Winter Chill Reduction from Climate Change

Future winter chill was quantified using the A2 IPCC greenhouse gas emissions scenario. Luedeling E, Zhang M, Girvetz EH (2009) Climatic Changes Lead to Declining Winter Chill for Fruit and Nut Trees in California during 1950–2099. PLoS ONE 4(7): e6166. doi:10.1371/journal.pone.0006166

I came across these papers discussing the reduction of winter chill due to climate change within the central valley of California:

1. Luedeling E, Zhang M, Girvetz EH (2009) Climatic Changes Lead to Declining Winter Chill for Fruit and Nut Trees in California during 1950–2099. PLoS ONE 4(7): e6166. 

2. Baldocchi and Wong (2008). Accumulated winter chill is decreasing in the fruit growing regions of California.  Climatic Change. March 2008, Volume 87, Issue 1 Supplement, pp 153-166.

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.

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5 thoughts on “Winter Chill Reduction from Climate Change

  1. I received a few comments questioning my intent about constructing this article. The comments address the aspect of climate change induced from greenhouse gas emissions and question its scientific merit. Climate science is not my area of expertise, but it is important to note that the selected articles (as well as several dozen more) are all peer reviewed by scientists who are experts in the respective fields. It is also important to note that green house gas is part of a theory on climate change, not a scientific law – so it is still being actively tested and debated.

    On the other hand, not any of the commenters would deny that changes are occurring within the climate. And my counter thoughts are that even if the theories on why the earth is getting warmer are wrong, does this change the point that it is getting warmer? And if it is getting warmer, doesn’t this suggest future limited areas for walnut and pistachio production within the SJV? This was the message I was trying to convey with the entry. If there was any interpretation that I was trying to push an agenda, I am sorry – that was not my intent.

    1. I found your article very relevant. We have just recorded our warmest September on record. Of the last 10 years 8 have been the hottest on record. Are there any low chill varieties in the pipeline.

      1. Ian,
        I spoke with our breeder, and he stated that they do consider chill requirements when selecting a variety. That said, almonds are considered low chill – requiring around 600 hours – and is not a driving force in varietal selection. There is some research in low chill rootstocks. This is important as some rootstocks have higher chill requirements than most scions. So to answer your question – yes, both varieties and rootstocks are being developed.

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