may have been a contributing source of colonizing seeds. Cross sections from an 

 800-foot-long section of the control area (Fig. 51) compared with the 1,200-foot sea oats 

 dune (Fig. 52) show that, since Hurricane Beulah in 1967 (the last destructive hurricane 

 surge in this part of the coast), the control section has grown to an area of mostly low, 

 unconnected hummock dunes, primarily of sea oats. The control area is shown in Figure 8. 

 Beach profiles of the sea oats planting and control area are compared in Figure 53. 



/. Invasion by Indigenous Flora. Natural invasion of plants into the artificial dune line 

 has been a slow process. Most species occurring on the foreslope of natural foredunes 

 (Table 8) do not presently occur on the foreslope of the artificial dune line, other than 

 those originally planted. The reason is partly that accumulating sand is a limiting factor for 

 many plants. Plants occur on the natural dunes after some stabilization has occurred. The 

 artificial foredunes are still actively accumulating sand on the foreslope, and is one reason 

 for the slow invasion rate. Another reason is the distance of over 1 mile for most plantings 

 from the experimental plots to the undamaged dune line, where most colonizing seed stock 

 originates. In November 1973, the 1,200-foot sea oats dune foreslope was sampled for 

 species composition and biomass (Table 56). Only one species (railroad vine) occurred that 



Table 56. Composition, by frequency and dry weight, of foreslope of 

 1,200-foot sea oats dune, north Padre Island, October 1973. l 



Species 



Relative frequency 



Dry weight (lbs. per acre) 



Sx 



Uniola paniculata 2 



82.4 



1,163.0 



85.6 



Panicum amarum 2 



6.2 



143.0 



64.1 



Spartina patens 2 



4.3 



16.5 



11.6 



Ipomoea Pes-caprae 



0.5 



2.7 





Bare ground 



13.8 









1. All species sampled, 55 months after planting, are included. Sampled plots are 

 0.25 square-meter plots; N = 210 random plots. Foreslope is 0- to 25-foot basal 

 of 50-foot-wide planting. 



2. Species initially planted. 



had not been planted. Woodhouse, Seneca, and Cooper (1968) reported that North Carolina 

 plantings of American' beachgrass are usually replaced by naturally invading sea oats within 

 10 years. This was explained by Seneca (1972a) as due partially to the greater temperature 

 tolerance of sea oats. In the Padre Island plantings, an eventual increase in species diversity 

 is expected, but the planted species should remain the dominant vegetation. 



Invading plants found on the artificial foredunes in spring 1974 are listed in Table 57 

 with an abundance ranking for the foreslope and backslope of each plot. The backslope has 

 more invading species and individuals than the foreslope, due to partial protection from salt 

 spray and wind; also the backslope receives a diminishing sand supply as the dune grows, 



158 



