The time scale for these sequences is mostly dependent upon the intervals between 

 storms, the severity of previous storm damage, the proximity of undamaged colonizing 

 species, and the precipitation cycle. Sequential aerial photos of Padre Island were examined 

 at the U.S. Geological Survey office, Corpus Christi, at the PAIS headquarters, and at the 

 University of Texas Marine Institute, Port Aransas. The photos showed that the area 

 presently containing the nortli Padre study plots was barren in 1937, presumably as a result 

 of the 1933 hurricane. However, by 1948, a fairly well developed vegetated foredune ridge 

 had appeared with, a vegetated plain to the west. The ridge and plain were more pronounced 

 in 1950, but by 1961 (before Hurricane Carla), they were less distinct but still present. By 

 1967, after Hurricanes Carla and Beulah, the dune ridge was absent, and the area was again 

 barren with a field of active sand dunes migrating west. Today (1974), in an unplanted 

 one-half-mile section north of the study plots, the dune field is reforming naturally, and 

 consists of mostly unconnected hummock dunes of sea oats, saltmeadow cordgrass and gulf 

 croton which are still actively growing (Fig. 8). Another area, extending south from 1 mile 

 south of the present Pan Am road, was also barren in 1937. In 1950, a foredune ridge was 

 forming from interconnected hummock dunes, but there was little evidence of a leeward 

 vegetated plain. In 1961, both ridge and vegetated plain were distinct; they apparently 

 weathered Hurricanes Carla and Beulah since both were present in 1967, although with 

 some active blowouts. The ridge is now mostly continuous and well vegetated. 



In mid-island, there is plant colonization and succession from the westward-migrating 

 barren dune fields, partly activated early in this century by a combination of overgrazing 

 and drought. The dunes are so unstable that colonization by any species does not occur. 



After the dune lias migrated past a given point, it leaves behind a zone of barren, moist 

 sand, usually about 5 feet above MSL. The rapidity with which a newly exposed young 

 deflation plain can be colonized during a wet cycle and after the elimination of grazing is 

 shown in Figure 9. Ten months after the second photo was taken, the area was sampled 

 quantitatively for species composition; the results are given in Table 10. The most important 

 colonizing species were feral bermuda grass (Cynodon dactylon) and red lovegrass 

 (Erogrostis oxylepis), with several species of sedges and forbs frequent. It is expected that 

 succession eventually will trend to a composition similar to the older deflation plain to the 

 east (mid-island transect). 



Foredune blowouts actively migrate and often encroach on the mid-island prairie and 

 smother the vegetation. When the barren dune field advances onto a vegetated mid-island 

 dune or dune ridge, it is often buried only to the crest, and species such as sea oats, gulf 

 croton, beach evening primrose and beach groundcherry, which can withstand moderate to 

 great amounts of sand deposition, continue to grow and even thrive as a vegetative island in 

 the midst of a sand sea. The dune may be partially stabilized in this manner, or it may 

 continue to migrate, eventually leaving behind the "island" as tire top of an inverted cone of 

 barren sand, which usually erodes. 



43 



