Chilling requirements of Mediterranean fruit crops in a changing climate

Fruit production in Europe may be hampered by climate change in terms of warmer winters resulting in insufficient chilling. This contribution provides a brief survey of winter chilling requirement of fruit crops. Therein, cherry and apple were identified as the fruit crops with the highest chilling requirement. Three chilling models are presented and the different ways of calculation explained. In the simplest model, Weinberger (1950) counts every hour between 0 and 7.2°C as one chilling hour (CH). The Utah model (1974) expands the temperature range to 1.4-12.4°C to calculate the chill units (CU). In this model, temperatures are weighted, namely, temperatures in excess of 16°C nullify previously accumulated CU. For the Dynamic model (1987), chill portions (CP) are calculated based on assumptions of metabolic processes at a temperature range of -2 and 12°C with an optimal chilling effective range from 6 to 8°C. First results applying the three chilling models are presented to calculate winter chilling in a cherry orchard at Klein-Altendorf (Bonn, Germany), and in an adjacent unheated greenhouse (average temperature increase by ca. 2°C to simulate global warming). When chilling was computed for the trees in the unheated greenhouse to simulate climate change, the available chilling at 50°N increased by 12% (CU), 15% (CP) and 20% (CH), respectively. In climate change predictions for other locations, the warmer winter temperatures, especially in the Mediterranean area, often induce insufficient chilling. Options and limitations of countermeasures in terms of cultivation methods such as microclimate manipulation, rest breaking agents, change of orchard location to a higher altitude and breeding are suggested and briefly discussed.