LESSONS LEARNED, FROM 30 YEARS OF RESEARCH BY NUMEROUS INVESTIGATORS, REGARDING MARSH VERTICAL ACCRETION

In Louisiana coastal wetlands and elsewhere, spatial patterns of vertical accretion demonstrate that marsh vertical accretion accelerates in response to relative sea-level rise (i.e., the combination of local subsidence and global sea-level rise) up to a limit. In Louisiana coastal wetlands, the elevation of many marshes with slow wetland loss rates is at mean high water. If relative sea-level rise exceeds the maximum marsh vertical accretion rate, then a vertical accretion deficit begins to develop and accumulates over time. Wetlands apparently persist, and vertical accretion deficits accumulate, while the surface gradually loses elevation until the surface is at the elevation of mean low water. At that time and elevation, or sooner, the vegetation dies from flooding stresses and wetland loss rates accelerate.

What limits accretion where it is inadequate to counter relative sea-level rise? Coastal wetlands on the northern Gulf of Mexico and in New England accrete via vegetative growth. Mineral sedimentation in these marshes increases soil bulk density rather than surface elevation. Bulk density is positively related to plant biomass; thus, mineral sedimentation probably is indirectly important to accretion via vegetative growth. Other factors that can limit wetland plant growth, such as salinity stress, flooding stress, nutrient availability, herbivory, and fire, probably also can limit vertical accretion via vegetative growth.

Vertical accretion via vegetative growth also can be affected by soil organic matter decomposition. Even a little soil drainage can reverse decades of vertical accretion. Nutrient addition rates typically observed in Louisiana coastal marshes have little effect on soil organic matter decomposition but nutrient addition rates typically used in agricultural fields doubled soil organic matter decomposition rates in wetland soils.