Holocene hydroclimate variability impacts on vegetation and carbon dynamics in the Great Dismal Swamp, southeastern Virginia, 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 18th century. Drainage and logging altered wetland hydrology, native vegetation, fire regimes, and resulted in peat compaction and decomposition. The Great Dismal Swamp National Wildlife Refuge was created in 1974 to protect and preserve a portion of GDS. We present new records of pollen, plant macrofossil, macroscopic charcoal, and carbon accumulation rates from five sites in GDS to examine the impacts of changing hydroclimate, fires, and ditch installation on vegetation and carbon accumulation rates. We selected sites in the four primary forest types: Taxodium-Nyssa swamps in the wetter western sections of the Refuge, Acer-Liquidambar in driest areas, pine pocosin forests, and Atlantic White Cedar stands. Preliminary results show that while wetland sediments began accumulating as early as 14,000 years ago, peat began accumulating ~10,000 years ago in locations closest to drainages and spread across the landscape in the mid-Holocene. In all cores analyzed, marsh vegetation comprised the basal peats and persisted until ~4,000-3,000 years ago, when the marsh was replaced by forested swamp peat. Rates of carbon accumulation were higher during the marsh phase, despite a higher frequency of fires than at any other period of the record. Evidence for diminishing fires at the southernmost sites ~4,000 years ago coincides with a transition of marsh-dominance to forested swamp, likely driven by a moister climate and a stabilization of the water table position. Carbon accumulation rates increased at the transition, from 4,000-3,000 years ago, before abruptly declining after 2,700 years ago, and remained low until present day. Questions remain about whether decades of artificially lowering the water table resulted in aerobic peat decomposition that caused the drastic decline in carbon accumulation rates of older peats, or if late Holocene climate can explain the decrease in carbon accumulation rate. Modern large fires in the forested swamps of GDS are consistent with drying from anthropogenic drainage and drought and are anomalous compared to the previous 3,000 years of the forested swamp's existence. The modern post-fire reversion to marsh suggests early to mid-Holocene marshes were maintained in part because of frequent fires.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.B22F..06J
- Keywords:
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- 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCES;
- 0428 Carbon cycling;
- BIOGEOSCIENCES;
- 0475 Permafrost;
- cryosphere;
- and high-latitude processes;
- BIOGEOSCIENCES;
- 4950 Paleoecology;
- PALEOCEANOGRAPHY