Carbon Storage Increases with Site Age as Created Salt Marshes Transition to Mangrove Forests in Tampa Bay, Florida (USA)

Coastal wetlands can sequester large amounts of organic carbon (OC), providing an additional motivation for the preservation and restoration of these ecosystems. In Tampa Bay (Florida, USA), created coastal wetlands are initially planted with Spartina spp., but nearly all sites naturally transition into mangrove forests. It was hypothesized that carbon storage in the created wetlands would increase with site age due to the accumulation of soil organic carbon and replacement of salt marsh vegetation with mangrove forests. Mature, mangrove-dominated sites had higher total organic carbon stocks (138.7 ± 13.8 Mg C ha−1) than middle-aged transitional sites (85.6 ± 25.5 Mg C ha−1) or young salt marshes (34.5 ± 7.7 Mg C ha−1). Mature sites consisted of tall trees (> 130 cm tall) and scarce salt marsh vegetation. Transitional sites contained mangrove scrubs (30–130 cm tall) and seedlings (< 30 cm tall) while still supporting salt marsh vegetation; younger sites were dominated by salt marsh vegetation and had no trees. Belowground OC constituted the greatest carbon pool (59.4% of the total OC stock), but belowground OC stocks were not significantly different among the site age classes, suggesting that aboveground OC stocks drove the difference in total OC stocks. The total carbon accumulation rate, including both aboveground and belowground OC, was 4.7 Mg C ha−1 year−1 across the 26-year chronosequence. This study has demonstrated that carbon storage in created coastal wetlands is correlated to wetland age, indicating that these ecosystems have the potential to become significant sources of OC storage.