Sample preparation of ring-less tropical trees for δ¹⁸O measurement in isotope dendrochronology

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Radial variation of δ¹⁸O before and after cellulose extraction was assessed in Acacia auriculiformis, Eucalyptus camaldulensis, and Celtis timorensis growing in northeastern Thailand to examine the necessity of extracting α-cellulose to detect annual rings from trees without visible rings. Optimum sampling resolution to detect peaks in the radial variation of δ¹⁸O values was also examined. Cored samples were sectioned into 0.2mm thickness in the radial direction. Each circular section sliced from a wood core sample were divided along wood grain into two semicircular sections, both of which were located at the same radial and longitudinal positions, and were side-by-side tangentially. One half was used for bulk analysis and the other for extraction. Peak positions were assigned from the seasonal variation of δ¹⁸O. The δ¹⁸O values cyclically changed in both bulk wood and α-cellulose. The correlation coefficient between bulk wood and α-cellulose δ¹⁸O was high in every species, and the offset was almost constant across the radial position. The mean cycle length of one sample was longer than those of the other two samples, although annual increment based on dendrometer monitoring was smaller than those of the other two samples. That is, the seasonal variation in δ¹⁸O values recorded in the xylem was not completely detected because of low amplitude or insufficient radial resolution. Therefore, we concluded that α-cellulose extraction is unnecessary for annual ring detection. It is necessary to determine an appropriate sampling resolution based on growth rate for effective peak detection.