Winter chill hours, which many tree nut and fruit varieties require to produce optimum yields the following year, will be halved during this century because of global warming.

A team of researchers from the University of California, Davis and the University of Washington predict that much of California's climate will no longer be suitable for growing many of the tree fruit and nut crops.

In some parts of California's 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," Minghua Zhang, a UC Davis professor of environmental and resource science, said in a new release.

The study is the first to map winter chill projections for all of California.

Fruit and nut trees occupy nearly 3 million acres and had a combined production value of $7.8 billion in 2007, the last year for which figures are available.

"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," Eike Luedeling, a post-doctoral fellow in UC Davis' Department of Plant Sciences.

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