Climate Change Effects on Chilling Hours for Yolo County's Fruit and Nut Trees
What are chilling hours and why are they important to orchard trees?
Plum orchard in Yolo County. Photo courtesy of Susan Ellsworth.
In order for most deciduous fruit and nut trees to break dormancy and begin flowering in the spring, they must first be exposed to chilling temperatures for a sufficient period of time each winter. This chilling requirement is typically measured in terms of "chilling hours", defined here as the cumulative number of hours below 7° C (45° F) each winter. The number of chilling hours required by various tree crop species and cultivars can range widely (Table 1).
Table 1. Selected tree crops grown in Yolo County and their annual chilling hour requirement. 
What are the effects of insufficient chilling hours?
Because they act to regulate plant life cycles, an insufficient amount of chilling hours can have major impacts to plants, such as:
- Lack of synchronization between opening of male and female flowers, leading to delayed pollination
- Delayed foliation
- Reduced fruit yield
- Reduced fruit quality
The effects of insufficient winter chill can vary among species. Walnuts and pistachios depend on a synchronization between male and female flowering that is regulated by the amount of chilling hours. For various stone fruit, a lack of winter chill results in delayed foliation, reduced fruit set, and poor fruit quality. In any case, insufficient winter chilling hours result in reduced tree crop performance for farmers.
How is climate change affecting winter chilling hours in the region?
While chilling hours can vary widely year to year, recent studies indicate that average chilling hours in the Southern Sacramento Valley have decreased by more than 20% since 1950 . Luedeling et al. (2009) modeled chilling hours in 4 regions of California based on historic climate data, an average of three global climate models and three scenarios of future emissions developed by the International Panel of Climate Change (B1, A1B, and A2). Under all scenarios the model predicted a significant decrease in the number of chilling hours in the Southern Sacramento Valley and throughout California by the end of the century (Table 2). Given the reductions in chilling hours which are expected, more research is needed to understand the how crop species of local importance might be impacted.
Table 2. Means and standard deviations of chilling hours modeled for four regions in California's Central Valley for 1950, 2000, 2041-2060, and 2080-2099 (Copyright permissions were not obtained). 
Figure 1. Distribution of annual winter chill estimates (in chilling hours) for Davis, CA, based on 100-year synthetic weather records for each point in time. In box plots, the central line indicates the median of the distribution, the edges of the boxes are the 25% and 75% quantiles, error bars are the 10% and 90% quantiles, and dots indicate outliers (Copyright permissions were not obtained). 
How will Yolo County adapt to decreasing chilling hours?
Given the wide range in chilling requirements among tree crop species and cultivars, perhaps the best strategy for adaptation is to identify varieties with lower chilling requirements. To facilitate the development of promising new cutivars, the National Clonal Germplasm Repository at Wolfskill Experimental Orchards located just outside of Winters maintains a extensive collection of fruit and nut trees grown in California. This collection includes dozens and in some cases hundreds of cultivars for a given crop species, some of which might have lower chilling hour requirements than the cultivars currently in use (Table 3). Germplasm repositories such as Wolfskill have the potential to give us a "head start" in preparing for climate change by helping to assess which cultivars or crop species are well-suited to future changes.
Table 3. List of crop types, number of species, and total number of cultivars available at the Wolfskill repository.
Where can I get more information on new varieties and other adaptation strategies?
|||Baldocchi, D. and S. Wong (2008). "Accumulated winter chill is decreasing in the fruit growing regions of California." Climatic Change 87(0): 153-166.|
|||(1, 2, 3) Luedeling, E., M. Zhang, et al. (2009). "Climatic Changes Lead to Declining Winter Chill for Fruit and Nut Trees in California during 1950-2099." PLoS ONE 4(7): e6166.|