Consistent δ 13C offsets between whole wood and α-cellulose of Pinus sylvestris confirmed across contrasting forest stand types and a climatic gradient

Whole wood can be used as a practical alternative to α-cellulose in tree-ring 13C isotopic (δ13C) analysis. However, the consistency of the relationships between δ13C of whole wood (δ13CW) and α-cellulose (δ13CC) and their climatic sensitivity can vary across contrasting forest stand structures in tree species composition and, along an eco-climatic gradient. We assessed whether δ13CW captures inter-annual variation and climatic sensitivity that are comparable to those obtained from δ13CC of Pinus sylvestris. The study was conducted across three monospecific and two mixed-species forest stands spanning Mediterranean, temperate and continental-temperate climates. Using 145 individual tree rings from 15 trees, we analyzed offset and correlation between δ13CW and δ13CC, centered around two site-specific pointer drought years (1983 and2014), including the two years before and after drought. Bootstrapped correlation and linear mixed-effects models were used to evaluate and compare the climatic signal strength between δ13CW and δ13CC across the study sites and among monospecific and mixed-species forest stands. Across sites and stand species composition types, δ13CW and δ13CC exhibited significantly different δ13C values, with a constant offset of 1.3‰ (±0.13‰ standard deviation). δ13C values were strongly correlated (91-98%) for the two substrates, showing comparable inter-annual variation and similar sensitivity to climatic factors. We conclude that δ13CW appears to be a reliable alternative to δ13CC under our eco-climatic and forest structural conditions. Our results confirm previously reported strong correlations and stable offset between δ13CW and δ13CC in P. sylvestris. By demonstrating this consistency across contrasting stand composition and wide climatic gradients, our study adds empirical support for the use of δ13CW when cellulose extraction is not feasible. δ13CW is a faster, less labour-intensive and more cost-effective alternative method to quantify, based on pointer years, the effects of water stress on tree growth in large-scale spatial dendro-isotopic studies at the level of annual resolution.