Phenotypic plasticity and inter-individual variability in Fagus sylvatica L. xylem traits challenge assisted migration

Climate change and its associated extreme events are significantly impacting European forest ecosystems, with European beech (Fagus sylvatica L.) being particularly vulnerable due to its high sensitivity to drought. This study investigates the role of phenotypic plasticity in xylem anatomical traits of beech from 15 provenances grown at three trial sites in Germany, representing different environmental conditions. Using quantitative wood anatomy, we assessed traits including mean ring width (MRW), mean vessel area (MVA), vessel density (VD), vessel grouping index (RVGI), and percentage of conductive area (RCTA). Our results reveal that site conditions have a stronger influence on MRW, MVA, and VD, while RVGI and RCTA are stronger affected by provenance. This suggests trait-specific plasticity, with some traits responding more to environmental factors than others. Notably, individual tree variability accounted for a substantial proportion of the observed differences, highlighting the role of intra-population genetic variation. Climate sensitivity was mainly site-dependent, with the northern trial site showing greater sensitivity to temperature, while the central and southern sites were more sensitive to drought. Interestingly, we found no clear relationship between the climatic origin of provenances and their xylem traits, challenging the assumption that provenances from warmer and drier regions are characterized by more drought-adapted xylem. The study underscores the complexity of predicting provenance performance under climate change and suggests that maintaining high genetic diversity may be more beneficial than selecting specific provenances for assisted migration. These findings contribute to improving forest management strategies for climate resilience.