4.3.3.1. Agriculture and Horticulture

The possible interactions among atmospheric CO₂, climate, soil, plants, animals, and humans in the agricultural system are very complex, which makes it difficult to draw firm conclusions about agricultural responses to projected climate changes (Warrick et al., 1996). However, research has indicated that increased CO₂ fertilization, higher temperatures, and changed rainfall characteristics are likely to lead to changes in productivity and crop yield; changes in grain quality and animal pasture feed quality; changes in the incidence of diseases, pests, and weeds; and changes in the suitability of districts for particular crop types and farming systems (IPCC 1996, WG II, Chapter 13). Furthermore, it can be expected that climate change impacts on other countries' agriculture will lead to changes in global food markets and that these external changes may have as great an effect on the region's agricultural industries as internal changes.

The yield responses reported for studies of Australasian crops and pasture (IPCC 1996, WG II, Section 13.6.4 and Table 13-7) cover a wide range of values, depending on the situation studied and the research assumptions. Considering atmospheric CO₂ concentration and temperature rises (but not necessarily rainfall changes) and ignoring adaptive measures, it appears that pasture productivity may generally increase (e.g., Greer et al., 1995), but annual crop yield may increase only slightly or remain about constant (e.g., Wang et al., 1992; Rawson, 1995; Wang and Connor, 1996), or even reduce slightly (Muchow and Sinclair, 1991). As long as slower-maturing grain cultivars are available, crop yields may benefit, with the beneficial effects of higher CO₂ concentrations outweighing the reductions in yield expected from shorter grain-filling periods associated with warmings (Wang et al., 1992; Wang and Connor, 1996). However, if sufficiently longer-maturing cultivars cease to become available due to greater warmings, yields may decrease. This would happen first at low latitudes, and the effect would increase as further warming occurs later in the next century (Pittock, 1995, 1996).

An empirically based study by Nicholls (1997) has attributed 30-50% of the observed increase in Australian wheat yield since 1952 (which was about 45%) to observed climatic trends, with increases in minimum temperature the dominant influence. The method excludes the direct physiological effects of increasing CO₂ concentration on yield but could be confounded if farmers used more fertilizer in climatically favorable years.

The interaction of climate change and increasing CO₂ concentrations on pasture quality and subsequent animal productivity remains uncertain (Owensby, 1993). Pasture plants grown under elevated CO₂ concentration always seem to have lower protein (Arp and Berendse, 1993) and higher nonstructural carbohydrate contents (Lutze, 1996), which implies an impact on the nutritional quality of the forage. In high-rainfall temperate regions, particularly early in the growing season, most forage is eaten as green sward (or products preserved from green material), in which case the increased soluble carbohydrate content would increase the digestibility and nutritional value of the forage (Smith et al., 1997). Reductions in feed quality may occur through reductions in digestibility and increase in lignin content of tropical grasses grown at higher temperatures (Wilson, 1982) and increased stem-to-leaf ratios of plants grown under high CO₂ levels (e.g., Morison and Gifford, 1984). Thus, rangeland forage quality is likely to decrease at some times of year-suggesting an increased need for feed supplements or legume pastures. However, higher plant water-use efficiency under elevated CO₂ levels may result in more "green days" each year, partly offsetting the above effects (McCown, 1980).

In Australia's rangelands, scenarios of climate change suggest alterations in the temporal and spatial components that are likely to increase variability and unpredictability in plant productivity and community composition. Increased competition from woody weed species in mixed pasture may have a significant negative effect on grass growth (Scanlan, 1992) and hence livestock production (Stafford Smith et al., 1994). It is anticipated that in temperate areas of Australia and New Zealand, encroachment of subtropical grasses into pastures may be exacerbated not only by the warming conditions but also by the strong response of C₄ growth during recurring periods of water stress (Campbell et al., 1996). This would affect the types of possible grazing activity and would require greater consideration of land management issues, such as the use of fire for woody weed control and pest animal management (Stafford Smith et al., 1994).

Aborigines have a large and increasing role in land management in the pastoral zone (Young et al., 1991; Ross et al., 1994). Aboriginal land supports a wide range of uses, including pastoralism, hunting and gathering, horticulture, conservation, cultural tourism, and mining. Climate change may affect native bush food supplies and sacred biota, as well as water resources for remote Aboriginal communities.

Temperature increases will very likely lead to an increase in pasture production in mid-latitudes, with corresponding increases in livestock production. However, if most of the increase in pasture growth comes in summer, winter feed may remain the limiting factor unless there is increased feed conservation or changes in livestock production systems. Increased pasture production may not occur if there is an increase in the frequency of extreme events, such as summer droughts or late-winter snowstorms in lowland sheep areas of New Zealand. Any increases in production also would depend on farmers' attitudes toward risk and on the signals reflected back to producers from the effect of economic and climate changes on customers and competitors (Martin et al., 1991).

Livestock-which in this region are largely unhoused-are expected to benefit from warmer winters, possibly extended growing seasons, and possible minor improvements in feed quality in temperate high-rainfall zones. In some circumstances, however, they may be negatively affected by lower nutritional quality of feed (Owensby, 1993) and greater summer heat stress (Russell, 1991; McKeon et al., 1993). There may be increases in diseases of tropical origin but decreases in cold-weather diseases (Sutherst et al., 1996). In the New Zealand high country, it is not uncommon for huge losses of livestock to result from cold, wet weather and heavy snowfalls during winter and early spring. Warmer winters may lead to reductions in the numbers of animals lost.

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