Chilling Challenges in a Warming World


Global warming affects perennial horticultural crops in many ways, including potentially by reducing available winter chill. For many fruit and nut species of the temperate and subtropical climates, such as apple, cherry, peach and walnut, fulfilment of cultivar-specific chilling requirements is a prerequisite for breaking dormancy, blooming regularly and ultimately for producing economically satisfactory yields. Global warming may jeopardize the trees’ ability to accumulate sufficient winter chill and become fully receptive to spring forcing. In our paper, we review recent evaluations of past and projected future changes in winter chill and discuss implications for the production of temperate fruits and nuts across Europe. Great differences both in historic trends and in future projections were identified when quantifying winter chill with different models. The commonly used Chilling Hours Model is highly sensitive to change due to hard temperature thresholds that are unlikely to be of biological significance. To a slightly lesser degree, this also applies for the Utah Model. Among the evaluated models, only the Dynamic Model instills confidence in its ability to reliably describe the response of winter chill to temperature, but even this model requires more validation. According to the Dynamic Model, changes in winter chill vary across climatic zones, with cold climates (e.g., in Scandinavia) experiencing an increase in winter chill, temperate climates (e.g., central Europe) seeing stagnation and warm growing regions (e.g., southern Europe) facing declining winter chill. In Europe, the warmest growing regions around the Mediterranean Sea are most threatened by reductions in winter chill. Besides highlighting the need to consider projected future climate conditions when selecting tree cultivars, our study stresses the need to improve our understanding of the dormancy-breaking process and to produce and validate quantitatively reliable models for projecting fruit and nut production in a warming world.