Causes of Vegetation Dieback and Subsequent Wetland Loss in Two Louisiana Coastal Marshes.

Abstract

Vegetation dieback is an important component of the wetland loss process in coastal Louisiana. To determine the factors causing vegetation dieback in a Louisiana salt marsh and oligohaline marsh, field monitoring and manipulative field experiments were conducted in both marsh types. In the salt marsh, a three-year field study was conducted in which biotic and edaphic characteristics were measured and the effect of elevation on four salt marsh plant species was tested in a manipulative field experiment. In the oligohaline marsh, a two-year field study was conducted which evaluated the relationship between biotic and edaphic characteristics. Additionally, the effects of increased salinity, increased submergence and their combination on oligohaline marsh vegetation was determined in a manipulative field experiment. Deteriorating salt marshes had more reduced soil conditions and higher sulfide and NH$\sb4$-N concentrations (which likely indicates decreased NH$\sb4$-N uptake by vegetation) than a nearby reference marsh. Soil bulk density and interstitial salinities did not differ between marsh sites. When salt marsh vegetation was elevated 20 cm, plant growth improved and soils became less reduced. Additionally, salt marsh soils elevated 20 cm had lower interstitial sulfide and NH$\sb4$-N concentrations. In an oligohaline marsh, deteriorating sites had higher interstitial salinities, sulfide and NH$\sb4$-N concentrations than the nearby reference marsh. Deteriorating oligohaline marshes had a change in species composition from a Sagittaria lancifolia-dominated community to a brackish marsh community dominated by Spartina patens and Paspalum vaginatum. When the elevation of S. lancifolia-dominated marsh vegetation was decreased 15 cm, soils became more reduced and plant growth decreased. The combination of a decrease in elevation and saltwater intrusion caused the most dramatic decrease in growth of S. lancifolia-dominated vegetation as well as causing a significant increase in interstitial sulfide and NH$\sb4$-N concentrations. An increase in salinities alone had no effect on the health of the S. lancifolia-dominated community. In both the salt marsh and the oligohaline marsh, increases in marsh elevation improved soil conditions and caused increased growth of marsh vegetation. To be successful, restoration activities in coastal Louisiana should focus on increasing marsh elevation, even with the threat of saltwater intrusion.