Blue rings in Bristlecone pine as a high resolution indicator of past cooling events

This study develops the use of ‘blue rings’ (BR), reflecting incomplete cell wall lignification, as a sensitive thermal indicator in bristlecone pine (Pinus longaeva D.K. Bailey). Using double-stained anatomical thin-sections, we explore the climatic and topographical constraints governing BR formation by developing a time-series from 83 cores and comparing BR occurrence with the full temporal span of available climatic data (1895–2008 CE). Lignification is temperature-dependent and continues at a cellular level post-radial growth completion. As BRs reflect incomplete lignification, they can serve as a higher resolution and more sensitive proxy for past cooling than previously established tree-growth indicators. Results indicate that blue ring formation is primarily induced by low September temperatures and responds more sensitively to cooling than the well-established frost-ring record. Additionally, the occurrence and intensity of blue rings decreases gradually below the upper tree line. Bristlecone pine BRs are demonstrated to have significant capacity to enhance the reconstruction of past cooling events in North America connected with both localized and hemispheric scale forcing over multi-millennial timescales. Given its unmatched longevity, the species offers an unparalleled potential for Holocene length climate reconstruction. Findings also highlight the potential for blue rings to provide a more nuanced understanding of past temperature fluctuations across multi-millennial timescales.