WOODY COVER: A KEY PROXY VARIABLE OF VEGETATION DISTRIBUTION IN THE FOSSIL RECORD OF HOMININ EVOLUTION (Invited Presentation)

Paleoanthropologists working on the record of human evolution in the Neogene of Africa require information about the ecology of early hominins, including the distribution of woody vegetation in the ancient ecosystems in which hominins lived. In tropical savanna ecosystems, defined by mosaics of grasslands with trees in varying densities, the mixing between C4 (grasses) and C3 (woody) vegetation provides an ideal isotopic proxy for the reconstruct the 2-dimensional, or lateral variation of woody cover preserved in sedimentary records of paleosols. The “paleo-shade” proxy provides a non-linear relationship of woody cover to the carbon isotopic composition of soil organic matter that accumulates from local biomass and leaves traces of its isotopic composition in fossil soils. This proxy is an improvement on earlier use of simple two-point mixing lines between endmembers of C3- and C4-biomass because (1) it is validated with robust data collected across a wide range of environments and soil types, (2) it empirically accounts for structural factors that affect the lateral variation in the accumulation of biomass in non-linear ways, and (3) provides a measure of a commonly used ecological parameter (woody cover) that can be related to other ecological variables. While this proxy has served well to distinguish major physiognomic habitats defined by woody cover (such as grasslands from wooded grasslands from woodlands), we discuss additional information on the vertical distribution of woody biomass at the forest-woodland transition and within woodlands, shrublands, and bushlands. Although additional information on vegetation structure is available in modern proxy calibration data sets, vertical structure is otherwise indistinguishable by the isotopic proxy of woody cover.