11.4.3.2 Horticulture
Australian temperate fruits and nuts are all likely to be negatively affected by warmer conditions because they require winter chill or vernalisation. Crops reliant on irrigation are likely to be threatened where irrigation water availability is reduced. Climate change is likely to make a major horticultural pest, the Queensland fruit fly Bactrocera tryoni, a significant threat to southern Australia. Warming scenarios of 0.5, 1.0 and 2.0°C suggest expansion from its endemic range in the north and north-east across most of the non-arid areas of the continent, including the currently quarantined fruit fly-free zone (Sutherst et al., 2000). Apple, orange and pear growers in endemic Queensland fruit fly areas are likely to have cost increases of 42 to 82%, and 24 to 83% in the current fruit fly-free zone (Sutherst et al., 2000).
In New Zealand, warmer summer temperatures for Hayward kiwifruit are likely to increase vegetative growth at the expense of fruit growth and quality (Richardson et al., 2004). Kiwifruit budbreak is likely to occur later, reducing flower numbers and yield in northern zones (Hall et al., 2001). Production of current kiwifruit varieties is likely to become uneconomic in Northland by 2050 because of a lack of winter chilling, and be dependent on dormancy-breaking agents and varieties bred for warmer winter temperatures in the Bay of Plenty (Kenny et al., 2000). In contrast, more areas in the South Island are likely to be suitable (MfE, 2001). Apples, another major crop, are very likely to flower and reach maturity earlier, with increased fruit size, especially after 2050 (Austin et al., 2000). New Zealand is likely to be more susceptible to the establishment of new horticultural pests. For example, under the current climate, only small areas in the north are suitable for the oriental fruit fly, but by the 2080s it is likely to expand to much of the North Island (Stephens et al., 2007).
Viticulture has expanded rapidly in both countries. Earlier ripening and reductions in grape quality and value are likely by 2030, e.g., in Australia, price per tonne drops 4 to 10% in the Yarra Valley and 16 to 52% in the Riverina (Webb et al., 2006). In cooler Australian climates, warming is likely to allow alternative varieties to be grown. With warming and a longer growing season in New Zealand, red wine production is increasingly likely to be practised in the south, with higher yields (Salinger et al., 1990). Higher CO2 levels increase vine vegetative growth, and subsequent shading is likely to reduce fruitfulness. Distribution of vines is likely to change depending upon suitability compared with high-yield pasture and silviculture, and with future irrigation water availability and cost (Hood et al., 2002).