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

• Published in: Plant, cell and environment Vol. 41; no. 12; pp. 2899 - 2914
• Main Authors: Klesse, Stefan, Weigt, Rosemarie, Treydte, Kerstin, Saurer, Matthias, Schmid, Lola, Siegwolf, Rolf T.W., Frank, David C.
• Format: Journal Article
• Language: English
• Published: United States Wiley Subscription Services, Inc 01.12.2018
• Subjects: Attenuation; Beech; Cambium - chemistry; Cambium - growth & development; Cambium - metabolism; Canopies; Carbon 13; Carbon isotopes; Carbon Isotopes - analysis; Carbon Isotopes - metabolism; Climate; Fagus - chemistry; Fagus - growth & development; Fagus - metabolism; Fagus sylvatica; Isotope ratios; Isotopes; light attenuation; long‐term trend; Offsets; Oxygen; Oxygen isotopes; Oxygen Isotopes - analysis; Oxygen Isotopes - metabolism; Picea - chemistry; Picea - growth & development; Picea - metabolism; Picea abies; Shade; stable isotope; Stable isotopes; stand dynamics; Stand structure; Temperate forests; tree ring; Tree rings; Trees; Trees - chemistry; Trees - growth & development; Trees - metabolism; Trends; long-term trend; stand dynamics; stable isotope; light attenuation; Picea abies; tree ring; Fagus sylvatica
• ISSN: 0140-7791; 1365-3040
• DOI: 10.1111/pce.13424

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. Tree‐ring stable isotope ratios have become an important paleoclimate proxy in temperate forests that are often characterized by heterogeneous stand structures. As the effects of stand dynamics on carbon (δ13C) and oxygen isotope ratios (δ18O) in tree rings are not well explored, we investigated 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 found positive trends in both stable isotopes ratios; however, the observed slopes in δ18O are less steep and much more consistent between trees of different ages and sizes compared with δ13C. Thus, we conclude that for shade‐tolerant species, δ18O should be in principle the superior proxy for climate reconstructions.