Sunday, April 14, 2013

Winter Chill: Fruit and nut trees feeling the heat of global warming

Climate change affecting safe winter chill for orchard trees.

A new study says that global warming will impact fruit and nut tree productivity in Australia and globally. Most fruit and nut trees have evolved in cool temperate climates and go through a dormant winter phase and require a certain amount of winter chill to trigger their spring growth and fruit production. But with strong trends in many regions for warming winter minimum temperatures and reduced number of 'winter chill' days, the heat is on our orchards.

Temperatures are projected to rise in most parts of the world with winter minimum temperatures projected to rise most rapidly. Our orchards have been established taking account of current climate conditions, which are now rapidly changing due to greenhouse gas pollution contributing to climate change. This will have a major impact on costs and productivity of fruit and nut orchards, an industry estimated to be worth about US$93 billion globally in 2005.

Role of Dormancy and Winter chill

Fruit and nut trees have evolved a period of dormancy to cope with cold winter temperatures, winter frosts and short winter warm periods. That period of dormancy is important for triggering new spring growth, flowering for cross pollination, and production of fruit. According to a 1995 paper by Dr Jill Campbell, "The major varieties grown in the world need 800 to 1500 hours of chill. Lack of chill makes buds slow to break in spring or not at all. The flowering period may be 4-6 weeks rather than 1-2 weeks, with very irregular bud break all over the tree. This may mean that the cross pollinating variety's flowers are over or not yet in bloom as different varieties have different requirements. Also fruit quality and maturity will be non-uniform on the tree, and harvesting and storage will be difficult."

Every deciduous tree species and cultivar has a slightly different chilling requirement and is selected for horticultural production in regions taking account of current climate conditions. If these trees don't receive adequate Safe Winter Chill (SWC), then flowering can be delayed with uneven flower and leaf bud development resulting in staggered bloom and anomalous growth with reduced crop, smaller fruit size, uneven size, a longer harvest period, and poor fruit quality. (See Chill Units of Stone Fruit) The biological process of dormancy and winter chill is still not fully understood by scientists.

The study by scientists at Melbourne University, CSIRO, and Victorian Department of Primary Industries - Impact of future warming on winter chilling in Australia - used three chill models to investigate changes in chill accumulation at 13 sites across Australia at projected localised temperature increases of 1, 2 and 3°C.

"Regional impacts and rates of decline in chilling differ among the chill models, with the 0-7.2°C model indicating the greatest reduction and the Dynamic model the slowest rate of decline. Elevated and high latitude eastern Australian sites were the least affected while the three more maritime, less elevated Western Australian locations were shown to bear the greatest impact from future warming." (Darbyshire et al 2013)

Unfortunately the full article is hidden behind a paywall for more detail. But other studies have been published to give a general indication of the situation facing Australia and globally.

In March 2008 Apple and Pear Australia Ltd made a submission to a Climate Change and Australian Agriculture Sector Inquiry in which they said "there is evidence that climate change is impacting on the apple and pear industries, with increased climate variability and reduced chilling hours, which are necessary for quality fruit production."

The submission highlighted that in 3 major fruit growing regions there has been a substantial increase in average minimum temperatures:

"The Huon Valley of Tasmania has shown an increase in average minimum temperatures from 1950 of about 0.6 C across both summer and winter. During winter, at this location, there has been a reduction in the number of days with a minimum below 7 C from 235 to 220 with a corresponding decrease in the chill hours received by the fruit trees which are essential for flowering and subsequent fruit set."

"In the Orange district of NSW, the winter (May to October) average minimums have also risen by 0.6 C although this increase in minimum temperatures was not observed in the summer months. At this location there has been little change in the number of days with minimums below 7 C. The Goulburn Valley area of Victoria shows the least change in minimums with no strong trend in either the annual or winter
average minimums, though there has been a decline in the average number of
days with minimums below 7 C from about 215 to just below 200 indicating a reduction in winter chill units. " (Apple and Pear Australia Ltd 2008)


Caption: Figure 8. Modelled and projected Safe Winter Chill in South Africa, Southern Australia and New Zealand, for 1975, 2000, the middle of the 21st century (middle), and the end of the 21st century (bottom). For each point in time, results are averaged over three greenhouse gas emissions scenarios and three Global Climate Models. Areas that are more than 5° away from the closest weather station, and areas with mean annual temperatures >20 or <°0C are shaded. Source: (Luedeling et al 2011)

Global assessment of Climate Change and impact on Winter Chill

A 2011 paper by Eike Luedeling et al on Climate Change Affects Winter Chill for Temperate Fruit and Nut Trees examined the impact of global warming on winter chill for 4,293 weather stations around the world for Global Climate Model projections.

"Warm regions are likely to experience severe reductions in available winter chill, potentially threatening production there. In contrast, SWC [Safe Winter Chill] in most temperate growing regions is likely to remain relatively unchanged, and cold regions may even see an increase in SWC." said the study.

