Blue rings are not light rings

Misperceptions persist between blue rings (BRs) and light rings (LRs) in tree-ring and wood-anatomical studies, despite their fundamentally different origins and implications. Both phenomena are associated with abiotic and/or biotic stress and consequent incomplete secondary cell-wall development. Yet, they are detectable at distinct resolution scales and carry different interpretive meanings. This communication study recognises BRs and LRs as complementary but distinct proxies and clarifies their diagnostic meaning. BRs are a wood anatomical feature visible on wood anatomical thin sections revealed by Safranin and Astra Blue staining that mark, in blue, a portion of a tree-ring with incompletely lignified cell walls that are not necessarily thinner. LRs, in contrast, are macroscopically visible as lighter latewood colour on polished or cut cross sections, reflecting low apparent latewood density that may result from reduced wall thickening, lack of lignification, or both. While BRs may contribute to the formation of LRs, not all BRs are macroscopically visible as LRs, and not all LRs involve unlignified tissues. Misinterpreting the two can lead to inconsistent identification of growth anomalies and introduce bias across paleoclimate reconstructions and other dendrochronological or ecological contexts. Future studies across climatic gradients and species, should strengthen our mechanistic understanding of BRs and LRs formation and their physiological underpinning to provide their robust interpretation in paleoclimate studies and beyond.