Warming climate threatens California fruit and nut production

July 22, 2009

Winter chill, a vital climatic trigger for many tree crops, is likely to decrease by more than 50 percent during this century as global climate warms, making California no longer suitable for growing many fruit and nut crops, according to a team of researchers from the University of California, Davis, and the University of Washington.

In some parts of California's agriculturally rich Central Valley, winter chill has already declined by nearly 30 percent, the researchers found.

"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.

Also collaborating on the study were Eike Luedeling, a postdoctoral fellow in UC Davis' Department of Plant Sciences and UC Davis graduate Evan H. Girvetz, who is now a postdoctoral research associate at the University of Washington, Seattle. Their study will appear July 22 in the online journal .

The study is the first to map winter chill projections for all of California, which is home to nearly 3 million acres of fruit and nut trees that require chilling. The combined production value of these crops was $7.8 billion in 2007, according to the California Department of Food and Agriculture.

"Our findings suggest that California's fruit and nut industry will need to develop new tree cultivars with reduced chilling requirements and new management strategies for breaking dormancy in years of insufficient winter chill," Luedeling said.

About winter chill

Most fruit and nut trees from nontropical locations avoid cold injury in the winter by losing their leaves in the fall and entering a dormant state that lasts through late fall and winter.

In order to break dormancy and resume growth, the trees must receive a certain amount of winter chill, traditionally expressed as the number of winter chilling hours between 32 and 45 degrees Fahrenheit. Each species or cultivar is assumed to have a specific chilling requirement, which needs to be fulfilled every winter.

Insufficient winter chill plays havoc with flowering time, which is particularly critical for trees such as walnuts and pistachios that depend on male and female flowering occurring at the same time to ensure pollination and a normal yield.

Planning for a warmer future

Fruit and nut growers commonly use established mathematical models to select tree varieties whose winter chill requirements match conditions of their local area. However, those mathematical models were calibrated based on past temperature conditions, and establishing chilling requirements may not remain valid in the future, the researchers say. Growers will need to include likely future changes in winter chill in their management decisions.

"Since orchards often remain in production for decades, it is important that growers now consider whether there will be sufficient winter chill in the future to support the same tree varieties throughout their producing lifetime," Zhang said.

To provide accurate projections of winter chill, the researchers used hourly and daily temperature records from 1950 and 2000, as well as 18 scenarios projected for later in the 21st century.

They introduced the concept of "safe winter chill," the amount of chilling that can be safely expected in 90 percent of all years. They calculated the amount of safe winter chill for each scenario and also quantified the change in area of a safe winter chill for certain crop species.

New findings

The researchers found that in all projected scenarios, the winter chill in California declined substantially over time. Their analysis in the Central Valley, where most of the state's and nut production is located, found that between 1950 and 2000, winter chill had already declined by up to 30 percent in some regions.

Using data from climate models developed for the Intergovernmental Panel on Climate Change Fourth Assessment Report (2007), the researchers projected that winter chill will have declined from the 1950 baseline by as much as 60 percent by the middle of this century and by up to 80 percent by the end of the century.

Their findings indicate that by the year 2000, winter chill had already declined to the point that only 4 percent of the Central Valley was still suitable for growing apples, cherries and pears — all of which have high demand for winter chill.

The researchers project that by the end of the 21st century, the Central Valley might no longer be suitable for growing walnuts, pistachios, peaches, apricots, plums and cherries.

"The effects will be felt by growers of many crops, especially those who specialize in producing high-chill species and varieties," Luedeling said. "We expect almost all tree crops to be affected by these changes, with almonds and pomegranates likely to be impacted the least because they have low winter chill requirements."

Developing alternatives

The research team noted that growers may be able change some orchard management practices involving planting density, pruning and irrigation to alleviate the decline in winter chill. Another option would be transitioning to different tree species or varieties that do not demand as much winter chill.

There are also agricultural chemicals that can be used to partially make up for the lack of sufficient chilling in many crops, such as cherries. A better understanding of the physiological and genetic basis of plant dormancy, which is still relatively poorly understood, might point to additional strategies to manage tree dormancy, which will help growers cope with the agro-climatic challenges that lie ahead, the researchers suggested.

Source: University of California - Davis

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2.1 / 5 (7) Jul 22, 2009

We are having unusually cool weather here in the mid-west, data that is exactly opposite to the dire predictions of Al Gore and the UN's IPCC.

I noticed that this news item mentions no observational data from California, other than the vague phrase that "winter chill has already declined by nearly 30 percent".

With kind regards,
Oliver K. Manuel
1.8 / 5 (5) Jul 23, 2009
The industry should move to Tennessee where, apparently, it is getting colder.

BTW SD hasn't had 'summer' as yet.

omatumr: "winter chill" is a precise technical term used by GW Conspiracy Believers. It means "what ever I want it to mean."
5 / 5 (3) Jul 26, 2009
SDMike2, if you don't understand what winter chill means, maybe you should look it up. It is used by fruit and nut growers around the world to select their cultivars and to anticipate yield potentials. Most of these growers are certainly not suspect of being "GW conspiracy" people.

omatumr: I don't know what study you're talking about, certainly not the one mentioned above. I did this study, and we used 50 years of observational data from about 200 weather stations. Sorry, but that phony argument falls apart. Check out the original paper, if you are interested in the facts (free at http://www.ploson...006166). If not, keep commenting on the press release, or someone's interpretation of it.

Both of you: the fact that this year is cooler than the last few is no proof of anything, and I think you probably know that. There is always statistical variation superimposed on the long-term trends. The fact that on a global scale, the 11 warmest years on record were all recorded in the last 15 years certainly hints at a warming trend. This was NOT made up by Al Gore, but found in detailed and thorough investigations by thousands of scientists.
1 / 5 (4) Jul 29, 2009

". . . the fact that this year is cooler than the last few is no proof of anything, and I think you probably know that."

Oliver K. Manuel
5 / 5 (3) Jul 29, 2009
omatumr: Is this the point where we should start dumbing the discussion down with out-of-context quotes and ALL CAPS one-liners? I find it pretty sad that some people, including some (ex-?)scientists like you, apparently want this to be the style. The point you brought up in your initial comment is 100% invalid, and there's really no defending it. Your last comment conveys the impression that you don't believe in statistics, which are essential for understanding climate trends.
Regardless of which side of the discussion you're on, climate change is one of the biggest scientific topics of our time, and thus worthy of a solid and informed scientific discourse. Those who aren't interested in this or aren't well informed enough to contribute more than pre-digested talking points should maybe stay away from the discussion.

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