Global warming could affect fruit and nut yield

In some countries, production could shift to areas where there is greater winter chill, but this could be prohibitively expensive.

Warmer winters could have a significant – and in some places, disastrous – impact on the global fruit and nut industry, which is worth over US$90 billion a year.

Research by World Agroforestry Centre scientist Eike Luedeling identifies future trouble spots. Luedeling got a glimpse of how the future could look for many farmers growing temperate fruits and nuts when he visited North Vietnam in 2010. "There were some 10-year-old plum trees that looked as though they were 100 years old," he recalls. "Some had flowers as well as fruit. They were clearly confused and experiencing inadequate levels of winter chill."

Like other temperate fruit trees, plum trees protect themselves in winter by shedding their leaves and sensitive tissue and becoming dormant. Each species needs a certain amount of chilling to break the dormancy and spring into leaf again. When the chilling requirement is not fully met – as was the case in the orchard Luedeling saw in Vietnam – the trees suffer from reduced or delayed flowering, strange growth forms and low yields. That is why fruit farmers in major growing regions choose species and cultivars adapted to local winter chill.

Drawing on data from over 4000 weather stations, Luedeling and his colleagues estimated winter chill for two past and 18 future scenarios, using three different global climate models and three greenhouse gas emissions scenarios. From these, they estimated the amount of winter chill that will be exceeded in 9% of all years. "We chose this figure because it makes sense for fruit and nut producers," says Luedeling. "If they don't get the required chilling requirements for more than 10% of the years, their operations will become unprofitable."

The projections suggest that warm regions will experience the greatest declines in winter chill over the next century. Cooler regions, in contrast, will see little change. This is disturbing news for fruit and nut growers in California's Sacramento Valley, China's Yunnan Province, parts of Australia and several other major growing areas. Already, fruit growers in North Africa are experiencing declining yields as a result of the reduced winter chill; matters are likely to get worse in the future.

So what is the solution? In some countries, production could shift to areas where there is greater winter chill, but this could be prohibitively expensive. A more viable approach hinges on the development of new cultivars that become more productive under lower chill conditions.

This has already been achieved for peach trees in Florida. Much more work needs to be done to develop suitable cultivars for a whole range of species, says Luedeling. "One thing that is clear from our research is that choosing the same cultivars that your grandfather planted may not be a good idea."

Luedeling E, Girvetz EH, Semenov MA and Brown PH. Climate Change Affects Winter Chill for Temperate Fruit and Nut Trees. http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0020155