Vol. 81 No: 6

Title:
Chill unit models and recent changes in the occurrence of Winter chill and Spring frost in the United Kingdom

Authors:
R.J. SUNLEY, C.J. ATKINSON and H.G. JONES

pp: 949-958

Abstract:
Following concern that a trend towards milder Winters may have a detrimental impact on bud break and fruit quality, we have investigated the choice of existing chill accumulation model that best explains year-to-year differences in the spread in flowering date in soft fruit, and have characterised the changes in Winter chill that have occurred in recent decades in the UK. As part of this study, we compared the major methods currently used to accumulate chill. As some of these models require hourly temperature data, we have also evaluated methods to estimate hourly temperatures from daily maximum and minimum records. All chill accumulation methods, with the exception of the ´Utah` model (which uses a complex weighting scheme for accumulation of chilling temperatures), showed a trend of declining Winter chill over the last three decades. The model that performed best in terms of its ability both to explain the variation in spread of bud burst in any year, and to describe differences in chill with geographical location and over time, was one that accumulated time below 7.2°C (the ´< 7.2°C` model). A significant declining trend in Winter chill has occurred in tandem with reduced Spring frost. Both have agronomic implications. The largest changes in these climatological factors have occurred in southern regions of England, with a much less pronounced decline in the North of the UK (Tayside).

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