The sand dune is a relatively inhospitable environment for vegetation 

 establishment. Environmental factors such as salt spray from saline waters of the Gulf 

 of Mexico, soil moisture deficiencies, limited nutrient supply, and soil instability may all 

 negatively affect coastal dune vegetation. 



Salt spray occurs when effervescence in the surf generates droplets into the air 

 where they are concentrated and transported inland by the wind (Boycel954). 

 Impingement on vegetation may result in chlorosis and subsequent death of plants. The 

 active agent of the salt spray is the chloride ion which enters the windward portions of 

 plant parts through cracks and lesions in the epidermis. The degree of injury is related to 

 the windspeed above the critical value of 7 m/sec where an abrupt increase in salt spray 

 intensity occurs as turbulent air flow increases. In addition to affecting growth, it has 

 been demonstrated that airborne salt spray is the primary environmental factor 

 determining the distribution, shape, and zonation of maritime plant species (Wells and 

 Shunk 1937; Costing and Billings 1942; Art 1971). Many of the grasses that grow on 

 foredunes are resistant to salt entry and hence can survive the intense spray zones of the 

 beach. Those plants that are less adapted are found in the lee of dunes or other 

 vegetation. Salt spray is an important factor preventing the establishment of annual 

 plants on the foredunes (Van der Valk 1974). As found along the Atlantic coast of the 

 United States, the salt spray effect only allows those plants specifically adapted to this 

 environment to inhabit the gulfward edge of Louisiana's barrier islands. 



The question of whether the dune environment presents a water deficiency to 

 plants has been greatly debated. Although the top few centimeters of a dune may be 

 completely dry, the sand below this level is often moist. It has been hypothesized that 

 the dry surface sand acts as a vapor trap which prevents deeper drying of the substrate. 

 The water table, per se, which depends on the size of the dune and may be several meters 

 from the active rooting zone, acts as an indirect source of water via vapor phase 

 diffusion upward to the rooting zone. Since the capillary rise of water from a free water 

 surface even in a very fine sand is not more than about 40 cm, the water table in a dune 

 only a few meters high can make no direct contribution to the moisture requirements of 

 most dune plants. Both rainfall and the condensation of soil water vapor provide 

 important sources of water to dune vegetation, but their relative contribution is 

 unknown. 



The dune plants themselves play an active role in controlling their water 

 requirements. This may be done by controlling water loss at the leaf surface, as in the 

 shallow rooting pennywort, Hydrocotyle bonariensis , by accumulating large amounts of 

 water in succulent tissue as in the dune elder, lva~imbricata , or by producing roots which 

 penetrate deep into the substrate, as in some of the dune grasses, e.g., Panicum , Uniola. 



Dune sands are generally deficient in nutrients essential for plant growth. The 

 major inputs to the dune system are salt spray and precipitation. The mineralization of 

 organic matter in the dunes is of limited importance since eolian processes remove most 

 lightweight organic matter. Fertilizer-addition tests have demonstrated that inorganic 

 nitrogen is the primary nutrient controlling the growth of dune vegetation (Woodhouse 

 and Haines 1966; Dahl et al. 1974). Phosphorus may become secondarily deficient after 

 the nitrogen deficiency has been ameliorated. Although nutrients would appear to be in 

 limited supply, some dunes support lush, productive stands of vegetation. Recently, it 

 has been demonstrated that dune grasses possess sand grain sheaths (rhizosheaths) around 

 their roots. Nitrogen fixation, a process by which microorganisms fix atmospheric 

 nitrogen into plant-available ammonia, is specifically associated with these sheaths 



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