High-Resolution Stable Oxygen Isotope Profiles of a Colophospermum mopane, from Northern Botswana

Tropical trees often lack distinct growth rings, excluding them from dendrochronological studies. However, over the past decade, several studies have suggested that the presence of distinct growth ring boundaries should not exclude topical trees from dendrochronological studies. In the absence of growth ring boundaries, a continuous high-resolution (intra annual) sampling combined with stable isotope analysis can be used to identify chemical rings as opposed to anatomical rings. The goal of this study is to determine the stable oxygen isotope profile of Colophospermum mopane, a tropical tree from Botswana and to investigate its potential for climate reconstruction. A polished mopane wood disc revealed dark and light bands, which under the microscope appear to be caused by changing vessel density. This anatomical feature of dark and light colored bands caused by changing vessel density is similar to the one observed in an unrelated species of mangrove wood (Verheyden et al., 2004, Annals of Botany, 94), in which the bands were proven to be annual. If we assume the bands in mopane are annual (one dark and one light band = 1 year), the ten-cm diameter sample would be between 15 and 20 years old. High-resolution samples for isotope analysis (15 um thickness) were obtained and due to such high resolution, cellulose was extracted from every fourth sample using a method modified from Anchukaitis (unpublished). Individual samples were then pyrolyzed in a High Temperature Conversion Elemental Analyzer (TCEA) coupled to an isotope ratio mass spectrometer (IRMS). The data are being investigated for annual cyclicity, as well as for their correlation with rainfall. Preliminary data show that the oxygen isotope ratios are high in June when the mopane was sampled. This corresponds to the cool, dry season of winter. The wood during time of sampling shows a higher vessel density, as well as a high stable oxygen isotope ratio. This pattern will be further investigated when additional data are available.