Remarkably high blue ring occurrence in Estonian Scots pines in 1976 reveals wood anatomical evidence of extreme autumnal cooling

Abstract Key message An exceptionally high number of blue rings were formed within and between Scots pine trees from Estonia in 1976: a year that is well known for its outstanding summer heatwave over Western Europe, but extreme autumnal cooling Eastern Europe has so far been neglected scientific literature. ‘Blue rings’ (BRs) are visual indicators less lignified cell walls typically towards the end tree’s growing season. Though BRs have associated with ephemeral surface cooling, often following large volcanic eruptions, intensity cold spells necessary to produce BRs, as consistency their formation still remains uncertain. Here, we report an BR occurrence ( Pinus sylvestris L.) at two sites Estonia, including first published whole-stem analysis BRs. Daily meteorological measurements nearby station allowed us investigate role temperature played since beginning twentieth century. The single which consistently amongst most was 1976. While 1976 exceptional Northwest mean September October temperatures remarkably low 3.8 °C below 1961–1990 our sites. Our findings contribute better eco-physiological interpretation further demonstrate ability reveal not captured by dendrochronological ring width latewood density measurements.