Sunday, June 28, 2020

A Tale of Three Salt Marsh Grasses

The salt marsh environment is interesting because it is composed of several layered environments, each of which have their corresponding population of species. The low marsh is closer to the sea, and is inundated daily by the rising tide, whereas the high marsh is only flooded during times of exceptionally high tides.

I have noted the ubiquity of Phragmites australis in Cheesequake State Park in earlier posts, but two other grass species are present in force, especially around the crabbing bridge that is one of the main attractions of the park.

S. patens in front of P. australis. Individuals of the latter shoot up in the midst of S. patens.
If one stands on the crabbing bridge in early Summer and looks toward the south side, one can see the rather beautiful mounds of  Spartina patens (Sporobolus pumilus), while behind them loom the much taller forms of P. australis. S. patens exists in the high marsh areas, and cannot tolerate the higher salt concentrations and daily invasion of the tides.

S. alterniflora along the water edges
Looking to the North, which leads towards the sea, one can see the taller forms of Spartina alterniflora (Sporobolus alterniflorus) hugging the shoreline, with S. patens and P. australis behind them. This species is a low marsh grass, and can handle higher salt concentrations and flooding as the tides come in daily. Interestingly enough, the species comes in two alternate forms: a taller form that takes root along the marsh edges, and a shorter form which sit on slightly higher ground.

P. australis lines the edges of lakes and ponds in the park
Looming above both are the distinctive stands of P. australis, which also dominates the edges of the nearby Hooks Creek Lake and Perrine Pond.

P. australis of course has been deemed the villain in this tale. It is the invasive, and it supplants the native salt marsh plants in its relentless march towards dominance, forming extensive monocultures of 4 meter tall grasses. Its dominance then causes changes in both the biotic and abiotic environment around it.

For example, because of its faster growth rate and investment in below ground structures, the dense mats of decaying organic matter that accumulate in tidal marches dominated by P. australis actually elevate the wetland surface. The very dense stands of this species also slow the movement of tidal water, such that flooding is less deep and less prolonged.

P. australis blocks view of a pond
Interestingly though, there have been some studies that show there are some positives to the spread of P. australis in these environments, and that its negative effects may not be as disastrous as first feared. For example, various studies have shown that animal diversity in the P. australis marshes was just as high as in marshes dominated by the Spartina spp.

P. australis looms behind a wading egret
In addition, P. australis provides some benefits that the Spartina spp. do not. Its ability to sequester carbon is significantly greater than the other grasses, a big plus in a world undergoing climate change. P. australis is also better in sequestering metals and other pollutants from the environment, including nitrogen, thus helping to prevent algal blooms. Finally, P. australis builds and stabilizes tidal marsh soils better than Spartina spp, another big positive in a world undergoing sea level rise.

So the next time you see dense stands of P. australis hogging a shoreline, don't immediately despair over the fate of that environment. Things may be more complicated than a simple black and white perspective on such invasives.

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