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Total Accumulated Positive Chill Units Featured
Issue FifteenDecember 2021

Climate-change guidelines for growers

A recent Hortgro-funded project focused on how climate change will impact growers in key production regions. By Jorisna Bonthuys.

Climate change poses a significant threat to the sustainability of the pome- and stone-fruit industries, according to Prof. Stephanie Midgley, Climate Change and Risk Scientist at the Western Cape Department of Agriculture.

Midgley collaborated with Prof. Roland Schulze from the University of KwaZulu-Natal and Nick Davis from Isikhungusethu Environmental Services in developing a guide aimed at informing climate-related decision-making on farms. The Scientific and Practical Guide to Climate Change and Pome/Stone Fruit Production in South Africa has now been published in two parts.

The researchers hope that this guide will become the go-to source for spatial information on climate-change risks, impacts and adaptation options.

Midgley and her co-workers concentrated on the Western Cape, and on the Langkloof region of the Eastern Cape. Together, these regions account for more than three quarters of South Africa’s pome- and stone-fruit production.

They compiled an atlas of key climate-related variables relevant to historical and future climate conditions, as well as high-resolution maps of 11 important production areas. Climatic conditions conducive to frost, chill accumulation, sunburn, red colour development, and insect-pest life cycles were taken into account.

Climate records from 1950 to 1999, and two sets of global climate models were used for rainfall and temperature predictions.

The maps reflect the current situation, and forecast the immediate – 2030s – and intermediate – 2050s – future, as well as showing the changes between these two time periods.

Figure #1

Total Accumulated Positive Chill Units Figure 1

Figure #2

Total Accumulated Positive Chill Units Figure 2

Total accumulated positive chill units for April–August calculated using the daily-positive model. Figure 1 shows the average based on historical climate data for 1950–1999. Figure 2 shows the average that is estimated will occur in the intermediate future, namely 2046–2065. The darker blue the area, the fewer the total accumulated chill units. It can be seen that very few of the current production areas are expected to accumulate more than 600–800 units per year

The heat is on

There is clear evidence that South Africa has been warming significantly in recent decades. Across all the production regions studied, temperatures are rising, with daily maximum temperatures already reaching record levels in certain places. These trends will persist in future.

Mid-century projections display marked increases in mean annual temperatures in the production regions included in this study. Changes of up to 2.0°C along the southern coast, and up to 2.6°C in the northern interior and northeastern production regions are expected.

“This anticipated increase in mean annual temperature is potentially a major concern for growers,” says Midgley. “It will likely lead to spatial shifts in production regions, and shifts in the fruit types and cultivars that are suited to specific regions.”

In broad terms, the warmer parts of the Elgin-Grabouw-Vyeboom-Villiersdorp region and Piketberg could become unsuitable for some pome-fruit cultivars, whereas important cooler apple-producing regions could become more like core present-day pear regions. Pear production in the warmer parts of the Klein Karoo is likely to become more marginal or uneconomical. In addition, stone-fruit production could become marginal in the warmest production areas in the Klein Karoo, the northern Berg River area, and parts of the Breede River valley.

The researchers foresee significant projected increases in hot and very hot days in the intermediate future that do not bode well for fruit growers in regions that are already hot. Large areas could experience up to 20 additional days per year where temperatures exceed 35°C. The worst-affected areas along the northwest coast, the northern border, and in the northeast – the Karoo – could experience 30 to 50 additional such scorching days per year, leading to impacts such as sunburn and increased irrigation requirements.

Two- and three-month-long dry spells are also projected to increase in the immediate future, until the 2030s. The highest risk of mild dry spells in this period is found in the southwestern coastal region, the Langkloof, Koo, Barrydale, and the Breede River valley.

“Generally, the models show a drying trend in the western side of the region,” says Midgley. “But we need to be careful when we look at rainfall projections because the models do not always agree. So, over the next 30 years, the patterns won’t necessarily be clear-cut in all regions.”

Seasonal changes

Overall, the global circulation models used in this study predict lower increases for mid-winter minimum temperatures than for mid-summer maximum temperatures.

One outcome could be less frost, but this may not be a win, depending on how trees respond to temperature cues. For example, trees of low-chill cultivars may flower earlier, which could increase the risk of frost damage during the early growing season.

The data show that autumn is becoming longer and warmer. Increasing autumn and winter temperatures reduce chill accumulation, with implications for dormancy development and spring bud-break. Increasing summer and autumn temperatures could reduce the number of days with conditions conducive to red colour development while increased temperatures during the growing season can cause more generations of moth pests.

Production of pome-fruit cultivars that have high chill requirements is most at risk in the southwestern coastal region and Wolseley, despite the use of rest-breaking agents. Challenges could also arise for stone-fruit producers in large parts of the Klein Karoo, Stellenbosch, and the Berg River and Breede River valleys. For cultivars with medium chill requirements, Elgin, Riviersonderend and Somerset West could become unsuitable.

Sunburn is already acknowledged as a major problem, so the increased risk brought about by climate change underscores the need for more rapid implementation of affordable and effective technologies to mitigate the effects of sunlight and high orchard temperatures.

Luckily, cultivars with different chill requirements are available – the severity of the situation will thus depend on whether new climates can be paired with suitable cultivars, says Midgley.

Using nets to soften the climate and to reduce sunburn will also become increasingly important, especially for apples. Where high- to medium-chill cultivars are concerned, growers are advised to identify the cooler microsites on their farms when establishing these trees.

“Growers should understand and utilise the microclimate conditions on their farms,” says Midgley. “Understanding seasonal and monthly changes is, and will increasingly be, key to making better-informed decisions about the future.”

Soil- and water-management practices will have to adapt to deal with climate-related impacts such as increased evapotranspiration, reduced rainfall, and changes in dry and wet spells, run-off, and streamflow. Growers could consider using irrigation technologies and scheduling methods to maximise water-use efficiency. In future, ground-water resources will become more important, and better management of these resources will be essential.

Midgley suggests using mulches and cover crops to reduce water losses from evaporation, and to improve the water-holding capacity of the soil by increasing soil organic matter.

“Due to the diversity of production regions, and of slopes, soils, etcetera, there will still be opportunities to continue with fruit farming,” concludes Midgley. “There may be shifts across the regions, and cultivar choices may change, but the industry as a whole should be resilient if we do good strategic planning now for the orchards planted in the next decade.”

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