I came across these papers discussing the reduction of winter chill due to climate change within the central valley of California:
Of interest is that the models based on the three widely accepted emissions scenarios by climate scientists indicate that by the end of the 21st century, the Central Valley of California will not receive enough chill within certain areas to support walnut or pistachio production (as well as other fruit crops). In many areas, lack of chill will occur by the middle of the century, negatively impacting pistachio and walnut bloom and thus production. This is due to the large amount of chill hours needed for for these crops (Almond~600, Pistachio >800, Walnut >900).
So, what are the impacts of low chill? Chill is needed for fruit bud development. In years with reduced winter chill, bloom is “strung out” and delayed, and pollinators may have poor overlap. Low chill years have been experienced in the past, especially in the southern San Joaquin Valley. In these cases, growers have applied “rest breaking agents” to even up bloom. This typically is a horticultural oil. Future research is focusing on the development of low chill varieties and rootstocks.
To think that almonds will be free of the negative effects of reduced chill is incorrect. In mild winters (think 2012), a “strung out” bloom usually means an uneven hull-split, and thus an uneven harvest. All of this influences the other issues including navel orangeworm (NOW), hull rot, and shaker damage. Let’s not forget that warmer winters also means a larger surviving population of NOW, other insect pests, and diseases, increasing our reliance on chemical control.