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 (δ C) and oxygen isotope ratios (δ O) in tree rings are not well explored. In this study, we investigated long-term trends and offsets in δ O and δ C 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 δ C that are best explained by the reconstructed dynamics of individual trees below the upper canopy, highlighting the influence of light attenuation on δ C in shade-tolerant species. We also detected positive trends in δ O 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 δ C to years when trees are in a dominant canopy position to infer long-term climate signals in δ C when relying on material from shade-tolerant species, such as beech and spruce. For such species, δ O should be in principle the superior proxy for climate reconstructions.