Circumferential and Longitudinal δ<sup>13</sup>C Variability in a <i>Larix decidua</i> Trunk from the Swiss Alps: Fid-Bau Portal

Circumferential and Longitudinal δ¹³C Variability in a Larix decidua Trunk from the Swiss Alps

Jan Esper / Dana F.C. Riechelmann / Steffen Holzkämper

Tree-ring stable isotopes are insightful proxies providing information on pre-instrumental climate fluctuations, yet the variability of these data within a tree trunk has not been fully explored. Here, we analyze longitudinal and circumferential changes in tree-ring δ¹³C values from 1991−2010, considering seven height levels from 1 to 13 m above ground and six sampling directions (radii) separated by 60° around the stem. The disk samples were taken from a 360-year old European larch (Larix decidua Mill.) that grew at 1675 m above sea level in the Simplon Valley, Switzerland. Results show that the circumferential δ¹³C variability, defined as the difference between the minimum and maximum isotope values within a single ring at a certain height, ranges from 0.5 to 2.8‱. These differences appear substantial as they match the range of year-to-year variations retained in long tree-ring δ¹³C time series used for climate reconstruction. The assessment of longitudinal variability demonstrated a systematic change of ~0.1‱ m⁻¹ towards isotopically heavier (less negative) δ¹³C values with increasing tree height, likely reflecting a vertical gradient towards isotopically heavier needle tissue due to changing microclimatic conditions and CO₂ stratification within the canopy. Calibration against regional climate data indicates no substantial signal changes in δ¹³C values within the trunk. We conclude that the longitudinal isotope gradient adds uncertainty to long δ¹³C chronologies derived from subfossil material of unknown (and changing) sampling heights. The large circumferential variability recorded in the sub-alpine larch suggests that more than two cores are needed to analyze absolute δ¹³C values representative for each tree.