High-resolution 14C bomb peak dating and climate response analyses of subseasonal stable isotope signals in wood of the African baobab - a case study from Oman

The African baobab, Adansonia digitata L., has great paleoclimatological potential because of its wide distributional range and millennial length life span. However, dendroclimatological approaches are hampered by dating uncertainties due to its unique, parenchyma-dominated stem anatomy. Here, securely dated time series of annual wood increment growth and intra-ring stable isotopes of carbon and oxygen of cellulose for a baobab tree from Oman covering 1941 to 2005 were established and tested for relationships to hydroclimate variability. Precise dating with the atomic bomb peak (ABP) using highly resolved C-14 measurements confirmed the annual character of the baobab's growth rings. (FC)-C-14 values of tree-ring cellulose were found up to 8.8 % lower than in the corresponding atmospheric CO2 for the period around the ABP, which in conjunction with a considerable autocorrelation of the delta C-13 series points to the incorporation of previous year's carbon contributing to the average age of intra-ring wood samples. (FC)-C-14 of terminal parenchyma bands, marking the tree-ring boundaries, were found to be considerably younger than their corresponding tree ring, indicating that parenchyma tissue is alive for many years, probably undergoing cell division and structural reorganization and contributing to secondary growth. In contrast to the delta C-13 time series, no significant autocorrelation was found in the delta O-18 series of tree-ring cellulose despite the enormous water storage potential of this stem-succulent tree species. Year-to-year variability in tree-ring width and stable isotope ratios revealed radial stem growth and the geochemistry of wood cellulose are influenced by fluctuations in the hydroclimate. In particular, delta O-18 was found to be a good climate proxy, followed by tree-ring width and delta C-13. Treering width and intra-ring delta O-18 min correlated well with each other and with precipitation amount for the period from premonsoon May to the end of the monsoon season in September/October. Intra-annual stable isotope courses were found to be rather similar for both delta C-13 and delta O-18. Years with particularly low monsoon rain were reflected by increased stable isotope values in the mid-section of intra-annual courses. Distinct patterns with low subseasonal isotope values seem indicative for years with heavy rainfall events from premonsoonal cyclones. Rain events from post-monsoonal cyclones may also be recorded; however, only 2 years of observation prevented a more conclusive evaluation.

Automated summary

The study established securely dated time series of annual wood increment growth and intra-ring stable isotopes for a baobab tree, examining their relationship to hydroclimate variability. It was found that the growth rings of the baobab tree exhibit annual character, with unique C-14 measurements providing precise dating. The study also revealed the influence of hydroclimate fluctuations on radial stem growth and the geochemistry of wood cellulose.