Salinity data from the artificial dune line are given in Table 19 for 1973, and for 1970-72 in 

 Table 20. The relationship of inundations and precipitation to subsurface soil salinity of a 

 backshore site on north Padre (1973-74) is shown in Figure 25. Periods of high rainfall are 

 related to low salinity, whereas inundations with low rainfall result in the highest values. 



Seneca (1969) found that the toxicity of substrate salinity to germination of three dune 

 grasses was due to an osmotic pressure deficit, and only slightly from cldoride ion; in sea 

 oats, germination may have been influenced by toxicity of the chloride ion. If osmotic 

 pressure deficit is a primary cause of toxicity, the actual amount of salt in the soil may be 

 less important than the concentration of salt in the available soil water. For example, a given 

 soil salinity content would be twice as concentrated and presumably more toxic in a soil 

 with 10 percent moisture than the same soil with 20 percent moisture. Saline soils are 

 defined as having a saturation extract conductivity of 4,000 micromhos per centimeter. 

 Since saturation is about 21.5 percent for Padre Island sand, this is equivalent to 430 

 micromhos per centimeter for a 2:1 dilution. Therefore, most soils of the backshore and the 

 elevated foredunes would not be considered saline. Other investigators (Kearney, 1904; 

 Oosting and Billings, 1942; Randall, 1970) have also concluded that beach soils ordinarily 

 contain no higher concentrations of salt than average cultivated soils of the area, and that 

 beach plants need not be obbgate halophytes. 



To determine critical sabnity levels for the survival of transplants, laboratory 

 experiments were conducted in the spring of 1973 with bitter panicum and in the spring of 

 1974 with shoredune panicum and sea oats. 



Transplants (250 to 500) of all species were harvested from a north Padre protected site, 

 and replanted, five culms per pot, in the greenhouse. After about 2 months, when the 

 surviving plants were actively growing, two cups of saline (sodium chloride) solutions of 

 various strengths (from to 6 percent) were added to randomly chosen pots. Subsequently, 

 the pots were watered weekly with one cup of tap water. Since the pots contained no 

 drainage holes, all salt added in the initial treatment remained in the sand. Six weeks after 

 the sabnity treatment, relative survival and conductivity of the soil in each pot was 

 determined. 



Shoredune panicum was least salt-tolerant, followed by sea oats; bitter panicum was 

 most tolerant. The point where live shoots are reduced 50 percent is a valid means to 

 compare the three species. Sabnity values required to reduce survival 50 percent for 

 shoredune panicum, sea oats, and bitter panicum were 1,830, 2,620, and 3,460 micromhos 

 per centimeter, respectively (Fig. 26). Another indication of relative salt tolerance is that 

 point where increasing salt concentration caused a significant (P 05 ) reduction in grass 

 shoot survival. These levels were 1,540, 2,120, and 2,720 micromhos per centimeter 

 respectively, for shoredune panicum, sea oats, and bitter panicum (Tables 21, 22, and 23). 

 However, Seneca (1972b) found that germination rates and seedling survival of sea oats were 

 more adversely affected by sabnity than were these characteristics of shoredune panicum. 

 Nevertheless, their relative habitats on Padre Island would support the evidence that sea oats 

 is more tolerant of saline conditions than shoredune panicum on the south Texas coast. 



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