Tree knowledge key to meeting climate challengeResearch & Extension
- Understand key risk periods in growth cycle
- Know which intervention gives best tree/profit outcome
- Manage water more effectively
- Forward-planning for increased variability and climate change
Understanding of the vulnerability and natural responses of trees to water and heat stress will better equip growers to manage future droughts and heatwaves, advises South African climate change and agricultural sustainability researcher/specialist Professor Stephanie Midgley, from Stellenbosch University.
Rethinking water use by shifting away from traditional land-based measures of yield and profit (T/ha or $/ha) to a resource-based measure such as per cubic metre of water (kg or $/m3), and implementing precise water budgeting, monitoring and scheduling will also be critical in both more efficient water use and future profitability.
Stephanie told growers attending the northern round of the Future Orchards winter orchard walks and the technical update sessions at Hort Connections in Melbourne in June that by understanding the risks of water or heat stress at each stage of the growth cycle they could make better informed decisions on managing the impact on their orchards.
“Climate is changing,” she said. “It is not just the disasters, there are gradual changes that are often overlooked, later onset of rain, higher temperatures and more hot days, loss of chilling and changes in the timing of key growth and fruiting processes.
“Hail causes more damage, but drought affects more people.”
Growers in South Africa’s Western Cape province have just emerged from their worst drought in over 100 years, during which Stephanie said growers who hit their ceiling of 50-60pc allocation were simply cut off from water regardless of the point in the growing cycle or their water needs.
“There were massive repercussions,” she said. “We almost got to the point that agriculture ran out of water. There were job losses and production declines, lower fruit quality and farmers had to make some very painful decisions.”
“A real concern we have is that there is less and less water, but the demand is going up.”
“We need to understand the tree and its natural responses and act, based on knowledge. We need to know how the tree responds to normal environmental fluctuations, to monitor closely and to recognise when it is under stress.”
Stephanie said apple trees had some resilience and response strategies to stress and could also acclimatise. Growers who understood these natural responses could better target the tools and resources at their disposal to achieve the best yield/profit outcome.
Water stress sensitivity
While apple trees generally use water efficiently and recover from moderate daytime internal water deficits, Stephanie said there were key periods in the growth cycle where adequate water was critical, such as early in the season (first 40-50 days after full bloom) during flowering, fruit set and cell division; flower bud initiation, and the final phase of fruit swell.
“Water stress affects fertilisation and cell division, leading to lower yield and fruit size,” she said. “The risk of stress early in the season is low, but you cannot catch up later if the stress is lifted.
“Bud initiation in summer is very sensitive to water stress. Bud development in late winter/spring is less sensitive to low or moderate water stress, but severe stress can lead to incomplete flower development.
“Fruit growth can recover from low to moderate water stress but is sensitive during the early and final growth phases.”
Water stress would first affect vegetative growth, leaves and shoots and then fruit growth, which could be used strategically to limit canopy growth in the period of rapid shoot growth, but slow fruit expansion (in early summer).
Managing Water Deficit
Strategies for managing water deficits included reducing demand and losses, using water more effectively and choice of rootstocks.
Reduce water demand and losses by
- Changing the microclimate – reducing temperature and wind through the use of shade netting;
- Reduce evaporation from soil through mulching, cover crop management, drip irrigation or irrigation overnight;
- Reducing transpiration/leaf area by pruning unproductive wood, removing limbs or in extreme circumstances, cutting back to rootstock;
- Reducing the crop – fruit thinning (don’t remove all due to impact on following season flowering), thin for size and quality, parking selected orchards with minimum water and focussing on productive high-value orchards to ensure get best financial outcome.
Use water more effectively by
- Precise monitoring and scheduling – using soil, tree and weather sensors and satellite imagery
- Water budgeting and accounting – knowing what water is available, planning precisely and removing old and unprofitable orchards
- Increase water productivity by focussing on yield and profit per unit of water input
- Reduce losses in on-farm system but upgrading infrastructure and drainage.
- Trees on dwarfing rootstock use less water than those on more vigorous rootstocks, but crop load is an important factor, so calculate water productivity (kg fruit/m3 water used) to inform decisions on the trade-off between reducing water costs/use and yield.
- Rootstocks differ in their water stress sensitivity, with some recovering better than others.
Higher temperatures and more hot days are also increasing the risk of sunburn, impacting on quality and packouts. Weather, cultivar, exposure in canopy and tree stress could all influence the risk of damage, with red cultivars less susceptible than Granny Smith or Fuji.
Stephanie said browning would occur at fruit surface temperatures of 46-49°C and as fruit surface temperatures could be 15°C higher than air temperature, there was a risk of damage once air temperatures rose above 32-35°C.
Ten minutes of fruit surface temperature above 52°C was enough to lead to necrosis. Water stress has been shown to dramatically increase the risk of sunburn as it heightened the fruit surface temperature.
Fruit grown in the sun could acclimatise and be less susceptible to sunburn than fruit grown in the shade and then exposed to strong sunlight.
“We have to avoid the sudden exposure of shaded fruit at all costs,” Stephanie said. “Sudden exposure will lead to sunburning, and is not dependent on temperature. We need to get the leaf to fruit ratio right.”
Heat stress sensitivity
Higher temperatures will impact fruit set, maturity and season length, storability and dormancy.
As with water stress, Stephanie said a key risk period was in the first 40-50 d.a.f.b where high temperatures impacted net carbon uptake and led to high losses to respiration in young fruit.
“There is a significant effect on daytime carbon balance in small fruit at temperatures above 30°C,” she said. “This can limit fruit growth potential and final fruit size.”
Although high light was part of the requirement for red colour development, temperature fluctuations could lead to a variation in colour and temperatures above 30°C could cause bleaching.
Managing heat stress
Strategies for responding to increased light and temperature include overhead evaporative cooling, protective netting and cultivar selection, both the use of improved red/blush cultivars less sensitive to climatic fluctuations and the use of early or late cultivars to avoid mid-season heat stress.
She said fruit colour was quite responsive to changes in temperature and cooling treatments, such as overhead water could lower surface temperatures by around 10-15°C and stimulate red colour, particularly if applied in the two to three weeks prior to harvest, but this increased water use.
Medium to long-term planning for replacement of trees with cultivars better suited to gradual warming, but growers would need to consider the implication of changing cultivars on harvest dates and pollination if flowering timing differed substantially from that of cross pollinators.
Beyond the farm gate
Stephanie said as climate change would impact beyond the farm gate, a collective public-private partnership approach would be needed to mitigate the social and economic impacts of climatic events on jobs, livelihoods and communities.
Included in this should be bringing climate change risk into all spheres of investment planning and establishing viable financial mechanisms to incentivise proactive disaster risk management and transition to resilience.
Stephanie Midgley visited Australia as part of the Future Orchards® program funded by Hort Innovation using the apple and pear industry levy funds from growers and funds from the Australian Government.