Improvement of chilling and forcing model to predict bud-burst

Timing of bud-burst for crop and forest species mainly depends on air temperature variation during the winter season. Exposure to a particular duration of cold temperature is needed to meet the chill requirement and break dormancy. The effectiveness of time-temperature combinations on meeting chilling requirements varies between species. Classical chill unit models were developed to predict breaking dormancy in spring (bud-burst) mainly as a function of cold temperature accumulation. Other more complicated models describe the sequential or parallel processes of breaking rest and forcing temperature to overcome quiescence. A sequential chill model for predicting bud burst based on the accumulation of chill days during rest and anti-chill days during quiescence was recently presented. It is based on the calculation of chill days and anti-chill days using the single triangle method to estimate degree days relative to a threshold temperature. The temperature threshold for calculating the chill and anti-chill days and the chilling requirement to determine when rest is broken were found by trial and error to minimize the root mean square error of predicted and observed bud-burst dates. The chill days model was superior to classical models when used to predict bud-burst for crop and forest species. Since the parameters of the Chill days model are determined by optimizing the prediction of bud-burst for a particular locations, they are site specific. The model was modified to include the effect on chill day accumulation of the temperature thresholds. This new approach should make the model more universal. Based on the first results, the model performed well and showed some promises.