Ecohydrology and causes of peat degradation at the Vasi peatland, South Africa

The Vasi peatland complex is situated in the northeast of South Africa. It is an unprotected area surrounded by pine (Pinus sp.) and blue gum (Eucalyptus sp.) plantations. Little is known about the conditioning factors for the area's development and causes of degradation. In order to understand the ecohydrological system of this complex, water tables and ionic composition of surface water and groundwater, as well as natural isotopes of oxygen, hydrogen and carbon, were measured. Macrofossils and radiocarbon dating of the peat layer were used to describe the historical environmental conditions. We found that peat accumulation started during the Early Holocene and was initiated by terrestrialisation of inter-dune lakes. Our results also show that the Vasi peatland complex is primarily dependent on a perched water table due to the presence of iron-rich deposits close to the surface (i.e. KwaMbonambi Formation). The Vasi peatland basins appear to be hydrologically connected and some basins show indications of cascading through-flow systems, which means that the water flows from a higher basin to a lower one through the permeable parts of the sand dunes and peat layers. The stratigraphy indicates continuous peat accumulation, and thus that the water table was perennially high for thousands of years, despite evidence of the occasional natural occurrence of fires. We conclude that the most likely cause of the observed signs of degradation is land use change, i.e. the recent establishment of plantations, which affect the groundwater system.