Vegetation classification and mapping of Padre Island National Seashore: Final project report

Diamond, David; Elliott, Lee; Treuer-Kuehn, Amie; Singhurst, Jason; Peterson, Arnie

Padre Island National Seashore (PAIS; Padre Island NS) circumscribes a section of Padre Island nearly 110 kilometers (68.3 miles [mi]) long and one to five kilometers (0.6 to 3.1 mi) wide. Padre Island is a barrier island along the Gulf of Mexico in southern Texas. The Gulf of Mexico forms one boundary to the east, and the Laguna Madre separates the western, landward side from mainland Texas. This report provides a map and analysis of vegetation of the seashore based on quantitative, plot-based field data collection and vegetation mapping. Barrier dunes form inland from the gulf, and generally rise 4 to 7 meters (13.1 to 22.9 feet [ft]) in elevation, with some reaching 12 meters (39.3 ft). These dune fields are usually less than 200 meters (656.1 ft) wide, and narrow to 50 meters (164 ft) or fewer in the south. Flats, sometimes with shallow troughs, are characteristic of areas landward from the dunes. These rise to more than 2 meters (6.6 ft) elevation in the north, but are less than 1 meter (3.3 ft) in the south. Narrow dune ridges, usually reaching a little more than 3 meters (9.8 ft) elevation, occur in generally linear patches perpendicular to the length of the island in some areas. A generalized vegetation transect perpendicular to the length of the island would intersect (1) unvegetated beach, (2) sparsely vegetated foredunes, (3) vegetated dune ridges and dune fields, (4) grasslands and herbaceous wetlands on broad flats and troughs that form the matrix of the back-side of the island in the north, and (5) irregularly shaped patches of tidally-influenced saline herbaceous vegetation on the Laguna Madre side of the island that form the back-side matrix in the south. Dune and foredune communities together made up 9.37% of the vegetated area of PAIS. Grasslands made up 48.00%, and tidal saline vegetation 33.19%. Herbaceous wetlands made up 8.74% of the vegetation of PAIS. A few shrublands and woodlands accounted for less than 1% of the vegetation. Visually, these communities stood out in the otherwise herbaceous-dominated landscape, and added to local structural diversity of the vegetation. Schizachyrium littorale (seacoast bluestem) and Paspalum monostachyum (gulfdune paspalum) were the most common dominant species of grasslands across large upland areas and stabilized dunes. Spartina patens (marshhay cordgrass) was the most common dominant of wetlands across a range of salinities and flooding regimes. Tidal saline, dune and foredune, and shrub-dominated areas each had their own unique flora. A total of sixteen plant associations were identified during the classification phase of this study, primarily from analysis of 151 vegetation plots. A total of seventeen unique map classes were identified during the mapping phase. These did not entirely match with the plant association classification based on plots, because some classified associations could not be mapped, and several new associations were discovered and mapped after the plot data were collected. A map accuracy assessment showed 64.78% accuracy overall, corrected for area, and average by-class accuracy of 73.63% and reliability of 71.72%. Variation in seasonal and annual precipitation, wind-influenced high tides (especially on the Laguna Madre side), wildfires, and storm events all influence vegetation dynamics at Padre Island NS. Relative dominance might shift within a given area based on a single wet or dry season or year. Erosional cuts extending from the gulf to the Laguna Madre might form due to a single storm event. Hence, the landscape and vegetation of this barrier island are susceptible to significant change over short time periods.

Vegetation classification and mapping of Padre Island National Seashore: Final project report

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