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Seasonal variation in dune vegetation at South Padre Island, Texas.

Article Excerpt
Abstract. -- Seasonal variation in vegetative cover, species richness, species composition and species importance were compared in the backshore and primary dunes topographic zones at a dune protection area and an unprotected site on South Padre Island, Texas, from May 2004 to April 2005. Cover was greatest in fall, but there was no significant variation among seasons in percent cover in the backshore or primary dunes at either site. Species richness was significantly greater at the dune protection site, but there was no significant seasonal variation in species richness in either topographic zone at either site. Species composition differed between topographic zones and sites, but was similar among seasons within a topographic zone at a given site. Heterotheca subaxillaris was either the dominant species or a co-dominant species in both the backshore and primary dunes at the dune protection site. There was significant variation in dominance in the backshore and primary dunes at the unprotected site. Ipomoea pes-caprae was the dominant species in warm seasons and Croton punctatus was dominant in winter.

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Padre Island is the longest of the barrier islands of the Texas coast. It extends 182 km southward from Corpus Christi to Brazos-Santiago Pass at Port Isabel. The southern third of the island is separated from the northern two-thirds by the Mansfield Channel. This southern segment of the island is known as South Padre Island, and the name also is applied to an incorporated town that occupies the southern 8 km of the island. South Padre Island is 55 km long and has an area of about 16,200 ha (Lonard et al. 1999).

The flora and vegetation of South Padre Island are relatively well known compared to other islands of the Texas coast (Dahl et al. 1974; Judd et al. 1977; Lonard et al. 1978; Lonard & Judd 1980; Lonard & Judd 1981; Judd & Sides 1983; Judd & Lonard 1985; Judd & Lonard 1987; Judd et al. 1989; Lonard & Judd 1989; Judd et al. 1991; Lonard et al. 1991; Everitt et al. 1991; Everitt et al. 1992; Lonard & Judd 1993; Judd et al. 1994; Lonard & Judd 1997; Judd et al. 1998; Lonard & Judd 1999; Everitt et al. 1999; Lonard et al. 1999; Summy et al. 2006), but except for the information provided by Lonard & Judd (1989) on flowering phenology, there are no data on seasonal variation in the vegetation. Sampling protocols in a study of the relationship between vegetative cover and elevation change provided an opportunity to assess seasonal variation in percent cover, species richness, species composition and species importance at two sites on South Padre Island which are presented herein.

MATERIALS AND METHODS

Based on ground observations and an aerial survey, two study sites were selected that appeared to differ markedly in vegetation abundance. Site 1 is in a "dune protection zone" where driving and pedestrian traffic in the dunes is prohibited. Vegetation at this site appeared to be abundant and diverse. The site is located at 26[degrees] 14' 34" N and 97[degrees] 17' 20" W. It included a broad area of secondary dunes and vegetated flats. Site 2 is about 7.4 km north of Site 1 at 26[degrees] 18' 37" N and 97[degrees] 17' 39" W in an unprotected area. It appeared to have markedly less vegetation than Site 1 and included only a narrow strip of secondary dunes and vegetated flats.

At each site, three parallel transects spaced 25 m apart were established extending 90[degrees] E from the margin of Highway 100 to the high tide line at the Gulf of Mexico shore. To facilitate relocation of transects, the starting points at Highway 100 were recorded using a survey grade GPS unit and marked with orange paint. The transects passed through three topographic zones and part of a fourth zone (Judd et al. 1977), i.e. foreshore, backshore, primary dunes, and part of the secondary dunes and vegetated flats. The foreshore is bare. It is the portion of the Gulf of Mexico shore over which the tide migrates daily. This study focused on seasonal changes in vegetation of the backshore and primary dune zones.

Vegetation was sampled at four periods: 1) early summer -- 30...

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