Reconstructing savanna tree cover from pollen, phytoliths and stable carbon isotopes

Abstract

Abstract Aim To calibrate a model of the relationship between bio‐proxies (pollen, phytoliths and δ 13 C of soil organic matter) and woody cover, measured as the leaf area index ( LAI ). This relationship, applied in palaeosequences, enables reconstruction of past savanna tree cover. Location The samples are from tropical A frica. Modern soil samples are from the Central African Republic and past samples are from sediments of lakes in S enegal and C ongo. Methods We analysed the pollen and phytolith content and stable carbon isotope values of 17 soil samples taken from three short transects in the Central African Republic; LAI was measured on the same transects. The indices used were the AP / NAP ratio of arboreal ( AP ) to non‐arboreal ( NAP ) pollen, the D/P ratio of ligneous dicotyledons ( D ) to Poaceae ( P ) phytoliths, and the δ 13 C of soil organic matter, i.e. the 13 C/ 12 C ratio. Results A multi‐proxy model was calibrated. The best model included only a combination of pollen and phytolith as proxies, excluding organic matter δ 13 C because of its long mean residence time in the soil. The model was then applied to two palaeosequences in Africa, and a time series of relative LAI changes was obtained, providing new information about vegetation changes. Conclusion This model can be applied in palaeosequences to reconstruct relative time series of LAI in African savannas and can help interpret vegetation changes quantitatively. This approach is complementary to the description of pollen and phytolith assemblages.