The “blue ring”: anatomy and formation hypothesis of a new tree-ring anomaly in conifers

Key message: Double-stained microsections from highaltitudePinus nigrawood cores highlighted unlignified latewood cells possibly linked to abrupt temperature reduction at the end of the growing season. More consolidated detection could increase their role in dendroecology. Abstract: Cambial activity and wood formation are influenced by environmental factors, primarily climate. During cell wall formation the lignification is mainly controlled by temperature. By staining with safranin and astra blue it is possible to distinguish cell walls richer in lignin (stained in red) from those richer in cellulose (stained in blue). Here we show an uncommon phenomenon observed in 41 transverse sections prepared for anatomical studies of young European black pine (Pinus nigra Arnold) individuals. We detected some layers of incompletely lignified cells that appear blue in safranin–astra blue-stained sections. Growth rings showing this anatomical feature were named “blue rings”. The aims of this preliminary study are: (i) to describe the features of this peculiar anatomical trait, (ii) to enhance its visualization, and (iii) to suggest possible drivers of its formation. First results indicate the influence of low air temperature causing a lack of lignification in latewood cells. The added values provided by the identification of “blue rings” within tree-ring series could be (i) their possible use as pointer year, (ii) cross dating improvement, and (iii) finer assessment of tree sensitivity to environmental and climatic factors.