Blue intensity of Swiss stone pine as a high-frequency temperature proxy in the Alps

Tree rings are widely used for climatic reconstructions and for improving our understanding of ongoing climate change in high-altitude sensitive areas. X-ray maximum latewood density is a very powerful parameter to reconstruct past climatic variations, especially if compared to tree-ring width, but this method is neither inexpensive nor timesaving. However, blue intensity (BI) has resulted in an excellent maximum wood density surrogate that measures the intensity of reflected light from latewood in the blue spectra. This methodology is still considered a prototype parameter, and more data are needed for validation of the method. We present the first BI values coming from Swiss stone pine (Pinus cembra L.) collected on the southern margin of the Alps. Analyses were performed by testing different solvents and polishing techniques, as well as different CooRecorder pixel percentage settings. The results demonstrate that solvents and software parameters have little influence on the final chronologies. Dendroclimatic analyses demonstrate that Swiss stone pine BI can be a useful tool to extract at least the high-frequency variations in July-August temperatures with a correlation coefficient of up to 0.6 (over the 1800-2017 time period). The immunity of Swiss stone pine to insect defoliator outbreaks further enhances the reliability of the BI values of this species in reconstructing past high-frequency temperature variations in high-altitude sensitive areas.