Quantifying peat carbon loss from centuries of drainage in the Great Dismal Swamp, USA

Abstract:

The Great Dismal Swamp (GDS) in southeastern Virginia and northeastern North Carolina (USA) consists of more than 430 km² of temperate forested peat swamp that has been diminished in size and ecological function by an order of magnitude since intensive efforts to drain and log the area began in the 18^th century. Currently, over 300 km of ditches and ditch roads interrupt the natural flow of water through the system. The combination of drainage and logging over nearly 300 years has altered wetland hydrology, native vegetation, fire regimes, which has resulted in peat compaction and decomposition. The Great Dismal Swamp National Wildlife Refuge was designated in 1974 CE to protect the remaining peat swamp. Modern vegetation communities consist of cypress tupelo, maple-gum, Atlantic white cedar, and pine pocosin. A significant portion of the Swamp burned in two fires (2008, 2011 CE), reverting most of this area to marsh. Here we use a combination of peat physical properties, loss-on-ignition (LOI), X-Ray Fluorescence (XRF), elemental carbon, and radiometric dating from 6 peat cores spanning the five dominant vegetation communities of the Great Dismal Swamp to quantify carbon stocks and losses in the uppermost peats. In these cores, we found the oldest and deepest peats occur along former drainages in the western part of the Refuge and become shallower and younger moving north and east. Using a calculation of ash-mass as a proxy for organic carbon loss, preliminary results suggest that ~2 Tg of carbon have been lost from the Great Dismal Swamp National Wildlife Refuge as a result of drainage. The largest losses were calculated from the maple-gum vegetation community, which occupies drier portions of the swamp and comprises the dominant forest community in the swamp today. These results suggest that carbon losses can be mitigated by increasing and maintaining water levels through active management of water control structures.