- Author: Brooke Jacobs
In January 2014 daytime temperatures were unseasonably warm throughout California. Winter temperatures are important because deciduous tree crops (including stone fruit, pome fruit and nuts) require cold during dormancy to bloom and leaf out in the Spring. Although warm January days were followed by more typical cold nights, it is possible that dormant season cold requirements for many California tree crops were not fulfilled this winter.
The first model developed to understand the relationship between cold winter temperatures, dormancy, and rest breaking in deciduous tree crops relied upon simple counts of the number of hours below 45oF during dormancy (chill hour model). Each hour below this threshold was counted as a "chill hour" accumulated, and the number of chill hours required to break dormancy in different species was estimated. Later, the dynamic model was developed to refine estimates of chill requirements in deciduous trees. The dynamic model counts chill accumulation during hours of the day below a temperature threshold. However, warm temperatures occurring later that day can cancel out some of the chill accumulation. According to the dynamic model, warm winter days, like those observed in January 2014, should eliminate any chill accumulated during cold winter nights.
When the traditional chill hour model and the dynamic model are used to calculate chill accumulation at the same weather station the difference between model predictions during the winter of 2013-14 is clear. For example, at the Modesto CIMIS station (#71) the traditional chill hour model estimates that chill accumulation this winter was slightly above average when compared to the previous five winters. However, the dynamic model estimates substantially lower chill accumulation than in previous years (Picture 1).
On Monday I will post a follow-up blog describing abnormal leaf out and bloom patterns we have observed this Spring. Stay tuned!