Factors affecting vegetation dieback of an oligohaline marsh in coastal Louisiana: Field manipulation of salinity and submergence

Abstract

Vegetation dieback is an important component of wetland loss in low salinity marshes of coastal Louisiana. A field experiment was conducted to determine the factors responsible for vegetation dieback within oligohaline marshes of Louisiana. Sections of marsh, dominated by Sagittaria lancifolia L., were transplanted into one of four locations depending on the treatment: (1) increased submergence—sods were lowered 15 cm below the donor marsh surface, (2) increased salinity—sods were transplanted into a higher salinity marsh and adjacent dieback pond, (3) increased salinity and submergence—sods were transplanted into a higher salinity marsh and adjacent dieback pond at 15 cm below the marsh surface, and (4) control—sods were exhumed and replaced at the ambient elevation of the donor marsh. Plant biomass and edaphic characteristics were measured after 5 mo. An increase in submergence caused decreased plant growth of the S. lancifolia‐dominated marsh community. An increase in salinities to 4–5 g/kg were not detrimental to plant growth. Although saltwater intrusion alone did not cause decreased growth of the S. lancifolia‐dominnled plant community, the combination of saltwater intrusion and increased plant submergence caused the greatest decrease in plant growth due to increased toxic sulfides and a likely reduction in the uptake of NH4‐N by the wetland vegetation. This illustrates that the dieback of oligohaline marsh vegetation can be alleviated by decreasing plant submergence even at salinities as high as 4.6 g/kg.

Coping with low nutrient availability and inundation: root growth responses of three halophytic grass species from different elevations along a flooding gradient

“…Secondly, reduced growth of salt marsh vegetation has also been ascribed to the accumulation of sulfide to concentrations above 1 mmol l -1 (Havill et al 1985;Koch and Mendelssohn 1989). However, sulfide concentrations in our pots were much lower (<6 µmol l -1 ) and even less than the average values found in most salt marsh soils exposed to contrasting flooding regimes (Howard and Mendelssohn 1995;Webb and Mendelssohn 1996;Baldwin and Mendelssohn 1998). By elimination of other factors, we speculate that oxygen limitation in root tips is the most likely cause of the observed reduction in growth, as it is known that O 2 transport is often insufficient for entirely aerobic respiration in roots growing in reducing sediments (Mendelssohn et al 1981).…”

A Landscape-Scale Assessment of Above- and Belowground Primary Production in Coastal Wetlands: Implications for Climate Change-Induced Community Shifts

“…Direct negative impacts of elevated salinity and prolonged flooding on production have been demonstrated for the fresh, intermediate and brackish species studied here (McKee and Mendelssohn 1989;Broome et al 1995;Webb and Mendelssohn 1996;Baldwin and Mendelssohn 1998;Gough and Grace 1998;Willis and Hester 2004;Spalding and Hester 2007). Furthermore, salinity and flooding have been shown to limit production even in the most salttolerant species (Naidoo et al 1992).…”

Alternative Stable States Driven by Density-Dependent Toxicity

“…In these vegetations, ammonium toxicity can be prevented because ammonium concentrations in the pore water are actively lowered, not only by uptake, but also by density-dependent oxidation of ammonium to nitrate due to high radial oxygen loss of the roots (Smolders and others 2002). Toxic effects of sulfide in salt-marshes (Webb and others 1995;Webb and Mendelssohn 1996), seagrasses (for example, Goodman and others 1995;Pedersen and others 2004) or sulfate-rich freshwater wetlands (Lamers and others 1998;Armstrong and Armstrong 2001; van der Welle and others 2006) may be prevented in a similar way. In these systems, sulphide can be oxidized to harmless sulfate if oxygen loss by the root system is sufficiently high.…”