HIGH-RESOLUTION STABLE OXYGEN ISOTOPE PROFILES OF A TROPICAL TREE, 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. Oxygen isotopes are often the preferred proxy in the high-resolution approach because they are predominantly influenced by precipitation and relative humidity, with very little effects from tree physiological processes. The goal of this study is to determine the stable oxygen isotope profile of Colophospermum mopane, a tropical tree collected in Botswana and to investigate its potential for climate reconstruction. Mopane is a heavy, termite-resistant species with an average lifespan of 100-200 years. According to the Plant Resources of Tropical Africa database (PROTA), Mopane is widespread in southern Africa and has varied uses, such as construction of buildings and fences, medicine, fertilizer, ropes, food for livestock, and carvings. 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 using a sliding microtome. Cellulose was extracted from the samples using a method modified from Anchukaitis (unpublished). Individual samples were then pyrolyzed in a High Temperature Conversion Element Analyzer (TCEA) coupled to an isotope ratio mass spectrometer (IRMS). The data will be investigated for annual cyclicity in the indistinct growth rings, as well as for their correlation with rainfall.