Assessment of stable isotope time series from Baobab trees (Adansonia digitata) as a high-resolution climate archive for (semi-)arid Africa

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According to the IPCC all of Africa is very likely to warm more than global average during this century. Especially (semi-)arid regions are endangered to experience particular high warming and possibly catastrophic droughts. The prediction of the climate change impact on these regions requires information about the past climate conditions in high temporal and spatial resolution. Large networks of multi-proxy data exist for Eurasia and North America, but there is a massive lack of transregional highly time resolved proxy data for the African continent. The aim of this study is to evaluate and establish the African baobab, Adansonia digitata L., as a new climate archive for (semi-)arid Africa. Baobab trees are widely distributed throughout (semi)-arid Africa and 14C-dating has revealed individuals reaching ages of up to 2000 years. We intent to core-sample numerous baobabs along two transects over southern Africa with sampling locations in Namibia, Botswana, Zimbabwe, Mozambique, and northern South Africa. The objective is to provide well-replicated tree-ring width and stable isotope (d13C, d18O) chronologies for each sampling location, to calibrate and verify them by monitoring- and climate-data and to reconstruct changes in precipitation regimes governing frequency and severity of past drought periods. In addition to this, we expect to obtain additional information about the climate response of baobabs and their adaptability to changes in water availability and atmospheric CO2 concentration (intrinsic water use efficiency, iWUE) during the Anthropocene from stable isotope analyses (d13C, d18O). As it is fundamental for dendroclimatology to better understand seasonal changes in tree growth, climate signal and stable isotope transfer from atmosphere, soil and leaves into the wood of tree-rings, comprehensive dendroecological monitoring will be undertaken. A monitoring site has already been setup in October 2012 so that observations for three vegetation periods will already be available for the project. In cooperation with South African partners it is planned to extend the monitoring to additional trees. By long-term investigations we are targeting a better understanding of the physiology that underlies varying baobab wood formation and stable isotope fractionation due to environmental impacts. In this project we shall: (task-1) study climate signal and stable isotope transfer in the baobab arboreal system from atmosphere through soil and leaves into the wood of tree rings; (task-2) assess the climate response of baobabs in terms of growth, intrinsic water-use efficiency (d13C) and tree-ring stable isotope ratios of oxygen; (task-3) establish well replicated proxy data series (for 200 years) of tree growth, d13C and d18O at 6 sites along two transects across southern Africa; (task-4) reconstruct precipitation regimes and past mega-droughts for the last Millennium for at least one site in sub-decadal time resolution.