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

Jan Esper,Steffen Holzkämper,Dana F.C Riechelmann

doi:10.3390/f11010117

Jan Esper, Steffen Holzkämper + Show 1 more

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https://doi.org/10.3390/f11010117

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Abstract

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.

Highlights

Tree-ring δ13 C is an important proxy in paleoclimate research that has been used to reconstruct temperature [1,2], precipitation [3,4], drought [5,6], and cloud cover variability [7,8] over pre-instrumental periods
The tree grew in a larch-dominated forest at 1675 m a.s.l. on a 65% steep, NW-facing slope. Larch trees from this sub-alpine belt have historically been used as construction timber [21], providing the source material for millennium-length climate reconstructions based on tree-ring growth and stable isotope parameters [17,19,45,46]
We used box plots to illustrate the range of correlations among the radii and calculated disc mean curves (DMCs) to produce time series

Summary

Introduction

Tree-ring δ13 C is an important proxy in paleoclimate research that has been used to reconstruct temperature [1,2], precipitation [3,4], drought [5,6], and cloud cover variability [7,8] over pre-instrumental periods. Records reaching further back in time, even covering the entire past millennium, are much rarer and often combine data from living and subfossil trees [12,13] The latter can be dead wood on the ground [8], material from logs covered in sediment [14], stems that fell into shallow lakes [7,15], and beams from historical buildings [16,17,18,19,20], such as the old huts and barns made of resistant Larix decidua boles in high elevation environments of the Swiss Alps [21]. Δ13 C variability among different radii of the same level, e.g., in 1 m above ground, can affect a reconstruction if such circumferential variability is large and not mitigated by sampling several radii from different trees for stable isotope measurement (overview in [26])

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