Climatic context of blue ring formation in high elevation bristlecone pine (Pinus longaeva D.K. Bailey).

'Blue Rings' (BRs) are distinct wood anatomical anomalies recently discovered in several tree species. Studies connect their occurrence to lower than normal temperatures during the cell wall lignification phase of xylogenesis which usually continue after radial growth is completed, following the growth season. BRs are also potentially more sensitive to temperature than frost rings which require freezing temperatures (linked with the forcing of volcanic eruptions) to form. Therefore, systematic analysis of blue rings can add another level of time resolution and/or sensitivity to dendroclimatic studies.  North American bristlecone pine is an invaluable resource for paleoclimatological reconstruction due to its extreme longevity (its specimens are reported to commonly exceed the age of 4000 years), high durability and favourable environmental conditions which hamper decay, enabling the construction of chronologies spanning more than 8000 years. Preliminary results for the last 1000 years reveal that BRs in bristlecone pine coincide significantly with major volcanic eruptions. Detailed analysis of recorded temperature conditions in the years of BRs formation can therefore provide additional information on the impact of volcanic eruptions on climate in periods where meteorological observations are unavailable. To establish baselines for this, we present the climatic context of BR occurrence in bristlecone pine for the period since 1895 where modelled surface mean monthly temperatures are available for the grid cell pertaining to the study area location (4km spatial resolution, from PRISM Climate Group, Oregon State University). A group of 83 cores of bristlecone pine (originating from the vicinity of the Sheep Mountain/Patriarch Grove area of the Ancient Bristlecone Pine Forest in the White Mountains of California, 37.5 W 118.2 N) was thin sectioned on a GSL 1 microtome and further prepared following Gärtner & Schweingruber (2013) safranine and astrablue staining procedures to reveal lignified cell walls in red and underlignified cell walls in blue. Scanned and digitized thin-sections were measured and cross-dated noting years of BR occurrence.  A generalized linear mixed-effects model (GLMM) was fit with mean monthly temperatures and distance from treeline (DTL) as independent variables, and a binary response variable representing the absence (0) or presence (1) of a BR in a particular year, in a particular sample. Results indicate that BRs positively correlate with mean monthly temperatures of February and October and negatively with April, June, August, September and DTL. BR formation most strongly correlates with September temperatures, and interestingly, also lacks correlation with July temperatures.