Frost risk climate change projections for sweet cherry (Prunus avium L.) in Slovenia

Sweet cherry (Prunus avium L.) is an important fruit tree for Slovenia, often threatened by spring frosts, especially the early flowering varieties. In recent decades, the occurrence of frost events has steadily increased, which has negatively affected cherry fruit production, especially in the Submediterranean part of Slovenia. Although fruit trees can tolerate severe cold during their dormant stage, the resistance of flower buds gradually decreases with phenological development in spring. Therefore, spring frost events that occur after the beginning of the growing season pose a major risk to fruit trees. An analysis of the probability of frost events in Slovenian cherry orchards was performed based on historical temperature data from the E- OBS database and climate model projections, in particular regionally downscaled EURO-CORDEX projections (ensemble of 6 models) for scenarios RCP4.5 and RCP8.5. The presented analysis is based on two phenological models for calculation of budburst - the growing degree days (GDD) model and the BRIN model. The projections showed an overall increase in frost risk for the majority of models and all selected sweet cherry varieties during the periods 2011–2040 and 2041–2071 compared to the reference period 1981–2010. For the end of the century (period 2071–2100), all of the ensemble projections showed a large increase in frost risk for the analyzed sweet cherry varieties. The projected increase in frost risk under the RCP4.5 scenario was calculated to be the highest for the Early Bigi variety, while under the RCP8.5 scenario the largest increase was calculated for the Germersdorf variety - a more than 40-fold increase in spring frost risk compared to the reference period.