Oxygen isotopes in tree rings are less sensitive to changes in tree size and relative canopy position than carbon isotopes

Stable isotope ratios in tree rings have become an important proxy for palaeoclimatology, particularly in temperate regions. Yet temperate forests are often characterized by heterogeneous stand structures, and the effects of stand dynamics on carbon (δ¹³C) and oxygen isotope ratios (δ¹⁸O) in tree rings are not well explored. In this study, we investigated long-term trends and offsets in δ¹⁸O and δ¹³C of Picea abies and Fagus sylvatica in relation to tree age, size, and distance to the upper canopy at seven temperate sites across Europe. We observed strong positive trends in δ¹³C that are best explained by the reconstructed dynamics of individual trees below the upper canopy, highlighting the influence of light attenuation on δ¹³C in shade-tolerant species. We also detected positive trends in δ¹⁸O with increasing tree size. However, the observed slopes are less steep and consistent between trees of different ages and thus can be more easily addressed. We recommend restricting the use of δ¹³C to years when trees are in a dominant canopy position to infer long-term climate signals in δ¹³C when relying on material from shade-tolerant species, such as beech and spruce. For such species, δ¹⁸O should be in principle the superior proxy for climate reconstructions.