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The phenological phenomena in terms of year day index (YDI) in South Korea were studied. The YDI was proposed here, because the remainer index such as Nuttonson´s index is unadequate for the interpretation on the phenological phenomena of early spring season in southern coastal area. The YDI was calculated by summing daily mean temperature of the yeat days (YD) above physical zero degree in centigrade, based on the data of the Monthly Weather Reports from 1967 to 1980 by the Central Meteocological Office. The pattern of YDI increase with the increase of YD was similar to that of the remainder index such as the Nuttonson´s index. The some YDI distribution maps were made by Yim and Kira (1975), dividing into 30´×40´ meshes, in latitude and longitude, on the topographical map(1:500,000) of the Korea Peninsula. According to the year day of different localities flowering dates of Prunus yedoensis and other phenological phenomena in various species delayed about 3.5 day as the increase of 1 degree of latitude, which coincides with the Hopkins bioclimatic law. It was found that the YDI is useful to interprete the phenology of plant and animal species and to select the optimum range of cultivars in South Korea.
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