Major barrier islands along the Louisiana coast

Chandeleur Islands
Grand Isle
Isles Dernieres

Zonation from the gulf side landward:

Open water of the Gulf of Mexico
Beach
Upper Beach
Dunes
Swales (dune slacks):  Areas of lower elevation and more moisture;
                                     Woody vegetation may be present; maritime forests present under certain conditions

Subtidal flats:  Salt marsh vegetation or black mangrove (Avicennia germinans)
 
Salt pans are areas of high salt concentrations (perhaps 3X salt conc. of sea water); usually devoid of vegetation.

Bays:  Submerged Aquatic Vegetation (SAV)
 

Major plants of barrier islands and coastal dunes

Beaches and dunes:

Cakile spp. "sea rocket"; upper beaches and dunes.   Succulent, fleshy herbs; alternate lvs; flowers with 4 white to purple petals;

Panicum amarum, "bitter panicgrass;" typical of the high dunes; most important in dune stabilization; transplants easily from cuttings.

Sporobolus virginicus, "seashore dropseed;" upper beaches

Sesuvium portulcastrum, "sea-purslane;" prostrate succulent herbs; lvs. opp.; flrs with 5 pink, showy petals, many stamens.

Ipomoea imperati (Ipomoea stolonifera), "beach morning glory;" Long, prostrate stems, rooting at the nodes;
                                lvs ovate to oblong and unlobed to 5-7 lobed; flrs. white with yellowish center.

Ipomoea pes-caprae, "goat’s-foot, railroad-vine;" long, prostrate stems, rooting at the nodes;
                                    lvs ovate to reniform, apices emarginate; flrs. lavender.

Croton punctatus, "beach croton;" shrub with greenish gray leaves.

Uniola paniculata, "seaoats;" grass of high dunes with large panicles of large flat spikelets

Iva imbricata ,W, "seacoast marshelder;" only on dunes, rare in Louisiana.

Spartina patens, "saltmeadow cordgrass" grows in various habitats; plants in marsh, dunes, and swales appear to be genetically different.

Iva imbricata Walt. "seacoast marshelder"

Strophostyles helvula (L.) Ell. "beach pea or trailing fuzzybean"
 

Vegetation of swales:

Higher water table may be responsible for more dense vegetation

Herbaceous plants are typical of such areas

In the absence of disturbance from storms, shrubs such as Myrica, Baccharis, and Iva frutescens may dominate.

Biodiversity in herbaceous vegetation

Sabatia camprestris "prairie rose gentian or Texas star"

Sabatia stellaris "saltmarsh rose gentian"

Dichromena spp. "whitetop sedge"

Andropogon spp.

Hydrocotyle bonariensis "seaside pennywort"

Oenothera spp. "evening primrose"

Phyla nodiflora "common frog-fruit"

Fimbristylis castanea "saltmarsh fimbry"

Supports a maritime forest dominated by Quercus virginiana "live oak" and Sabal minor "palmetto." Grand Isle supports a remnant maritime forest. Data from the mid-1800s indicates that the area was mainly forested.

Isles Dernieres supports, "black mangrove," Avicennia germinans on the bay side of the islands back from the beach. Black mangrove is viviparous, i.e., its seeds germinate before being dispersed from the parent plant. This allows the young root (the radicle) to rapidly penetrate the ground preventing the seedling from being swept away by the tides.

Plants found in the sounds behind from the barrier islands, especially, the Chandeleur Islands include seagrasses such as Thalassia. This plant requires salt for growth, its leaves are 3-4 cell layers thick and lack stomates, and pollination takes place underwater. Other seagrasses in the area are listed in the next lecture.
 
 

Environmental factors controlling vegetation on barrier islands and coastal areas:

1. Salt spray effects. Wave action and wind create an aersol of seawater that causes cell death in shoots on the seaward side of plants resulting in less growth than on the leaward side. Plants subjected to such conditions exhibit uneven growth and appear to be growing away from the ocean.

2. Sand burial. Wind and water action such as overwash readily move sand that can bury plants and propagules. Plants adapted to these environments often have shoots that can grow up through the sand. Larger seeds are also an adaptation for growing in habitats subjected to sand burial.
3. Soil moisture. With adequate rainfall and dew formation sand may not be highly deficient in moisture. Long, extensive roots systems allow plants in deep sands to tap the water supply. Succulent, thickened, hardened, and waxy leaves are adaptations seen in plants in arid coastal environments.
4. Herbivory. Animals have dramatic affects on coastal vegetation. Nutria in high densities are responsible for "eat-outs."
5. Nutrient deficiencies. Nitrogen and phosphorus are limited in sandy coastal soils and constrain plant growth.
6. Hurricanes. Hurricanes and other violent storms can readily alter coastal environments. Dunes may be flattened by wind and water action. Breaches in dunes and barrier islands will increase salt concentrations on back dunes and in swales.
7. Humans. Development for housing, industry, and agriculture replace natural habitat. Pipeline installation and other disturbances can be the nucleus for a blow-out where dunes and barrier islands can be severely eroded.