Latitudinal population transfer reduces temperature sum requirements for bud burst of European beech

The phenology of many woody plants is controlled by an interaction of chilling requirements, photoperiod and temperature forcing. Predictions of leaf unfolding and connected ecosystem processes in global warming should thus consider not only increasing temperatures but also require a thorough understanding of the cumulative effects of daylength and temperature. In the present study, bud burst of six populations of European beech was assessed at two provenance trial sites across a latitudinal gradient of 6° within two consecutive years. Significant differences in bud burst were found among populations, trial sites and observation years. If flushing was related to the temperature forcing at trials sites, populations at the southern trial required similar temperature sums in both observation years, though the average flushing date differed by 6 days. At the northern trial site, bud burst occurred approximately 14 days later, but here the trees required 43 % degree days less to reach the same flushing state. This indicates a significant effect of the photoperiod on the temperature requirement for bud burst. The flushing sequence among populations was stable across trial sites and observation years, suggesting that similar genetic pathways regulate bud burst throughout the beech distribution. Also, it indicates that the environment of the trial location rather than the genetic origin of the populations determines its reaction to increase spring temperatures in climate change. Significant interactions of the velocity of flushing were observed between populations and trial sites, but not between populations and observation years at the same site. Together with the high variation among populations, this points to local adaptations to specific temperature-photoperiod regimes, however, adaptations to local temperature forcing seem stronger than to the light conditions.