Assessment of stable isotope time series from Baobab trees (Adansonia digitata) as a high-resolution climate archive for (semi-)arid Africa
Palaeontology
Final Report Abstract
This project has underlined the great potential of baobabs as climate and environmental archive. Nonetheless, the dating uncertainties owing to the very specific wood anatomy remain most challenging for unlimited use of baobab trees as a paleoprecipitation archive. Tree-ring width and stable isotope ratios have revealed significant climate sensitivity. δ18O was found to be a good climate proxy followed by tree-ring width and δ13C. However, tree-ring width cannot be used if they cannot be identified. Multi-centennial reconstructions from stable isotopes can help assessing climate trends. Reconstructions of extreme events currently suffer from the lack of accurate dating, although intra-annual stable isotope courses were found well suitable for detecting not only years with particularly low rainfall, but also for indicating heavy rainfall events, e.g. from pre-monsoonal cyclones. More studies at various sites are advised. The larger sample size will provide better estimates of past spatial and temporal precipitation variability. To gain a better understanding of the signal transfer and to help developing an improved sampling strategy, ecophysiological monitoring and additional intra-annual analysis of stable isotopes may also help to assess the vulnerability of baobabs to future climate and environmental change. This project resulted in significant progress in dendroclimatolgy of the African baobab, a widely distributed and long-lived tree species with a very specific and complicated wood anatomy, that can provide palaeoclimate information for in areas where long tree-ring records from “conventional” trees are lacking. Tree-ring width cannot be used a proxy, because tree ring boundaries cannot be unambiguously identified. Hence, radiocarbon dating is crucial, because conventional dendrochronological dating has failed. Stable isotopes from the baobab archive were found particularly useful for reconstructing trends in rainfall variability. Extreme values point to extreme drought or pluvials, however, high quality reconstructions of extremes seem to impossible because of the lack of accurate dating methods. Besides paleoclimate reconstructions from the baobab archive, carbon and oxygen isotope records can help to better understand the ecophysiology of baobabs. Particularly in terms of isotope-derived estimates of intrinsic water-use efficiency and transpiration trends relative to other tree species. In this respect, a combined approach of monitoring, as established in this project, and ecophysiological modelling of isotope fractionation in the arboreal system is suggested for assessing the resilience of baobabs to ongoing climate change.
Publications
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2017. A guideline for sample preparation in modern tree-ring stable isotope research. Dendrochronologia 44, 133-145
Schollaen, K., Baschek, H., Heinrich, I., Slotta, F., Pauly, M., Helle, G.
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2017. Baobabs on Kubu Island, Botswana – A dendrochronological multi-parameter study using ring width and stable isotopes (δ13C, δ18O). Erdkunde 71, 23-43
Slotta, F., Helle, G., Heußner, K.-U., Shemang, E., Riedel, F.
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2019. Improved tree-ring visualization using autofluorescence. Dendrochronologia 55:33-42
Godoy-Veiga, M., F. Slotta, P. C. Alecio, G. Ceccantini, M. S. Buckeridge, and G. M. Locosselli
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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, Biogeosciences Discuss. 2019
Slotta, F., Wacker, L., Riedel, F., Heußner, K.-U., Hartmann, K., and Helle, G.
