Potential tree and soil carbon storage in a major historical floodplain forest with disrupted ecological function

Floodplain forests are extremely productive for agriculture and historical floodplain forests have been converted to prime agricultural land throughout the world, resulting in disruption of ecosystem functioning. Given that flooding may increase with climate change and reforestation will increase resiliency to climate change, we tested whether reforested floodplains also have great potential to store carbon and the effects of even modest increases in forested acreage on carbon storage. To calculate potential aboveground biomass in the Lower Mississippi River Alluvial Valley (LMAV) of the United States, we determined current and historical tree biomass used density estimates and diameter distributions from tree surveys and relationships between diameter and biomass from current forests. To calculate potential soil organic carbon if the landscape was forested, we used soil organic matter from soil surveys of the agricultural landscape, and multiplied the carbon by a factors of 1.25, 1.5, and 1.75 based on published reports of soil carbon increases due to afforestation. Our results showed that area-weighted mean biomass density (trees ≥12.7 cm in diameter) for historical forests was 300 Mg/ha, ranging from 228 Mg/ha to 332 Mg/ha by ecological subsection, based on the most conservative diameter distribution. Mean biomass density for current forests was 97 Mg/ha, ranging from 92 Mg/ha to 111 Mg/ha. Mean carbon density for agricultural soils was 96 Mg/ha, whereas combined tree and soil carbon densities varied from 169 Mg/ha to 317 Mg/ha; soil carbon accounted for 0.5–0.7 of total carbon density. Historical forested carbon storage in the Missouri LMAV was about 234 TgC, with the most conservative diameter distribution and assuming 80% forest coverage. Current forested carbon storage in the Missouri LMAV is about 2% of historical storage, at 5 TgC in 30,000 ha of forests, but may reach 23 TgC if forested extent almost triples, with the addition of 50,000 ha of marginal agricultural land, and carbon storage increases in trees and soil. The entire LMAV currently stores 97 TgC in forests and reasonable carbon storage for the entire LMAV may be about 335 TgC, based on increased carbon storage and reforestation of 600,000 ha of marginal agricultural land, which would double the current forested extent. Although 335 TgC storage for the LMAV is only about 1.5 times greater than historical carbon storage of the Missouri LMAV, doubling the forested extent will increase other ecosystem functions, including carbon storage, flood abatement, and reduction of fertilizer pollution in the Gulf of Mexico.