Phenological shifts of buds and flowers caused by early heat recognition in citrus in Jeju, Korea

Abstract

The objectives of this study were to trace the shift in bud burst and full bloom dates and provide a countermeasure for the suitable prediction of these representative phenological indices for citrus trees. The heat accumulation value was calculated by the growing degree day (GDD) model with six base temperatures (Tb) from 5 to 17.5℃ in 2.5℃ intervals, and the estimation accuracy of this value for both phenological stages was evaluated. The optimal Tb and heat requirement of Miyagawa Satsuma mandarin trees were 7.5℃ and 108.6℃·days for bud burst, respectively, and 10.0℃ and 196.2℃·days for full bloom, respectively. The data showed that bud burst and full bloom have advanced by 0.3−0.5 days per year since the 1960s, and this advancement was more drastic over the last 10 years, which reached 1.4 days per year. Moreover, the theoretical calculation of heat accumulation started on the date when fruit was harvested in the previous year, but the practical heat accumulation affecting bud burst started much later because of the negative effect associated with values below the Tb by the heat accumulation model. The start date of heat accumulation also gradually advanced in recent decades by 0.5 days per year. The dates of full bloom at specific study sites were predicted via the dates of full bloom at Jeju and Seogwipo, which are close to the northern and southern coasts and represent sites where recent meteorological data have been collected. The prediction equations had common components, and the value of the southernmost site could be used as a low variable threshold by dropping the coefficient; thus, the equation was dependent on the large variability of the northernmost site, which was represented by a higher coefficient.