Research paperComplexity in chill calculations: A case study in cherries

- This study questions the validity of a fixed chill threshold value given the complexity found in chill calculations.
- Comparison of sites across Australia showed a difference in chill required.
- Imposing a fixed start date for chill accumulation excluded varying amounts of chill compared with using a self-regulating start time, increasing the variation of chill requirement values.
- Method of determining chill requirement will impact on practical decision-making in the orchard, and on assessment of regional suitability both now and into the future with a changing climate.

This study seeks to highlight the range of chilling requirement values that can be obtained for one cherry cultivar ('Lapins') across different Australian locations by using the same data but different approaches in calculating chilling requirement (CR). We seek to test the assumption that chill thresholds are fixed. The physiological and in-field implications for management are explored. All sites and years showed a steady increase in chill accumulation during the year. Bud burst in relation to accumulated chill portions (CP) was not consistent between sites or years, but all sites exhibited a sharp increase in bud burst after 40 chill portions (CP). Using a 50% bud burst threshold performed statistically better than a 30% bud burst threshold, and observed CR values performed slightly better than modelled CR values. The spatial analyses of safe winter chill for the range of CR determined in this study confirm the variability in values between the 30% and 50% bud burst approach. Imposing a fixed start date for chill accumulation excluded varying amounts of chill compared with using the self-regulating start time and increased the variation of CR values. The implications of using different methods to estimate CR, using projected estimates of chill accumulation under climate change, are provided. This study has e xposed the complexity in not only comparing CR values from previous studies, but the inherent confusion in the communication of such knowledge, especially with the additional consideration of a warming climate.