Productivity definition of the chilling requirement reveals underestimation of the impact of climate change on winter chill accumulation

Evaluation of chilling requirements (CR) of cultivars of temperate fruit trees provides key information to assess regional suitability, according to winter chill, for both industry expansion and ongoing profitability as climate change continues. Traditional methods for calculating CR use climate controlled chambers and define CR using a fixed budburst percentage, usually close to 50% (CR-50%), without considering the productivity level associated to this percentage. This CR-50% definition may underestimate the real CR of tree crops for optimal productivity. This underestimation is particularly important to consider as winter chill accumulation is declining in many regions due to climate change. In this work we used sweet cherry to analyse the traditional method for calculating CR in many Rosaceae species (CR-50%) and compared the results with more a restrictive, productivity focused method, with CR defined with a 90% bud break level (90%, CRm-90%) close to the optimal budburst which assures productivity. Climate projections of winter chill suitability across Europe using CR-50% and CRm-90% were calculated. Regional suitability landscape was highly dependent on the method used to define CR and differences were found for a wide area of the European geography, both cold and mild winter areas. Our results suggest a need to use an optimal budburst level for the assessment of CR for sweet cherry. The use of traditional methods to determine CR can result in an underestimation of productivity CR with negative consequences for the fruit industry, particularly as climate change advances.