Peat Accumulation and the Success of Marsh Plants

Accumulated belowground plant debris in salt marsh habitats forms a compact peat substratum that generally increases in density from the littoral edge of marshes landward. To examine the influence of peat accumulation on the success of marsh plants, unvegetated substratum varying in peat density was transplanted over a tidal gradient in a New England salt marsh, and colonization of the plots was followed for 3 yr. Manipulated plots were established in the tall— and short—form Spartina alterniflora zones and near the littoral border of the Spartina patens zone. Substratum type markedly influenced plant colonization of plots in each zone. In the tall— and short—form S. alterniflora zones, the vigor of S. alterniflora invading plots decreased which increasing peat density. In both these zones, S. alterniflora production in experimental plots lacking peat exceeded even that in unmanipulated control plots, suggesting that natural peat accumulation in these habitats reduces plant production. Experimental plots in the S. patens zone also were colonized as a function of peat density. All plots in the S. patens zone were heavily colonized by S. alterniflora, suggesting that S. alterniflora is normally excluded from this habitat by the presence of high marsh vegetation. S. alterniflora invasion of the S. patens zone plots, however, precluded assessing the influence of substratum type on the success of the grasses normally found in this habitat. A physical parameter inventory of the experimental plots indicated that plant growth in the plots was not correlated with many edaphic conditions believed to favor marsh plant growth. For example, plots lacking peat had lower redox and total nitrogen, higher salinities, and lower permeability to water than peat substratum, yet supported more vigorous plant growth than plots containing peat. The basis of peat inhibition of marsh plant growth, therefore, is unclear and needs further work. These data suggest that peat accumulation in marsh habitats may inhibit plant growth, lead to autogenic changes in plant performance, and contribute to within— and between—marsh variation in plant production.