Shifts in walnut (Juglans regia L.) phenology due to increasing temperatures in Slovenia

Bud-breaking date (BBD) is one of the most important traits in walnut cultivars for commercial cultivation. In regions with frequent late-Spring frost injuries, BBD affects the stability, quantity, and quality of walnut yield. Due to global warming, shifts in walnut phenology are expected in the future, and the suitability of some cultivars for certain climatic regions may be affected. In the present study, the influence of increasing Winter and Spring air temperatures on BBD was analysed. The late BBD cultivar 'Franquette' and the medium BBD cultivar 'G-1.39' were observed over two time-periods: 1984-1990 (P1), and 2000-2006 (P2). During P1, the mean air temperature from January to April was 3.7 degrees C, and the average BBDs were 23 April and 5 May for 'G-139' and 'Franquette', respectively. During P2, the mean air temperature from January to April increased by 0.9 degrees C and the BBDs were 3 days (d) earlier for 'G-139' and 7 d earlier for 'Franquette'. Phenological models showed that the mean air temperatures in the preceding 1-2 months (T-Mar and T-Apr) were important for predicting BBD and, due to increasing temperatures in Winter and early-Spring, BBDs will be shifted to earlier in the year. Depending on walnut cultivar, the average BBD could be advanced by as much as 4 weeks by 2060. Consequently, the frost hazard will probably increase.