The link between δ18O in precipitation and tree-ring cellulose across time, space and species

The oxygen isotope composition (δ¹⁸O) of precipitation is strongly linked to climate and becomes integrated into tree-ring archives. However, this climatic information is only partly preserved in tree rings, as it is modified by hydrological processes prior to root water uptake and by physiological processes before cellulose synthesis. This complicates tree-ring isotope-based climate reconstructions. Nevertheless, direct links between δ¹⁸O in precipitation (δ¹⁸O_P) and tree-ring cellulose (δ¹⁸O_C) have been rarely tested, largely due to the lack of long-term precipitation δ¹⁸O records. Over the past two to three decades, numerous δ¹⁸O_C chronologies have been established, and they can now be combined with δ¹⁸O_P data from AI-supported models with high spatiotemporal resolution. This provides a unique opportunity to systematically evaluate the linkage between δ¹⁸O_P and δ¹⁸O_C.

In this study, we used a network of 45 annually resolved δ¹⁸O_C chronologies across Europe starting in 1950 and compared them with monthly time series of Piso.AI modelled δ¹⁸O_P for the corresponding locations. Our main research questions were: (1) which seasonal δ¹⁸O_P signals are recorded in δ¹⁸O_C? (2) Is the relationship between δ¹⁸O_P and δ¹⁸O_C stable over recent decades? (3) Which factors (species, geography, climate) control the strength of this relationship across the network?

We found that correlations between δ¹⁸O_C and monthly δ¹⁸O_P were strongest for June, July and August of the current year at most sites. Significant correlations were also observed for other months, including months from the previous year in some cases, but without a consistent pattern across the network. To further examine these relationships, we calculated seasonal and annual mean δ¹⁸O_P values. Compared with unweighted δ¹⁸O_P mean values, precipitation-amount weighting reduced correlations with spring, early summer and annual means, thereby narrowing the dominant signal window to the summer period (May–August mean δ¹⁸O_P). We found that the δ¹⁸O_P–δ¹⁸O_C relationship was stable across sites over recent decades, with no systematic change in correlation strength over time. Ongoing analyses use (1) the correlation coefficients (r values) between δ¹⁸O_C and δ¹⁸O_P and (2) the δ¹⁸O offset between cellulose and precipitation, both considering annual and June-July-August δ¹⁸O_P values. These metrics are used to investigate the role of species, geography, climate in controlling the observed δ¹⁸O_P–δ¹⁸O_C linkage. Our findings improve the understanding of site- and species-specific isotope signal transfer from water sources to tree rings and help identify spatial and temporal climate signals reflected in tree-ring δ¹⁸O.