Tom Coleman is busy pruning branches off pistachio trees that aren't budding at an orchard just north of Fresno, Calif. He farms and manages more than 8,000 acres of pistachios across the state.

"Here's an example of some hanging down nuts from last year that just wouldn't come off because of the position on the tree, so we want to remove that," says Coleman.

Coleman worries these trees won't get enough sleep this winter. Crops like pistachios, peaches and almonds need a certain amount of cold weather every year. This is what the agricultural industry refers to as chill hours.

Frigid temperatures between 32 and 45 degrees help set buds that will turn into flowers in spring, then into fruits and nuts in summer. The problem is that there is a decrease in the amount of hours needed for tree crops to reach these temperatures. Coleman's trees need more than 700 hours of sleep every winter, but for the past four years, many have slept less than 500 hours.

"And as result of that, they do not bloom uniformly. When they don't have uniform bloom, it can dramatically reduce the yield," says Coleman.

This is a problem that farmers are facing across California, and if it continues, the prices for these products could go up. Multiple University of California studies predict that within 30 to 50 years it may be too warm to grow many tree crops where they now flourish.

Agricultural scientist Eike Leudeling found that climate conditions in California by "the middle to the end of the 21st century will no longer support some of the main tree crops currently grown." He says farmers will either need to find alternative crops or establish ways to mitigate warming temperatures.

UC Davis researcher Hyunok Lee, whose study was published in the journal California Agriculture, found that winter temperatures are increasing more than at any other time of year. Her model looks at the year 2050 in Yolo County.

"Our agriculture will continue," Lee says. "But if you look at . . . 20 years or 30 years, the pattern may change a little bit, crops may move a little bit north."

She says tree crops like walnuts would be harmed the most, but annual crops like tomatoes could benefit from rising temperatures. For growers with huge investments in trees that have life cycles of 25 years or more, this is a big deal.

Farm adviser David Doll, of the University of California's Division of Agriculture and Natural Resources, is trying different things to get the trees more sleep. He has experimented with overhead sprinklers and even painting the trees white with liquid clay to reflect sunlight.

"This is something that could impact a lot of farmers over the next 10 to 40 years," Doll says. "It's already impacting farmers on random given years across the state."

This problem is so prevalent that climate-change deniers really can't get away from it. Crops have failed because of warming temperatures. (Doll explains more about chill hours here.)

In 2015, California's pistachio industry was hit hard by a lack of chill hours. As a result, the crop was nearly split in half. The UC system and the pistachio industry have invested about a million dollars to figure out how to cope with warming temperatures. UCANR farm adviser Craig Kallsen is trying to breed a pistachio tree that needs less sleep.

"We're trying to use the other species of pistachios to see if we can come up with something that has a low-chill requirement. It's pretty hypothetical at this stage," Kallsen says. "We made quite a few crosses this spring and we actually hope to put a trial in a low-chill area."

It's not just pistachio trees that aren't getting enough shuteye. Kern County farmer Steve Murray says his cherries may suffer this year because of a warmer winter.

"Initially it was looking like 2017 was going to be a disastrous year, because not only were the trees not getting chilling, they were actually heating up," Murray says. "When the sun hits the wood, the temperature of the wood can be 20 and even 30 degrees warmer."

Murray says the only real solution is for the temperature to drop. And to complicate the matter, many researchers and farmers say there isn't enough understanding about why the trees need sleep. Plus, a decrease in the amount fog in the region also keeps trees from staying cool. That has to do with rain.

Back on Coleman's 160 acres of pistachios, it has rained so much that a large creek has formed in the middle of his property. He's hoping the ground is saturated enough for fog to form this winter, which would lower the temperature around his trees.

"The fog is a good thing because it keeps a uniform cooler temperature on the ground, but we just haven't seen that over that last several years — even with the rain," says Coleman.

For Coleman the facts are evident: The climate is warming and he has to adapt his practices.

"I know that there are people that think that global warming is not man made, but regardless we have to deal with it. I think that making plans around it is necessary," says Coleman.

These trees that he's prided himself on since the '70s could live well beyond him and his children. He's taking warming temperatures seriously, because what happens with the climate today could mean severe cuts or total crop loss in the coming decades.

The story comes to us from KVPR, an NPR member station in Central California.

Copyright 2017 Valley Public Radio. To see more, visit Valley Public Radio.

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