The oxygen isotope composition in tree rings is commonly used to assess the impact of climatic factors on tree growth. However, the relationships between environmental variables and δ¹⁸O in whole wood and the wood components (e.g., cellulose, lignin) are still not completely clear - particularly for deciduous species. It is currently common procedure to consider only one wood constituent, a process that saves time. To test whether the δ¹⁸O of cellulose relates better to environmental variables than that of whole wood, we measured the oxygen isotopic composition of tree ring whole wood and cellulose, for the period 1916-1950, for two different broad-leaved species (Fagus sylvatica L. and Acer pseudoplatanus L.) on Monti Picentini (Southern Italy). Data show that δ¹⁸O in cellulose is enriched (4.7 ± 1.1‰) compared to the corresponding whole wood. The relationship between the δ¹⁸O of the cellulose and of whole wood is generally poor, especially for A. pseudoplatanus suggesting that the sources of variability for δ¹⁸O differ strongly between the whole wood and the cellulose constituents.
Furthermore, we find that the cellulose fraction correlates strongly with climatic variables, while the whole wood fraction generally does not. Monthly temperature during the growing season is strongly correlated with δ¹⁸O of cellulose for F. sylvatica (r = 0.82) and A. pseudoplatanus (r = 0.63) but not with δ¹⁸O of whole wood.
We found a relevant difference in the isotopic signals of the two species that could be related with difference in seasonal timing of growth. In contrast to F. sylvatica, early spring conditions have an influence on A. pseudoplatanus at our study site. This is evidenced by the high correlation between δ¹⁸O of cellulose and April and May precipitation, and the positive relationship between tree ring width and March and April mean annual temperature. The different climatic sensitivities of the two investigated species suggest that physiological properties, i.e. root system and stomatal conductance responses, or a variation in the proportions of wood constituents play an important role for the oxygen isotopic signal.
We conclude that, even when partially obscured by site parameters (soil depth and structure, nutrient availability, etc.), δ¹⁸O in the cellulose compartment strongly reflects climate information. Our study suggests that cellulose extraction is a necessary step in studies seeking to investigate the climatic signatures in the δ¹⁸O of broad-leaved species.