the USA, it might encourage
a tendency to switch from maize to soybean (C3)
for forage.
Many of the pasture and forage grasses of the
world are C4 plants, including important prairie
grasses in North America and Central Asia and in
the tropics and subtropics (Edwards and Walker
1983 ). The carrying capacity of the world's major
rangelands is thus unlikely to benefi t substan-
tially from CO 2 enrichment. Much, of course,
will depend on the parallel effects of climatic
changes on the yield potential of these different
crops.
The actual amount of increase in usable yield
rather than of total plant matter that might occur
as a result of increased photosynthetic rate is also
problematic. In controlled environmental studies,
where temperature and moisture are optimal, the
yield increase can be substantial, averaging 36 %
for C3 cereals such as wheat, rice, barley, and
sunfl ower under a doubling of ambient CO 2 con-
centration (Table 4.4 ).
Little is also known about possible changes in
yield quality under increased CO 2 . The nitrogen
content of plants is likely to decrease, while the
carbon content increases, implying reduced
protein levels and reduced nutritional levels for
livestock and humans. This, however, may also
reduce the nutritional value of plants for pests, so
that they need to consume more to obtain their
required protein intake.

Fig. 4.2 Typical photosynthesis response of plants to
CO 2 . Net photosynthesis of wheat is about 70 mg of CO 2
dm h compared with maize (about 55 mg of CO 2 dm h) for
equivalent light intensity (0.4 cal cm min). Maize is satu-
rated at a lower CO 2 concentration (c. 450 ppmv) than
wheat (c. 850 ppmv) (Adapted from Akita and Moss 1973 )

benefi t temperate and humid tropical agriculture
more than that in the semiarid tropics and that if
the effects of climatic changes on agriculture in
some parts of the semiarid tropics are negative,
then these may not be partially compensated by
the benefi cial effects of CO 2 enrichment as they
might in other regions.
In addition it should be noted that although C4
crops account for only about one-fi fth of the
world's food production, maize alone accounts
for 14 % of all production and about three-
quarters of all traded grain. It is the major grain
used to make up food defi cits in famine-prone
regions, and any reduction in its output could
affect access to food in these areas.
C3 crops in temperate and subtropical regions
could also benefi t from reduced weed infestation.
Fourteen of the world's 17 most troublesome ter-
restrial weed species are C4 plants in C3 crops
(Morison 1989 ). The difference in response to
increased CO 2 may make such weeds less com-
petitive. In contrast, C3 weeds in C4 crops, par-
ticularly in tropical regions, could become more
of a problem, although the fi nal outcome will

4.2.2 Impact on Water Use by Plants

Just as important may be the effect that increased
CO 2 has on the closure of stomata, small openings