The impacts of climate change for the main fruit crops are positive with yields, fruit size and numbers not expected to be compromised. The risk of frost damage is not expected to increase but the use of water and pressure from pests and disease are likely to be greater. More extreme weather events such as high temperatures and increased storm damage will pose a risk to production.
Seasonal Temperature Profiles
The impacts of changes to the temperature profiles across the season and in different regions are complex and do not consistently result in negative or positive changes to production.
Changes to average temperature are expected to impact on the winter chilling (vernalistion) requirement to break winter dormancy of deciduous woody fruit trees and vines. A lack of accumulated chilling has a flow on effect because prolonged dormancy will result in poorer fruit quality and lower yields, which in turn might result in changes to production costs and profits.
The seasonal pattern of temperature affects the (phenology) timing of bud break, flowering and harvest for fruit crops.
Higher summer temperatures affect tree and vine crops differently:
There are expected to be trade-offs in plant responses and costs:
the current costs of chemical and hand thinning apple flowers might be reduced because of the lack of winter chilling
warmer temperatures during fruit development might result in larger fruit sizes but more risk of sunburn damage.
There is predicted to be larger areas in New Zealand that are free from spring and autumn frosts. There also might be fewer frosts in other areas. If this happens, an increase in growing period each year is expected. This could result in an expansion of the economic production of grapes into new areas.
Rainfall - Precipitation
The nature of the rainfall processes in New Zealand are complex. The factors contributing to how, why and where precipitation falls in New Zealand is not always straight forward because the two main islands are long, narrow, with high axel mountain ranges surrounded by ocean. Rain, hail or snow can fall almost anywhere, any time.
In general the western side of the country is expected to get wetter and eastern regions are expected to become drier, but the timing and intensity of the precipitation events is likely to be less predictable.
The two main consequences of the changes to rainfall patterns are:
changes to irrigation needs
changes to ground water recharge as a result of changes to drainage patterns
Since crops like grapes are usually grown in the dry eastern regions of New Zealand the water requirements for irrigation and frost protection is likely to coincide with decreasing rainfall and groundwater reserves. The timing of when, during the growing season, rainfall occurs can influence the fruit maturation rates, the quality of the fruit and the overall yields of crops. It is difficult to assess the changing pattern of hail in the future, but similarly, the timing of hail events can have serious impact on the profitability of horticulture crops because flowers and fruit are vulnerable to damage by hail.
Increasing levels of Carbon dioxide
The prime impact of rising CO2 levels on plants is to increase the biomass produced. The focus of horticultural crops is on producing flowers and fruit, therefore the balance of vegetative and reproductive growth is important to these industries. The types of plant responses measured in studies of plants growing in a CO2 enriched atmosphere have been different depending on species.
A study of orange tree rooting found increased amounts of fine root biomass but there was not difference in the root: shoot ratios. In other studies tree root zone area increased. With grapes there was no impact on grape phenology because temperature is the main driver of grape phenological development. Some studies on vegetable crops have reported a shortening of time taken until flowering, others reported a change in the sex ratio of flowers because of a difference in the branching architecture of the plant. Increased fruit yields in cucumber, squash, and tomato have also been reported.