Many people recognize the firey hue glasswort brings to a salt marsh as it turns red in the fall. Even from a distance, those with a discerning eye can pick out the bands of low marsh that fringe the water’s edge. Real connoisseurs of drive-by marsh plant identification can even pick out the dusty grey-green of spike grass (Distichlis spicata) interwoven with the backdrop of salt marsh hay (Spartina patens), but why do these plants grow in the places they do?
An ecotone is a region of transition between two biological communities. In a marsh system these boundaries are associated with local tide heights, often more accurate to that very spot than those based on a close by tide station. Since cordgrass (Spartina alterniflora) is the only species that can tolerate being flooded twice daily, a commonly known ecotone is the boundary between high and low marsh. This interface indicates local Mean High Water (MHW) level. However from the water’s edge to the upland there are several other ecotones that are a vegetative indication of local tide height (see diagram below).
Implicit in a salt marsh, all plants that survive there are halophytic or salt tolerant. Elevation, and consequently exposure to flooding, is the primary driver of ecotone distribution within tidal marsh systems. However, within the marsh ecotones, plant morphology, physiology, and phenology broadly determine species spatial patterns.
Cordgrass is the only plant to survive in the anoxic soil of the low marsh because it contains an internal pathway of air spaces. This means air can flow from the tips of the leaves to the ends of the roots allowing plants to oxygenate the soil. Dense stands of cordgrass do better than sparse areas because the increased oxygen makes the soil more hospitable.
When wrack is deposited on the high marsh, over time, it usually shades out and kills the vegetation underneath. This creates bare spots of mud that are fully exposed to the sun causing extreme evaporation and so hypersaline conditions. The fleshy stems of glassworts (Salicornia spp.) make them the most salt tolerant plants in the high marsh so they are the first to colonize these recently denuded areas.
Next along the successional chain in the high marsh comes spike grass, demonstrating plant morphology can affect competitive ability. This plant species has stems that are separated by long underground runner systems making it a relatively sparsely growing species, but one well adapted to spreading into new areas. New stems can colonize relatively saline sites because they connect via rhizome to receive water from surrounding plants growing in less saline soil. As spike grass begins to shade the soil it reduces evaporation, and consequently salinity, so allowing salt marsh hay and black grass to begin to move in.
Black grass and salt marsh hay have dense turf root systems which take longer to establish, but within two to four years they ultimately outcompete the colonizing species. Black grass can outcompete salt marsh hay closet to the terrestrial border by emerging much earlier in the spring. Looking at plant patterns in the marsh can make you much more than a botanist, but allows an ecological translation to help understand the nuances of local tides and microtopography too.
Rachel Stevens, Stewardship Coordinator & Wildlife Ecologist