Linking leaf phenology with xylem anatomical traits to better understand Fagus sylvactica L. responses to climate variability

Phenological traits are among the key features to disclose the effects of climatic change on living organisms. In deciduous trees of temperate ecosystems, the timing of bud-burst is one of the most investigated phenological traits. Although temperature explains most of year-to-year variability, early or late flushing behaviours of single trees are partially under genetic control. In a global change scenario, this can either determines a selective advantage of a longer growing season or a greater risk to late frosts. We applied a multi-trait, long-term and retrospective investigation linking a 20-y long series of bud-burst phenological observations with dendro-anatomical analyses to understand whether for beech it is more beneficial to be early or late. In a natural beech stand in the northern Apennines (Italy), we selected more than 100 trees with early and late phenological behaviors. Among different phenotypes, we focused on 16 trees at the extremes of the bud-burst timing distribution for an in-depth xylem-traits quantification. All trees show similar responses to temperature and maintain their ranks in bud-burst phenology over time with early individuals usually opening their buds 9-10 days before late ones. Extremely late and early individuals reflect such phenological difference in several anatomical traits with, for instance, smaller vessel dimension in early trees. Our approach highlights the role of bud-burst phenology as one of the most evident factors explaining growth variability in beech. Such functional link could help to assess potential short-term responses to climate change and to increase our understanding of its upcoming effects on forests.