"The extent of projected changes in winter chill in many major growing regions of fruits and nuts indicates that growers of these commodities will likely experience problems in the future. " the study continued.

Orcharding practices will need to change on a global level. While the climate for some regions will become newly available for production horticulture and orcharding, and some will maintain their productivity, many current temperate climate regions will face extensive losses in productivity. This study indicated that Safe Winter Chill would increase in cold regions affecting fruit growing regions in Canada, Southern Scandinavia and Eastern Europe.

Orcharding requires investment in trees over lengthy time spans measured in decades. It takes at least 3 to 4 years before orchard trees even start being productive, and their productive life spans decades. There are major lead times associated with changing orcharding practices, choice of cultivars or relocating orchards.


Caption: Figure 5 - Modelled and projected losses in Safe Winter Chill compared to 1975 for the year 2000 (top), the middle of the 21st century (middle), and the end of the 21st century (bottom). For each point in time, results are averaged over three greenhouse gas emissions scenarios and three Global Climate Models. Source: (Luedeling et al 2011)

Germany, the United Kingdom, the Midwestern United States, Northern China and Central Asia are projected to see little change in Winter Chill levels.

Slight changes are projected in winter chill for Southern France and New Zealand but loss of orchard production is likely to be insignificant.

Warmer regions, in particular around the Mediterranean Sea and in South-western North America will be adversely affected. South Africa, Southern Australia and Northern Africa are projected to lose most of their winter chill regions during this century.

"The Sacramento Valley in California, the Southeastern United States, Chile's Valle Central, Yunnan Province in China, as well as South and Southwestern Australia are all projected to lose winter chill. This will likely require growers to transition to different species or cultivars than are grown today or to develop management practices that can help overcome shortages in winter chill. The highest losses relative to current winter chill levels occurred in Morocco, Tunisia, Israel, in the Cape region of South Africa and, for some GCMs, in the highlands of Kenya and Ethiopia. In these regions, climate change is likely to severely challenge current production systems, some of which already rely on cultural measures such as rest-breaking chemicals and artificial defoliation." (Luedeling et al 2011)

These changes are also dependant on how much greenhouse gas mitigation is done to reduce temperature increases and climate change. The A2 scenario - or the business as usual emissions scenario - indicated the greatest changes in winter chill, which the study says will result in "many subtropical regions will see chilling declines up to 40 CP, which can be expected to disrupt production systems."

If moderate greenhouse gas mitigation is undertaken on a global level, as in the B1 scenario, then "few growing regions are likely to see decreases by more than 20 CP by the end of the 21st century."

Climate mitigation by reduction of emissions and adaptation strategies

The study recommended the importance of reduction in greenhouse gas emissions to reduce the long term climate change impacts on this food industry, plus climate adaptation measures such as breeding tree cultivars for lower chill requirements and relocating orchard production where possible to maintain climate productivity.

But adaptation measures may not prove suitable or fully achievable. Orchard relocation to a climate zone offering safe winter chill may be appropriate, but may also pose many difficulties:

"For example, the ecological niche of many fruits and nuts in the Western United States is likely to move north, from California's Central Valley towards Northern California, Oregon and Washington. These new potentially suitable areas have adverse topography, poorer soils, and limited water availability compared to the Central Valley, making the economic viability of production there questionable. Similarly, other regions, such as parts of Scandinavia, Canada and Siberia, that could potentially become more suitable for these tree crops, may be limited by cold winters, lack of summer heat, or adverse photoperiodic conditions." (Luedeling et al 2011)

A 2009 study by researchers at the University of California Davis Campus found that some parts of California's agriculturally rich Central Valley, winter chill had already declined by nearly 30 percent. "Depending on the pace of winter chill decline, the consequences for California's fruit and nut industries could be devastating," said Minghua Zhang, a professor of environmental and resource science at UC Davis in a 2009 media release.

This 2009 study warned that winter chill had already declined to the point that only 4 percent of the California Central Valley was still suitable for growing apples, cherries and pears. With climate projections, by the end of the 21st century, the Central Valley might no longer be able to grow walnuts, pistachios, peaches, apricots, plums and cherries.

Agri-chemicals have been developed to artificially break a tree's dormancy. This may not be cost effective on a wide scale, especially in developing countries. Changes to irrigation and shading may also be partially effective in influencing orchard micro-climates. Developing breeding programs for low safe winter chill cultivars will definitely be important.

While adaptation measures can be undertaken, they will all come with a cost that will ultimately be passed on to consumers through increased food prices. Many small orchardists may need substantial financial assistance to implement adaptations or to relocate. If funding comes from Government primary industry programs, this increases Government debt. Ultimately you and I are going to pay more for our apples, apricots and almonds; our cherries, peaches and nectarines.

It is part of the strain that climate change is placing on global food security this century. For more detail on Food Security and Climate Change, read John Vidal's excellent article in The Observer on 13th April 2013: Climate change: how a warming world is a threat to our food supplies.

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