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Global warming threatens Central Valley’s fruit and nut crops

Winter tule fogs in decline; no rest for the orchards

A peach orchard in Palisade, Colorado in full bloom.

A peach orchard in Palisade, Colorado in full bloom.

STAFF REPORT

FRISCO — The winter tule fog in California’s Central Valley may be fading with climate change, threatening part of the region’s multibillion dollar agricultural industy, according to a new study by University of California, Berkeley researchers,

High-value crops like almonds, pistachios, cherries, apricots and peaches all need a winter dormant period that is triggered and maintained by cold temperatures, but those are becoming less reliable as the global climate warms. The new study, published May 15 in the journal Geophysical Research Letters, found a 46 percent drop in the number of fog days between the first of November and the end of February during the 32-year study period.

“The trees need this dormant time to rest so that they can later develop buds, flowers and fruit during the growing season,” said biometeorologist and study lead author Dennis Baldocchi, whose father grew almonds and walnuts in Antioch and Oakley. “An insufficient rest period impairs the ability of farmers to achieve high quality fruit yields.”

The findings have implications for the entire country since many of these California crops account for 95 percent of U.S. production, the authors noted.

The researchers paired NASA and National Oceanic and Atmospheric Administration satellite records with data from a network of University of California weather stations, covering 32 consecutive winters. While there was a lot year-to-year variability, the long-term trend was pretty clear.

“The year-to-year variability we saw was likely influenced by whether the season was relatively wet or dry,” said Baldocchi, professor of Environmental Science, Policy and Management. “Generally, when conditions are too dry or too wet, we get less fog. If we’re in a drought, there isn’t enough moisture to condense in the air. During wet years, we need the rain to stop so that the fog can form.”

Other studies have marked the decline in the Central Valley of winter chill – the number of hours between 0 and 7 degrees Celsius. The number of hours of winter chill has dropped by several hundred since the 1950s, the study authors noted.

But ambient air temperature alone may not adequately reflect the heat experienced by the crops, said Baldocchi. Direct sunlight can heat the buds so that they are warmer than the surrounding air temperature. As a result, fog is important in shielding the buds from the sun and helping them accumulate winter chill.

Climate forecasts suggest that the accumulation of winter chill will continue to decrease in the Central Valley. Baldocchi said that fruit developers are already trying to develop cultivars that can tolerate less winter chill.

“Farmers may also need to consider adjusting the location of orchards to follow the fog, so to speak,” said Baldocchi. “Some regions along the foothills of the Sierra are candidates, for instance. That type of change is a slow and difficult process, so we need to start thinking about this now.”

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