Beach Sand Dune Formation by Grasses

Panicum amarum at Sandy Hook Beach, New Jersey

It's a common sight in beaches: sand dunes that rise behind the beach goers like towering castle ramparts. Most people barely pay any notice to these natural formations, but in fact these bulwarks act to protect the existence of the beach itself by preventing beach erosion. They also protect against coastal storms by absorbing the initial impacts of storm surges, thus preventing or delaying inland flooding.

Grasses like Ammophila breviligulata ("beach grass") and Panicum amarum ("bitter panic grass") play major roles in the formation and maintenance of coastal sand dunes. Sand dunes form when strong winds drive sand particles towards an obstruction on the beach, where the particles are stopped and start accumulating over time.

Grasses such as A. breviligulata and P. amarum  act as large obstructions, and sand soon accumulates at their bases (whether gradually or rapidly during storms). In this way, these grasses slowly help in the formation of the huge sand dunes that line our beaches.

Panicum amarum at Sandy Hook Beach, New Jersey

However, the forces that build dunes can also threaten to bury them. Fortunately, both grasses have evolved mechanisms to help them survive such burials. In the case of C3 grasses like A. breviligulata, the plants increase allocation to root biomass. C4 grasses like P. amarum on the other hand allocate more energy towards above ground structures like stems and leaves (Brown and Zinnert, 2018). But both practices result in the maintenance of photosynthetic rates and the continued survival of the plant.

The difference in their response stems from the photosynthetic methods employed by both. Grasses with C4 photosynthesis have higher photosynthetic rates at low CO2 levels, high light, and warm temperatures, which allows greater efficiency in their use of resources such as water and nitogren (that is, C4 grasses can fix more carbon per unit of water and nitrogen). This means that under burial stress, the limiting factor for C4 grasses is not nutrients or water, and they can concentrate instead of producing more above ground biomass to capture light.

Ammophila breviligulata at Sandy Hook Beach, New Jersey

These grasses also help maintain the structure of the dune due to their extensive fibrous root and rhizome system, which hold the sand particles together (Brantley et al, 2014).

I briefly visited Sandy Hook Beach  in New Jersey (Parking Lot D), and saw quite a lot of  flowering P. amarum, with a few flowering A. breviligulata interspersed in the dunes. This was interesting because the latter is normally the dominant dune species in the area, so I would be interested in revisiting the area again in more detail sometime in the near future.

REFERENCES:

Brantley, S.T., S.N. Bissett, D.R. Young, C.W.V. Wolner, and L.J. Moore. 2014. Barrier island morphology and sediment characteristics affect the recovery of dune building grasses following storm-induced overwash. PLoS ONE 9: e104747

Brown, J.K., Zinnert, J.C. 2018. Mechanisms of surviving burial: dune grass interspecific differences drive resource allocation after sand deposition. Ecosphere, 9:e02162.