Method
Chill Units
The Daily Positive Chill Unit (or PCU), also known as the Modified Utah Chill Unit model or the Infruitec model, was used to produce maps of mean seasonal or monthly chill units. The optimum temperature range for chilling accumulation is between 7.2 and 9.1°C, with temperatures >15°C negating chilling. In contrast to the Utah model, the PCU model assumes that high temperatures can only negate the chilling received on the day of occurrence and do not affect the chill units accumulated previously.
The techniques by which PCUs were computed from hourly temperature values are outlined in detail in Schulze (2011). From the hourly PCU calculations, daily PCUs were accumulated, from which monthly and seasonal totals of PCUs could be computed for the period April to August.
The modelling for the intermediate future (2050s) was conducted using five different CMIP3 GCMs (Schulze, 2011).
In addition, the month in which a certain level of chilling has accumulated was computed and mapped for the historical climate and the intermediate future. The levels were set at 250 (low), 500 (medium) and 700 (high) PCUs.
Maps
Map Information
The eleven pome and stone fruit regions of the Western Cape – Langkloof have historically distinctive seasonal (April to August) accumulated chill units. These have played a large role in determining the current distribution and dominance of fruit species and cultivars between and within specific regions. With climate change, chill units are projected to shift to a lower seasonal total.
The range of mean seasonal PCUs experienced historically within each of the production regions (sometimes divided into sub-regions), together with the projected intermediate future seasonal PCUs, are listed in Table 1. As regions become warmer and seasonal PCUs are reduced, they become more like currently other warmer production regions, or they may accumulate fewer PCUs than any current production region. Estimates were made of what each region / sub-region could in future resemble, in comparison with the historical situation in other regions.
In Table 1, the ticks indicate current and projected future suitability for pome and stone fruit production. The absence of a tick for future suitability indicates that current cultivars and production methods may not allow for production in future, but that with additional adaptation e.g. low chill cultivars, this may still remain possible.
We caution that these results should be interpreted broadly, giving only an indication of a possible future. The results are specific to the set of GCMs used for the modelling (CMIP3), together with the future scenario (A2), and slightly different results may be expected with other GCMs and scenarios. Further modelling with new sets of GCMs and scenarios, and long-term instrumental on-farm observations, are required to confirm the projections and trends, and to guide possible responses at farm level.