Isotope dendroclimatological studies on Juniperus procera from Ethiopia: Towards a reconstruction of Blue Nile baseflow

High-resolution climate reconstructions are fundamental to improve the understanding of the climate system. Tree rings are an excellent high-resolution proxy, but currently the spatial coverage of tree-ring chronologies is mostly limited to temperate regions. Here, it is aimed to extend the tree-ring networks into Northwest Ethiopia, the main source area of the Nile River tributaries Blue Nile and Atbara. Juniperus procera trees were sampled at seven sites in South Wollo and North Gondar. The samples were crossdated and a few sub-samples from the post bomb era were radiocarbon dated. The trees in South Wollo did not form annual growth rings. Though multiple false, missing and indistinct growth rings were present, growth rings formed in the main wet season could be identified in North Gondar. Various growth-ring variables were measured, including ring width, minimum blue intensity, and carbon and oxygen isotope ratios, and examined for environmental and physiological signals. A combined chronology of ring widths and carbon-isotope values was developed over three sites in North Gondar. This chronology was calibrated against instrumental records of river discharge (r=0.75, p<0.01, EPS=0.85). A highly promising, preliminary reconstruction of Blue Nile baseflow was developed. From AD 1836, the reconstruction has low dating (± 1 year) and detrending uncertainties, while being statistically accurate (e.g. SSS mostly above 0.85) and reliable, comparing well to other proxy records from East Africa. The Southern and North Atlantic Oscillations were identified as forcing, or at least associated, factors. The 11-year sunspot cycle was also recognised. Feedback mechanisms involving vegetation cover occurred at several periodicities, whereas the influence of increasing atmospheric CO2 concentration and Indian Ocean SSTs seemed minor. In conclusion, a solid foundation has been laid for dendrochronology in Ethiopia.