Stable carbon isotope ratios in tree rings of co-occurring species from semi-arid tropics in Africa: Patterns and climatic signals

Although several proxies have been proposed to trace the course of environmental and climatological fluctuations, precise paleoclimate records from the tropics, notably from Africa are still sorely lacking today. Stable carbon isotopes (delta C-13) in tree rings are an attractive record of climate. In this study, the patterns and climatic signals of delta C-13 ratios were determined on tree rings of deciduous (Acacia senegal, Acacia tortilis, Acacia seyal) and an evergreen (Balanites aegyptiaca) species, from a semi-arid Acacia Woodland in Ethiopia. delta C-13 inter-annual patterns are synchronous among the co-occurring species. A declining trend with time was observed in delta C-13, notably for B. aegyptiaca, which Could be due to anthropogenic increases in atmospheric CO2 concentration and decrease in atmospheric delta C-13. Tree ring delta C-13 values of all the species revealed significant negative correlation with precipitation amount but not with temperature and relative humidity. The PC series of the deciduous species shows a higher correlation (r = -0.70 to -0.78) with precipitation than the evergreen species (r = -0.55). A master delta C-13 series, composed of the average of the three Acacia species, displayed stronger significant correlation (r = -0.82) than any of the individual species delta C-13 series. The weak relationship between temperature and delta C-13 in this study indicates that photosynthetic rate is not a significant factor. Moisture stress, however, may have a direct impact on the stomatal conductance and explain the strong negative relationship between delta C-13 and precipitation. The results demonstrate the potential of delta C-13 in tree rings to reflect physiological responses to environmental changes as a vehicle for paleoclimatic reconstruction, which is important to understand tree response to past and future climate change. (c) 2009 Elsevier B.V. All rights reserved.