STABLE ISOTOPIC STUDY OF SOIL ORGANIC MATTER: UNDERSTANDING DIFFERENCES IN VEGETATION STRUCTURE AND CANOPY COVER IN THE NECH SAR NATIONAL PARK, SOUTHWESTERN ETHIOPIA

Understanding parameters of vegetation structure, such as distribution of woody cover and canopy height is crucial for reconstructing past ecosystems of hominins and other vertebrates in the Neogene of Africa. Although global and regional scale variation in woody cover have been addressed previously, there is a need for high spatial resolution studies that look at habitat-scale variation. As paleosols are well-preserved in the fossil record and are commonly used in reconstructing past ecosystems, a detailed study and a more comprehensive understanding of modern soils at high spatial resolution is important in establishing a baseline for interpreting paleosols and their associated vegetation structure. Thus, here we report on variation in stable carbon isotopic composition (δ¹³C and δ¹⁵N values) of soil organic matter recovered from different habitat types with varying canopy cover and height from the Nech Sar National Park, Southwestern Ethiopia. Surface soil samples were collected from four different habitats (riverine forest, woodland, wooded grassland and grasslands) and hemispherical photographs were taken at each sampling location. Soil samples were analyzed using an EA-IRMS and woody cover was calculated from the hemispherical photographs using GAP light analyzer imaging software. The δ¹³C values show significant differences of the soils collected from the four different habitat types while woody cover calculated from the hemispherical photographs also shows significant differences. δ¹⁵N values also appear to indicate differences among similar habitats with similar woody cover, reflecting habitat-scale processes of nitrogen cycling. In addition to the spatial variations in vegetation cover, differences in the vegetation height between the woodlands and riverine forests was observed and need to be further explored for its potential implications for the reconstruction of factors such as primary productivity. More detailed study of differences in vegetation structure and height across undisturbed woody ecosystems will allow as to reach more refined and reliable proxies of past ecosystems.