Abstract. Recent developments of on-line methods have provided another boost to the determination of stable isotope ratios in organic material. Along with a significant increase in sample throughput, the sample sizes decrease, both of which are necessary conditions to acquire long time series from limited wood amounts. In view of this new technique we reconsidered the most important factors influencing the measured isotopic signature which are (1) pooling, (2) homogeneity, and (3) cellulose extraction. In most cases, pooling (i.e., mixing wood of the same year from different trees) can be made in a simple way by mixing the whole wood available because mass-weighted and unweighted isotope measurements were the same within the error. More attention must be paid in homogenizing the sample. Theoretical considerations underpinned by experimental results suggest a fineness of 0.15 mm (115 mesh) if cellulose is extracted and 0.1 mm (165 mesh) for direct wood analysis. Many of previous studies did not achieve this fineness. We find that wood is as good a climate proxy as cellulose. This is shown by comparing correlations of wood and corresponding cellulose isotope values with meteorological data, which are identical within the uncertainty.
The measurement of δ18O in cellulose of tree rings is a potential means to reconstruct δ18O variations of precipitation and climate. We present δ18O data from cellulose of 3 beech trees (Fagus silvatica) growing on a relatively dry site in Switzerland where the roots do not have access to ground water, as well as data of 2 other groups of 4 beech trees each, one from a dry site and the other from a semi‐dry site, respectively. The measurements cover the time period from 1934 to 1987 in 3‐year‐groups for the 1st site and 1965 to 1992 with a 1‐year resolution for the other 2 sites. We find a high degree of common variance (61%) between the δ18O variations of the 3 trees from the 1st location suggesting a common external cause. The comparison with climate data indicates that spring temperature (April/May/June) is the main influence for the long‐term isotope variations, with a temperature coefficient of 0.33‰ per °C, whereas the short‐term variations are mainly influenced by the relative humidity, with a coefficient of – 0.13‰ per %. This latter value is about ⅓ of the expected model value which points to leaf water pools with different δ18O values influencing the isotopic composition of synthesised carbohydrates or to oxygen exchange with stem water. Tree ring δ18O is correlated with δ18O in precipitation, and the slope of the linear regression for different months most probably yield information about the growth rate function of the particular tree group. Our results confirm the potential of δ18O measurements in tree ring cellulose for climate reconstruction, in particular information about δ18O in precipitation can be gained.
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