Sea Level-Driven Marsh Migration Results in Rapid Net Loss of Carbon

Abstract

Sea level rise alters coastal carbon cycling by driving the rapid migration of coastal ecosystems, salinization of freshwater systems, and replacement of terrestrial forests with tidal wetlands. Wetland soils accumulate carbon (C) at faster rates than terrestrial soils, implying that sea level rise may lead to enhanced carbon accumulation. Here, we show that carbon stored in tree biomass greatly exceeds carbon stored in adjacent marsh soils so that marsh migration reduces total carbon stocks by ∼50% in less than 100 years. Continued marsh soil carbon accumulation may eventually offset forest carbon loss, but we estimate that the time for replacement is similar to estimates of marsh survival (i.e., centuries), which suggests that forest C may never be replaced. These findings reveal a critical C source not included in coastal C budgets driven by migrating ecosystems and rapidly shifting allocations between carbon stored in soils and biomass.

Plain Language Summary

Forests along the upland edge of salt marshes are being killed by rising sea levels and replaced with salt marsh in a process called marsh migration. Marsh soils, unlike soils in forest, quickly accumulate carbon in their soils. This indicates that marsh migration could possibly increase carbon storage across the landscape. Here, we show the opposite, that sea level rise reduces coastal carbon stocks through the loss of woody aboveground biomass. This loss is partially offset with expanding organic soil in the newly formed marsh, but the amount of carbon lost from forest mortality is far greater than that gained by the growing marsh soils. Continued carbon accumulation in wetland soils may eventually compensate for forest carbon loss, but the time it would take for marshes to replace that lost carbon is at the scale of centuries, which is approximately the same amount of time predicted for marshes to drown from rising sea levels. This suggests that forest carbon may never be replaced and reveals a critical source of carbon under climate driven landscape changes.