Climate change is expected to influence plant productivity particularly through changes in the timing of budburst. Nonetheless, knowledge about the intraspecific variation of the timing of budburst and its relationship with climate is insufficient for most tree species. Based on the common garden experiments of Fagus crenata, we investigated the interrelationships between the day of budburst, cumulative degree-days (temperature sum), chilling duration, and photoperiod at the timing of budburst for the trees of different combinations of 11 sites of seed origin and seven experimental sites in Japan. We found that the relationship between the latitude of experimental sites and the timing of budburst differed for the trees of different latitudes of origins. The timing of budburst was earlier for the trees of more northern populations throughout the latitudes of experimental sites. Variation in the timing of budburst among the trees of different seed origins was smaller for more northern experimental sites. Such patterns were caused by directional changes in the relationships between temperature sum, chilling duration, and photoperiod among the trees of different origins: the asymptotes of the curvilinear relationship between chilling duration and temperature sum, chilling duration and photoperiod, and temperature sum and photoperiod, decreased for more northern populations. With the northward expansion of species distribution, the responses of budburst to climate probably changed genetically in such ways in this species. Our results suggest that intraspecific variations in the relationships between the timing of budburst and associated meteorological factors inevitably influence the overall pattern of the timing of budburst at the geographic scale, and the timing of budburst might deviate from predictions when intraspecific variations are not considered.
Keywords: Chilling duration; Global warming; Intraspecific variation; Provenance; Spring phenology; Thermal time.