12.5.3. Cropping and Horticulture
Cropping in Australia recently has undergone great diversification, from predominantly
wheat and barley to include a variety of other crops, including rice, cotton,
pulses, and oilseeds. Cane sugar is grown extensively in coastal areas of Queensland.
Many of these crops are subject to frost limitations on seasons and to water
stress in dry spells; some are subject to direct heat stress or deterioration
during heat waves. For example, wheat grain protein composition deteriorates
after several days above 35°C (Burke et al., 1988; Behl et al.,
1993), making it less suitable for high-value uses such as pasta and breadmaking.
However, climate warming may allow earlier planting and faster phenological
development, resulting in little change in heat shock risk up to a 4°C mean
warming (Howden et al., 1999c). Independently, increasing CO2
can result in a decrease in wheat grain protein contentalso leading to
a decrease in breadmaking quality (Rogers et al., 1998). A potential
complication of these impacts is water stress that results in decreased yield
and potentially increased protein.
Figure 12-3: Percentage change in average annual total Australian
wheat yield for doubling of actual CO2 (to 700 ppm) and a range
of changes in temperature and rainfall. Yield response is shown for rainfall
changes of +20% (white), 0 (stippled), and -20% (black), for warmings of
Howden et al. (1999c) report a comprehensive study
of global change impacts on Australian wheat cropping. Studies were conducted
of changes in wheat yields, grain quality, and gross economic margins across
10 sites in the present Australian wheat belt. Results were scaled up to provide
national estimates, with and without varying planting dates. Response surfaces
were constructed across the full range of uncertainty in the CSIRO (1996a) scenarios
and are shown in bar-graph form in Figure 12-3. The estimated
increase in yield resulting from physiological effects of a doubling of actual
atmospheric CO2 is about 24%. The analysis assumes that the regional
distribution of cropping is unaffected. (This is not completely accurate, but
changes at the margins of present areas would not change the total yield much.)
The best variety of wheat is used under each scenario, with current planting
dates (a) and optimal planting dates (b) for each scenario. Note that yield
reaches a maximum at about 1°C warming with current planting dates but about
2°C with optimal planting dates and that yield drops rapidly with decreases
in rainfall. Under the SRES scenarios, warming in Australian wheat-growing areas
would exceed 2°C and could be well in excess of 6°C by 2100; actual
CO2 concentrations could be between 540 and 970 ppm.
Doubling CO2 alone produced national yield increases of 24% in currently
cropped areas, but with a decline in grain nitrogen content of 9-15%, which
would require increases in the use of nitrogen-based fertilizer of 40-220 kg
ha-1 or increased rotations of nitrogen-fixing plants. Using the
mid-range values from the CSIRO (1996a) scenario (which includes both slab-ocean
and coupled GCMs and is now supersededsee below), climate change added
to CO2 increase led to national yield increases of 20% by 2100 under
present planting practices or 26% with optimum planting dates (Howden et
al., 1999c). Regional changes varied widely.
Howden's response surfaces show that for doubled CO2 but no
change from historical rainfall, a 1°C increase in temperature would slightly
increase national yield (see Figure 12-3a) when the best
variety was used with the current planting window. However, the slope of the
temperature curve turned negative beyond 1°C, so the yield at 2°C was
predicted to be similar to that at present temperatures, and the total yield
declined below the current value for greater warmings. Adoption of earlier planting
windows with climate change extended the yield plateau to 2°C warming before
the slope of the temperature curve became negative (see Figure
12-3b). This was based on the present regional distribution of cropping,
although cropping could expand into drier marginal areas with higher CO2but
this may be countered by substantial reductions in rainfall or land degradation
in currently cropped areas. Yield decreases rapidly with decreases in rainfall.
These response surfaces were used by Howden et al. (1999d) with the
SRES scenarios (see Section 126.96.36.199), which use
only recent coupled-ocean GCMs that show reductions in rainfall over most of
mainland Australia in summer and winter. Results indicate that for mid-range
scenarios (A1-mid and B2-mid) extended to 2100, national yields are reduced
3% without adaptation compared with current yields and increase only 3% with
adaptation. Yields decline in western Australia and south Australia but increase
in the eastern states. With the A2-high scenario, there are much larger negative
impacts, with cropping becoming nonviable over entire regions, especially in
western Australia. These results highlight the importance of the more negative
rainfall scenarios found with the coupled-ocean GCMs.
In New Zealand, generally drier conditions and reductions in groundwater will
have substantial impacts on cereal production in Canterbury (east coast South
Island), the major wheat and barley production area of New Zealand. Other grain-producing
areas (primarily Manawatu and Southland) are less likely to be affected. Grain
phenological responses to warming and increased CO2 are mostly positive,
making grain filling slightly earlier and decreasing drought risk (Pyke et
al., 1998; Jamieson and Munro, 1999). Although grain-filling duration may
be decreased by warmer temperatures, earlier flowering may compensate by shifting
grain filling into an earlier, cooler period.
Maize production is mainly in Waikato (upper middle North Island) and Bay of
Plenty and Poverty Bay (east coast North Island), with some production (more
likely to be for silage than grain) further south in Manawatu and Canterbury.
Rising temperatures make this crop less risky in the south, but water availability
may become an issue in Canterbury.
Sweetcorn is grown mostly on the east coast of the North Island (Poverty Bay
and Hawke's Bay) but increasingly in Canterbury. Climate warming is decreasing
frost risk for late-sown crops, extending the season and moving the southern
production margin further south. In the South Island, production is irrigated
and is vulnerable to changes in river flows and underground water supply.
Horticulture in Australia includes cool,temperate fruit and vegetables in the
south and at higher elevations, extensive areas of tropical fruits in the northeast
and in irrigated areas in the northwest, and a rapidly expanding viticulture
industry in cool and warm temperate zones. Many temperate fruits require winter
chill or vernalizationwhich in some cases can be replaced by chemical
treatmentsand are strongly affected by disease and hail. Other more tropical
fruit are subject to disease outbreaks and severe damage from hail, high winds,
and heavy rain from tropical storms. These fruits are all likely to be affected
by climate change, but few studies have been made (but see Hennessy and Clayton-Greene,
1995; Basher et al., 1998).
In New Zealand, climate change may have mixed results on horticulture. Kiwifruit
require some winter chill (Hall and McPherson, 1997a), and studies by Salinger
and Kenny (1995) and Hall and McPherson (1997b) suggest that some varieties
in some regions will become marginal; warmer summers and extended growing seasons
may benefit others but may adversely affect timing for overseas markets.
Chilling requirements for most cultivars of pip-fruit are easily satisfied.
However, some common cultivars have shown an adverse reaction to excessively
warm conditions, with problems such as sunburn, water-core, and lack of color.
There has been a southern expansion of grapes in New Zealand over the past
few decades, sometimes into more climatically marginal land. The New Zealand
wine industry to date has shown a largely beneficial response to warm, dry conditions,
which are expected to become more dominant in the east, but limitations on groundwater
for irrigation may become a problem. Warmer conditions also are assisting expansion
of the citrus industry in the north of New Zealand and are particularly beneficial
for mandarins. However, this region would be susceptible to any increase in
the location-specific frequency of subtropical storms reaching New Zealand.