Confocal fluorescence microscopy reveals subtle lignification variations in a Scots pine blue ring

Blue rings (BRs) are anomalies in tree rings revealed by a double staining technique. They have been thought to reflect reduced cell wall lignification during ephemeral end-of-growing-season cold periods, often following significant volcanic eruptions. BRs have been distinguished visually according to qualitative descriptions and are considered a promising proxy for cold extremes at the intra-seasonal scale. However, reduced cell wall lignin content has never been successfully measured in BRs, limiting understanding of their relationship with thermal (or other) conditions. Further, difference in lignin content between BRs and ‘standard’ rings has not been confirmed independently of the double staining method. Here, using confocal microscopy of samples of Scots pine, we show that BRs are indeed associated with marked reduction of lignin in latewood cells. We also identify subtle variations in lignin across rings that are not well resolved by the double staining method. We draw on these findings to propose a new framework to visualise and measure lignin changes in tree rings. Our findings emphasise the potential of BRs as a paleoproxy of intraseasonal climate variability.