The Effect of Hurricane Allen on the Near-Shore Vegetation of South Padre Island

Abstract

The effect of Hurricane Allen (Aug. 10, 1980) on the near-shore vegetation of South Padre Island was assessed by comparing data obtained in line transects before and after the hurricane. Eight species of plants were present in the Backshore Zone prior to the hurricane; after the hurricane, only three species were present and percent cover was only 11% of that before the hurricane. The foredunes of this zone were completely leveled and a belt of Sesuvium portulacastrum was obliterated. Conversely, plants of the taller Primary Dune Zone were not greatly disturbed by the hurricane. Numbers of species and percent cover were similar in pre- and post-hurricane transects. However, before the hurricane, Ipomoea stolonifera was dominant on both the windward and leeward slopes. After the hurricane, Uniola paniculata was dominant on the windward slopes andSchizachyrium scoparium was dominant on the leeward slopes. Apparently, low-growing, shallow-rooted species such as Sesuvium portulacas- trum and Ipomoea stolonifera are more greatly affected than grasses because they are more easily dislodged or covered by sand transported by water surges. The predominant land form along the Gulf Coast of Texas consists of a series of barrier islands. Padre Island is the longest of these islands, it extends from Corpus Christi on the north to the Brazos-Santiago Pass at Port Isabel on the south; a distance of about 182 km. The Mansfield Channel separates the northern two-thirds of the island from South Padre Island. The mechanism(s) of origin of the barrier islands is a subject of considerable controversy (Hoyt, 1967; Fisher, 1968), but workers agree that Padre Island began forming about 4,500 years ago (Dahl, et al., 1974), and that hurricanes are important factors in the development of the barrier islands. Major hurricanes have occurred at South Padre Island six times since 1844. The most recent was Hurricane Allen on August 10, 1980. During hurricanes, the storm surge or overwash moves large volumes of sand from the beach, foredunes, primary dunes, and offshore areas to the interior and bay-tidal flats and into the bay. In the process large expanses of vegetation are denuded or covered. Study of the effects of hurricanes on vegetation of barrier islands has been devoted primarily to the revegetation of overwash areas (God- frey and Godfrey, 1972, 1976); and little is known of the immediate consequences for vegetation. However, Harris and Chabreck (1958) reported on the immediate effects of Hurricane Audrey on marsh vegetation of Marsh Island, Louisiana and Chabreck and Palmisano (1973) reported on the immediate effects of Hurricane Camille on the marsh vegetation of the Mississippi River delta. Both studies showed that the hurricanes resulted in a drastic reduction of vegetation due to the sweeping action of wind and water, that either uprooted or ripped apart plants which were subsequently carried away by the current. Herein we present data on the immediate effect of Hurricane Allen to the near-shore vegetation of South