Abstract. 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 14C measurements confirmed the annual character
of the baobab's growth rings. F14C 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 δ13C series points to the incorporation
of previous year's carbon contributing to the average age of intra-ring wood
samples. F14C 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 δ13C time series, no
significant autocorrelation was found in the δ18O 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, δ18O was found to be a good climate proxy, followed
by tree-ring width and δ13C. Tree-ring width and intra-ring
δ18Omin correlated well with each other and with
precipitation amount for the period from pre-monsoon May to the end of the
monsoon season in September/October. Intra-annual stable isotope courses
were found to be rather similar for both δ13C and δ18O.
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 pre-monsoonal cyclones. Rain events from post-monsoonal
cyclones may also be recorded; however, only 2 years of observation
prevented a more conclusive evaluation.
High-resolution <sup>14</sup>C bomb peak dating and climate response analyses of subseasonal stable isotope signals in wood of the African baobab – a case study from Oman
