Quantifying Post‐Colonial Peat Carbon Loss From a Drained Forested Peatland, Great Dismal Swamp National Wildlife Refuge, USA

Peatland carbon storage is increasingly threatened by the combination of land‐use change and climate variability, though carbon losses from land‐use changes that span centuries are difficult to quantify, particularly in systems where little undisturbed area remains. Here we use a combination of vegetation change, fire history, and calculations of excess ash mass to quantify carbon loss in the Great Dismal Swamp National Wildlife Refuge (GDS NWR), USA, a highly impacted oligotrophic temperate peat swamp. Our results indicate that ditch construction that began in the Colonial Era in the late 1700s and continued into the mid‐20th century across the swamp resulted in shifts from cypress‐tupelo swamps to a combination of maple‐gum and pine pocosin forests, consistent with drying surface conditions. Two large smoldering fires (2008, 2011) that were exacerbated by surface drainage, shifted vegetation from swamp to marsh, consumed peat over 25 km2, and caused losses of 1.05–1.34 Tg C due to peat burning. Across the Refuge as a whole, up to 48.2 Tg C has been lost to peat oxidation since ditch construction. Both stocks and rates of carbon loss remain higher than post‐disturbance accumulation across most of GDS NWR, suggesting that existing efforts to block drainages to elevate water tables may not be enough to offset carbon losses. Rewetting heavily impacted surface peats may reduce peat oxidation and carbon loss, and shift vegetation toward hydrologic conditions preferred by pre‐disturbance cypress‐tupelo swamps. Plain Language Summary Peatlands sequester a globally significant pool of carbon despite covering only 3% of the terrestrial earth's surface. However, this carbon can be released to the atmosphere as the greenhouse gases carbon dioxide and methane due to changes in climate and land use. In this study we quantify carbon loss from Great Dismal Swamp, a temperate peat swamp with a history of drainage that began with George Washington in the late 18th century. Using ash content as a proxy for peat loss, we found that Great Dismal Swamp has lost a substantial amount of carbon (∼50 million metric tons) since Colonial‐era ditching, with more than 1 million metric tons lost from two recent catastrophic smoldering peat fires. Our calculations suggest that the swamp continues to lose carbon, despite efforts to slow water loss from nearly 240 km of drainage ditches constructed over the last >200 years. More aggressive mitigation methods may be needed to return the G