The region is a key fruit and nut-producing region in the United States, containing 1.2m hectares of orchards that rely on winter chilling, producing crops worth an estimated $8.7bn a year.
Cool winter temperatures are needed for all economically important crops that have temperate origins in order to ensure uniform flowering and sufficient yields. But in California recent observations have shown increased winter temperatures, and they are projected to continue to rise due to climate change, the researchers wrote.
The study, from the University of California, Davis and the University of Washington and published online by PLoS One, suggests that winter chill could decrease by up to 50 percent by the end of the century, making the state unsuitable for fruit and nut production – and in some parts of the Central Valley winter chill has already decreased by 30 percent.
Professor of environmental and resource science at UC Davis Minghua Zhang said: "Depending on the pace of winter chill decline, the consequences for California's fruit and nut industries could be devastating.”
Crops at risk
Crops most likely to succumb to warmer winters include apples, cherries and pears, which have a very high reliance on winter chill, and the area suitable for growing these crops had already declined to just four percent of the Central Valley – which produces most of the state’s fruit and nuts – by 2000. The findings suggest that none of the Central Valley’s area will be suitable for growing apples, cherries or pears by mid-century. Many nuts and stone fruits are also at risk.
“Our projections showed that for many tree crops that now cover large areas within the Central Valley, climatic conditions will become less suitable and in many cases potentially prohibitive for production,” the authors wrote. “Areas where safe winter chill exists for growing walnuts, pistachios, peaches, apricots, plums and cherries…are likely to almost completely disappear by the end of the 21st century.”
The situation could be particularly severe for the walnut and pistachio industries that rely on male and female flowering at the same time to allow for cross-pollination.
Calculating future temperatures
To provide temperature projections that are as accurate as possible, the researchers looked at daily and hourly temperatures measured across California from 1950 to 2000 and also took into account 18 different climate projections for the rest of the 21st century. Average temperature projections were made over 20-year periods in order to avoid excessive influence of year-to-year fluctuations.
Growers currently use mathematical models to match trees’ chilling requirements with the climatic conditions in their area, but the researchers assert that these models are based on out-of-date temperature conditions.
"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.
Source: Open-access online journal PLoS One
http://www.plosone.org/article/info:doi/10.1371/journal.pone.0006166
“Climatic Changes Lead to Declining Winter Chill for Fruit and Nut Trees in California during 1950-2099”
Authors: Eike Luedeling, Minghua Zhang, Evan H. Girvetz