Stable isotopes in tree rings reveal the role of genetics and phenotypic plasticity in shaping water-use strategies of sessile oak across Europe

Quantifying the relative contributions of evolutionary mechanisms to tree water-use strategies is critical for predicting species’ responses to climate change and supporting forest management strategies. Common garden experiments can explicitly address the contributions of genetics and plasticity in physiological-related traits. However, these experiments typically focus on young trees, and long-term physiological measurements from common garden experiments are largely lacking. Stable isotope analysis of tree rings bridges this gap by enabling the reconstruction of long-term water-use strategies of mature trees growing in long-term common garden experiments.

In this study, we investigate the evolutionary mechanisms underlying long-term water-use strategies in Quercus petraea across its distribution range by analysing annually-resolved stable isotope ratios (δ¹³C, δ¹⁸O, δ²H) from tree-ring cellulose. We sampled 234 individuals originating from nine provenances grown in four European common gardens (Denmark, France, Poland, and the United Kingdom). For the period 2012–2021, we derived annual carbon isotope discrimination (∆¹³C), intrinsic water-use efficiency (iWUE), and isotopic enrichment relative to precipitation (∆¹⁸O and ∆²H). Linear mixed-effects models were used to quantify the contributions of genetic variation, phenotypic plasticity, and its interaction (i.e. genetically-based plasticity) to variation in iWUE, ∆¹⁸O, and ∆²H. The dual-isotope approach (δ¹³C and ∆¹⁸O) was applied to investigate the provenance-specific adjustments in photosynthetic rate and stomatal conductance across sites.

Our results revealed significant genetic and genetically-based plasticity effects on all isotope ratios whereas phenotypic plasticity had a significant effect only on ∆²H. ∆¹⁸O and ∆²H exhibited distinct patterns related to genetics and phenotypic plasticity effects. Notably, ∆²H variability across sites exceeded provenance-level variation. These results could be indirectly related to the link of ∆²H to primary C metabolism. The dual-isotope analysis (δ¹³C and ∆¹⁸O) further identified adjustments in stomatal conductance as the main plastic response to contrasting environments. The provenance with the least plasticity (originally from the United Kingdom) also showed reductions in photosynthetic rates, indicating a limited capacity to adjust to contrasting environments. Overall, these findings highlight strong genetic and plastic control in water-use traits and demonstrate the potential of stable isotopes in tree rings to unravel evolutionary mechanisms in tree water-use strategies.