California Chilling - Overview

The Central Valley of California is one of the most productive growing regions for temperate tree crops, such as almonds, pistachios, walnuts, prunes and peaches. One of the major reasons for the high productivity of these species is the local climate, which is ideal for meeting tree requirements. This is critical, because trees from temperate and cold climates fall dormant in the winter to avoid frost damage to sensitive growing tissue. During winter dormancy, most physiological functions are suspended or severely slowed, allowing the trees to survive the harsh winter conditions of their native ranges in Central Asia and the Middle East. To resume growth in spring, winter dormancy must be broken, and trees have evolved sophisticated mechanisms to detect the end of winter. They monitor the progression through the cold season by recording how long it has been how cold. Duration and 'amount' of coldness is integrated into winter chill, and trees are only receptive to warming in spring, after they have fulfilled their chilling requirements. This chilling requirement is thus a major factor that determines where certain tree species can be cultivated. In California, for example, chilling requirements constrain the suitable areas of walnuts, prunes and cherries, which can only be grown commercially in the colder parts of the Central Valley.

According to climate model projections, temperatures in California are likely to rise during the 21st century, and this can be expected to reduce available winter chill. This in turn may shift suitable ranges for commercially cultivated tree crops. Due to the long productive life spans of trees, anticipating future winter chill can be crucial for ensuring that the current high level of productivity can be sustained in the future. This website therefore provides winter chill projections for several climate scenarios, including historic changes, as well as changes projected by global climate models.

The full methodology and citations for all background information is published here (freely accessible):

The information presented in the paper, as well as the dataset available from this website, can be used for any non-commercial purpose, but please ensure that the following reference is added in any derived work:
Luedeling E, Zhang M and 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.

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