Crossing the threshold: adaptation tipping points for Australian fruit trees

Photographer: Liese CoulterBackground

Fruit tree orchards are particularly vulnerable to climate change, with changes likely to affect variables on which orchard productivity is reliant such as cold winter temperatures and extreme summer heat. In order to best prepare for climate change, the fruit tree industry needs targeted information on likely climate impacts and suitable adaptive management options. However there is little baseline physiological data against which impacts and adaptation can be evaluated.

This project collated field information on key physiological phases of fruit trees across the major growing regions of Australia. The data enabled researchers to predict lead times until key tipping points are likely to be crossed under future climates, and to measure the efficacy of adaptation options in managing these impacts.
 

Project outline

Researchers collected physiological data and historical grower records on apple, pear and cherry in Queensland, New South Wales, Victoria, South Australia, Western Australia and Tasmania. The project focused on four temperature-reliant processes that affect fruit productivity: 

Winter chilling

Perennial fruit trees require a prescribed ‘sum’ of cold weather through winter to break dormancy, known as ‘winter chill’. Researchers identified where, when and for which cultivars projected warmer winters are likely to lead to insufficient winter chill, and therefore poorer flowering and lower production.

Spring flowering

Fruit trees flower in response to temperature, but the relationship is not well understood. Changes in temperature are likely to shift timing of flowering, which may affect frost risk and synchronisation of cross-pollinating cultivars. Researchers used historical flowering and temperature data to improve prediction of flowering timing.

Summer extreme heat

Extreme heat events are common in many fruit growing areas, and are expected to increase under future climates. Researchers measured the fruit surface temperature at which sun damage occurred in pear and apple cultivars, and investigated how netting reduced the risk of sun damage under current and future climates.

Potential yield

Potential fruit yield is affected by both temperature and radiation, but there is limited understanding of how climate change will influence potential yield. Researchers investigated whether a model developed in America – MaluSim – could be applied to Australian conditions to accurately predict apple yield. 

Results

Winter chilling

The analysis showed that future risk of low winter chill accumulation varied across Australia. Warmer sites, including Donnybrook and Manjimup in WA and Applethorpe in Queensland, are at risk of insufficient chill by 2030 for higher chill cultivars. In contrast, Batlow in NSW, and Huonville and Swansea in Tasmania, were safe out to at least 2050.

An evaluation of chilling requirements for Pink Lady® apples suggested that, historically, sufficient chill was only received 56% of years in Applethorpe, Queensland, in the last 50 years. However, the lack of low chill impacts observed in Applethorpe suggest greater research is required into the determination of chilling requirements and specifically physiological triggers.

Spring flowering

A delay in flowering is likely in many growing districts by 2050 and in most by 2090, so frost risk may be lowered in the future. However, the complexity in predicting frost makes future projections of flowering frost risk difficult to assess.

The sequential model of flowering timing, which assumes winter chill and spring heat combine sequentially and independently to lead to flowering, was shown to be a poor fit for of Pink Lady® flowering. The newly developed chill overlap model, which incorporates the positive effect of ‘extra’ chill received above the minimum chill requirement, was shown to be a better fit across Australia’s growing districts. The team’s development of this model will allow climate projections to be applied to Pink Lady® apples with more confidence.

Summer extreme heat

Fruit surface temperature threshold for browning damage of Deliza® pear was found to be 47.1°C, with necrosis damage manifesting at 50°C. Air temperature thresholds for potential sunburn browning damage for Royal Gala apple were 37.9°C and 34.1°C for netted and non-netted fruit, respectively, while necrosis damage occurred at 38.7°C for non-netted fruit.

The risk of sunburn damage to apples can be reduced by at least 50% with the installation of netting at warm apple growing sites such as Donnybrook (WA), Tatura (Victoria) and Young (NSW). Cooler Tasmania sites showed only low risk of heat damage out to 2090.

Potential yield

The evaluation of the MaluSim model indicated it was not able to appropriately represent final Pink Lady® fruit yield under Australian conditions, and so is not suitable for projecting changes in fruit yield under future climates. Simplifications to the model could allow for application to within season management decisions. 

Outcomes

The data collection and modelling efforts have provided the first detailed analysis of likely climate change impacts for the Australian apple, pear and cherry industries. The identification of lead times until critical temperature thresholds are crossed will allow growers the necessary time to plan for adaptations to orchard blocks. The insights provided will enable growers to better understand and manage likely climate change impacts, using region and crop-specific adaptation strategies.

The project has also identified a range of priorities for future research, particularly in relation to:

  • understanding winter chill and flowering physiology to better manage current orchards and plan for future conditions
  • evaluation of alternate and combined measures to manage extreme heat damage (canopy design, over-tree sprinklers, stress reduction chemicals)
  • streamlining and automations of weather and climate metrics to assist better intra- and inter- season management decisions.
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