Benchmarking Blue Intensity from drought limited Pinus sylvestris using tree-ring anatomy

Robust, high-resolution proxy information on hydroclimate is critical for understanding the recent European drying in the context of natural variability. This is particularly true for Fennoscandia and the surrounding northern high latitude regions, where such records are scarce, and little is known about the physical drivers of these rare events. Recently, the potential of tree-ring Blue Intensity (BI) of Pinus sylvestris L. from cool drought-prone sites has emerged as a source of high-resolution hydroclimate information. Limitations to these proxies may however be linked to the analytical measurement resolution of the technique as well as potential color-related biases caused by the heartwood-sapwood transition and decay-associated discoloration of the tree-ring samples. These proxy uncertainties need to be addressed before the methodology can be used more systematically in the context of hydroclimate reconstructions. Here, we evaluate the reliability of BI tree-ring parameters, specifically regarding their ability to accurately track both low-frequency variations as well as hydroclimate extremes. This is accomplished by benchmarking these parameters against newly developed quantitative wood anatomical (QWA) data from Pinus sylvestris trees in southeast of Sweden. The ΔBI parameter shows a high similarity to its QWA analogue across scales ranging from interannual to centennial, suggesting that the Δ conversion efficiently overcomes the long-term trend biases seen in the raw BI data. The earlywood and latewood BI parameters record largely the same strong spring-summer precipitation signal, together explaining over 50% of the variance in May-July precipitation, as do their QWA counterparts. However, both BI and QWA show a pronounced asymmetric response to precipitation extremes such that dry extremes are captured better than the wet extremes. This dry-biased extreme value capture needs careful consideration in future reconstruction efforts. We conclude that tree-ring BI holds significant potential for contributing to regional reconstructions, especially given the untapped opportunities to utilize the region’s extensive collection of historical pine material for paleoclimate research.