Holocene palaeovegetation of the Tigray Plateau in northern Ethiopia from charcoal and stable organic carbon isotopic analyses of gully sediments

A long history of supporting sophisticated but unsustainable kingdoms makes the Tigray Plateau of the northern Ethiopian highlands a promising region for the study of relationships between palaeoenvironmental change and the trajectories of human civilizations. The natural vegetation above 2200 m elevation is thought to be forests dominated by Juniperus procera. Nonetheless, this hypothesis is not supported in the vegetation cover now and is scarcely studied in the palaeorecord. To examine changes in vegetation, climate, and land use, we identified buried charcoalized wood and estimated the percentage of organic carbon from C4 plants (% C4 carbon) from δ13C values of bulk organic matter in the soils of gully walls in the boundaries of the ancient Aksumite Empire. Charcoal ranged in age from ca. 13,700 to 110 cal yr BP. Juniperus procera occurred in even the youngest samples, although at lower percentages of the total charcoal than in older samples. Nevertheless, rapidly regenerating angiosperms usually dominated or co-dominated charcoal even in some of the oldest strata. A shift towards higher % C4 carbon and % total organic carbon (%TOC) in soils younger than 3300 cal yr BP began during a period when agricultural uses of land may have increased in order to support the needs of growing societies. A shift towards higher % C4 carbon but lower %TOC began at ca. 6000 cal yr BP, however, during a period when no charcoal was found and no changes in human societal complexity are known. These results indicate that juniper forest types have long been present at > 2200 m in the Tigray Plateau but that they have rarely been the dominant natural vegetation. Furthermore, lack of repeatable correspondence between factors suggests that the causes of similar shifts in vegetation composition were not always the same.