(Wullstein and Pratt 1980) and may be a primary pathway by which nitrogen is provided 

 to dune plants. In addition, some plants such as beach pea, Strophostyles helvola , possess 

 nitrogen-fixing nodules which serve the same function as rhizosheaths. 



Soil instability is another problem that dune vegetation must overcome. Plants 

 have a more difficult time establishing themselves in shifting windblown sand than in a 

 stable substrate. In addition, vegetation is often buried by drifting sand. Dune plants 

 have adapted to this environment by having the capacity to grow upward through 

 considerable accumulations of sand. Burial has a stimulatory effect on the growth of 

 dune grasses; too much sand burial can cause plant death, however. The resistance to 

 sand burial varies with species. The grasses are most resistant, while dicotyledonous 

 plants are more susceptable to sand burial. On the outer banks of North Carolina, sand 

 burial was the major factor preventing the establishment of most annual plants on the 

 foredune or any other area of shifting sand (Van der Valk 1974). These plants can survive 

 sand burial of no more than 16 cm. Accumulations of 20 to 30 cm are normal in this 

 foredune. 



In Louisiana, the dominant dune vegetation includes salt meadow hay, Spartina 

 patens , bitter panicum, Panicum amarum , seashore dropseed, Sporobolus virginicus , and 

 beach morning glory, Ipomea stolonifera . Of secondary importance, as indicated by their 

 frequency of occurrence, are beach tea, Croton punctatus , seashore paspalum, Paspalum 

 vaginatum , dune elder, Iva imbricata , seaside goldenrod, Solidago sempervirens , sea oats, 

 Uniola paniculata , and pennywort, Hydrocotyle bonariensis . Figures 2, 3, and 4 

 demonstrate the distribution of these and other species on three Louisiana barrier 

 systems. 



Certain species of dune plants are more efficient dune builders than others. For 

 example, in Louisiana species such as panicum, croton, and sea oats can build dunes from 

 I to 5 m high, while salt meadow hay normally generates dunes of relatively low profile, 

 less than I m. Also, the shape of the dune produced can vary depending upon the 

 vegetation type. For example, beach tea and dune elder produce large hummock dunes, 

 while panicum more frequently generates dune ridges. Even different species of grasses 

 produce different dune forms. For example, in North Carolina, American beachgrass 

 produces a gently sloping dune while sea oats generates a steep dune front; panicum 

 builds a dune intermediate in shape (Woodhouse et al. 1977). 



Although most of Louisiana's dune vegetation is ubiquitous, found on all of 

 Louisiana's barrier islands and beaches, there are two notable exceptions. Sea oats is 

 primarily found on the barrier islands east of the Mississippi River delta, specifically the 

 Northern Chandeleur Islands. Sea oats is almost completely absent west of the delta, 

 except for three small populations on the Camlnada-Moreau coast and a few plants on 

 Grand Isle. On the other hand, panicum is very prevalent on Louisiana's barrier islands 

 west of the Mississippi delta, but almost nonexistent on the Chandeleur islands. The 

 reasons for these disjunctions are unclear. There are two plausible hypotheses for why 

 sea oats is not appreciably found west of the delta. Since the islands west of the delta 

 are of a much lower profile than the northern Chandeleurs, these islands tend to be 

 overwashed more frequently. Sea oats may not be able to recover from the effects of 

 overwash as rapidly as other species and hence has lost its prominence on these low-lying 

 islands. Because sea oats growing on dunes of lower elevation are closer to the 

 watertable, it has been hypothesized that this plant, which is apparently highly adapted 

 to dry beach sands, is stressed by excess soil moisture which reduces its vigor. The 

 reasons for the panicum disjunction is an even greater mystery. Nonetheless, both plants 

 are potentially good dune builders and sand stabilizers. 



193 



