Growth and wood-structural responses to moderate drought in young Quercus robur with contrasting heartwood production

Broadleaf afforestation increasingly requires planting material that combines valuable wood properties with resilience to climatic stress. High heartwood content is a desirable trait in pedunculate oak (Quercus robur), but it is unclear whether selecting for it could be a trade-off with resistance and resilience to drought. We addressed this question in a 24 year old progeny trial in southern Denmark where trees were ranked by heartwood production and the 2018 drought was used as a natural stress event. Trees with contrasting heartwood content were assessed using multiple indicators of drought response, including annual ring width, vessel anatomy, wood density, and dual stable isotopes (δ¹³C and δ¹⁸O) measured separately in earlywood and latewood over a five-year period. The 2018 summer drought was of moderate intensity at the trial site, with SPEI3 approximately -1.5, but it was clearly reflected in isotopic signals, indicating that trees perceived the water deficit. In contrast, radial growth did not decline, and ring widths remained above average in 2018. Across all metrics, trees with lower and higher heartwood content showed broadly similar physiological and wood-structural responses. Within the environmental conditions and drought intensity observed in this study, our results provide no evidence for a strong trade-off between heartwood content and response to a drought of moderate severity. They therefore support, with appropriate caution, the continued consideration of heartwood content as a selection trait in breeding and afforestation programs in comparable environments.