Warming Climate Jeopardizes California Fruit and Nut Crops

In some parts of California's agriculturally rich Central Valley, winter chill has already declined by nearly 30 percent, the researchers found, and they say more warming is on the way.

"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.

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 2100, using data from climate models developed for the most recent assessment of the UN's Intergovernmental Panel on Climate Change.

The research team found that by the year 2000, winter chill had already declined to the point that only four percent of the Central Valley was still suitable for growing apples, cherries and pears, all of which need winter chill.

The researchers project that by the year 2100, the Central Valley might no longer be suitable for growing walnuts, pistachios, peaches, apricots, plums and cherries.

The study is the first to map winter chill projections for all of California, which is home to nearly three 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," said Eike Luedeling, a postdoctoral fellow in UC Davis' Department of Plant Sciences.

The warmer California climate has not damaged the 2009 crops, says the head of the organization that handles fruit marketing on behalf of California's more than 1,100 fresh peach, plum and nectarine growers.

"The crop outlook for 2009 is positive for our industry with adequate chilling hours this winter we expect exceptional fruit quality and flavor this summer, always key to consumer satisfaction," says Sheri Mierau, president of the California Tree Fruit Agreement.

Peach orchard in Fresno County, California (Photo by Michael Layland courtesy Fresno County Blossom Trail)

Most nontropical fruit and nut trees avoid cold injury in the winter by losing their leaves in the fall and entering a dormant state. To resume growth, the trees must experience a certain amount of winter chill, traditionally expressed as the number of winter chilling hours between 32 and 45 degrees Fahrenheit.

Insufficient winter chill plays havoc with flowering time, especially 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.

Each species or cultivar of tree is assumed to have a specific chilling requirement and to select tree varieties whose winter chill requirements match conditions of their local areas, fruit and nut growers use established mathematical models.

But those models were calibrated based on past temperature conditions, and the researchers say they may not remain valid in a warmer future.

"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 climate 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.

The researchers found that in all projected scenarios, the winter chill in California declined over time.

"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."

Growers may be able change some orchard management practices such as planting density, pruning and irrigation to alleviate the decline in winter chill, the study suggests. Or growers might transition to different tree species or varieties with less demand for 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 might point to additional strategies to manage tree dormancy, helping growers cope with future climate warming.

Funding for this study was provided by the California Department of Food and Agriculture and The Nature Conservancy. The findings appear in the online journal PLoS ONE.

Copyright Environment News Service, ENS, 2009. All rights reserved.