Method
Heat Units
The concept of heat units (HUs, also known as growing degree days, GDDs) is based on the fact that plant development stages depend on the accumulated heat experienced prior to a critical phenological milestone. In general, it holds that the lower the temperature and thus accumulated heat, the slower the rate of growth and development of plants and their organs. Also, the amount of accumulated heat required to complete a given developmental stage does not vary substantially from year to year. For this study, a lower temperature threshold (base) of 10.0°C was used.
Maps
Map Information
Mean annual HUs under historical climatic conditions range from < 400 HUs in the high-altitude colder mountain areas to > 3 500 HUs in the north-west. Over most of the fruit production regions they are in the range 800-2500, but up to 3500 HUs are received in some stone fruit production regions. Heat units are essentially a summer season phenomenon, and the HU range in summer in the fruit regions is mostly 600-2500. Winter HUs are less than 800 in the fruit regions.
The impact of climate change on HUs is expressed as a ratio change between the intermediate future and the present. For the mean annual scenario, the ratios are generally up to 1.4 in the fruit areas, implying a 40% increase with climate change. Higher ratios of up to 1.5 are projected for the north-western and Langkloof production areas. The colder high-lying areas are more responsive to climate change, displaying ratios up to 1.8, or a projected 80% increase. Regarding projected seasonal HU changes, these are generally < 1.3 (30%) in summer in the southern production areas, and up to 1.5 (50%) in the north-western and Langkloof production areas. The greatest changes by mid-century are seen in winter, with projected ratio changes of 1.3-1.5 in the southern areas and up to 1.8 in the Langkloof and the higher altitude areas of the north-west.