Vegetation Survey

At each transect station, the following was recorded in the field: plant species (dominant listed first), percent cover, range in vegetation height, average height of "mass"of vegetation, and water regime (e.g., dry, moist, wet, very wet, depth of standing water). Digital aerial photographs were used in conjunction with field notes to classify the stations into one of the following habitats:

Beach
Dune
Vegetated barrier flat
Fresh-water and non-tidal marsh
Fresh and non-tidal pond
Salt- to brackish-water marsh
Wind-tidal flat
Estuarine water

Habitats

Beach

Beaches lie along the Gulf shoreline of Mustang Island and include the forebeach, which is subject to daily wave swash and tidal inundation, and the backbeach, which is inundated less frequently by spring and storm tides (diagram, photograph). Scattered vegetation may occur along the back beach, but this unit is typically barren of vascular plants. In the Cowardin and others (1979) classification that is used by the U.S. Fish and Wildlife Service (USFWS) in their National Wetland Inventory (NWI) program, Gulf beaches are classified and mapped as marine intertidal unconsolidated shore (M2US). Water regime modifiers used in this classification would be regularly flooded, N, for the forebeach (M2USN), and irregularly flooded, P, for the backbeach (M2USP).

Dune

Along the back beach are isolated coppice dunes behind which are well-vegetated fore-island dunes and dune ridges (schematic, photograph, photograph). In addition, past active dunes have migrated bayward and have become vegetated and stabilized at various locations on the island. Mid- and back-island dunes were encountered on the two transects. Typical vegetation on the vegetated fore-island dunes include Bitter Panicum (Panicum amarum), Sea oats (Uniola paniculata), beach tea (Croton punctatus), sea purslane (Sesuvium portulacastrum), goatfoot morning-glory (Ipomoea pes-caprae), and fiddleleaf morning-glory (Ipomoea stolonifera). Vegetation on the back side (bayward side) of fore-island dunes and on stabilized mid- and back-island dunes include saltmeadow cordgrass (Spartina patens), dune paspalum (Paspalum monostachyum, bushy bluestem (Andropogon glomeratus), little bluestem (Schizachyrium scoparium var. littorale), camphor weed (Heterotheca subaxillaris), Indian blanket (Gaillardia pulchella), prickly pear cactus (Opuntia sp.) and others. Dunes are classified as uplands (U) on USFWS NWI maps.

Vegetated-Barrier Flat

Vegetated-barrier flats are hummocky, and grass-, forb-, and shrub-covered sandy areas of low relief that generally lie between the fore-island dunes and bay marshes and tidal flats (schematic, photograph, photograph). The hummocky nature of this land reflects its origin as low dunes and mounds, deflation flats, accretionary flats, and washover deposits. Vegetation includes dune paspalum, bushy bluestem, little bluestem, marshhay cordgrass, gulf cordgrass (Spartina spartinae), camphor weed, penny-wort (Hydrocotyle bonariensis), white-topped-sedge (Dichromena colorata), sea-oxeye (Borrichia frutescens), bigleaf sumpweed (Iva frutescens), frog-fruit (Phyla sp.), Indian blanket, marsh fimbry (Fimbristylis castanea), bristle grass (Setaria sp.), prickly pear, and others. Vegetated-barrier flats are generally mapped as uplands (U) on NWI maps.

Fresh-Water and Non-Tidal Marsh

Fresh-water marshes, or interior non-tidal marshes, form in depressions typically surrounded by dunes and vegetated-barrier flats (photograph, photograph). The depressions may be formed by natural processes such as wind-deflation and scouring of sand during storms, and artificial processes such as quarring for sand. Those marshes that are regularly flooded and wet with standing water are typically classified as low marshes (topographically). On NWI maps the low marshes are classified as palustrine emergent wetlands, persistent vegetation, semipermanently flooded (PEM1F) or seasonally flooded (PEM1C). Common vegetation in these habitats includes cattail (Typha spp.), three-square bulrush (Scirpus pungens), seashore paspalum (Paspalum vaginatum), spikerush (Eleocharis spp.) coastal-water hyssop, (Bacopa monnieri), drummond's rattle-bush (Sesbania drummondii), seashore dropseed (Sporobolus virginicus), frog-fruit, marsh fimbry and others. Fresh-water marshes flooded less frequently are classified as high marsh, or in the NWI classification as temporarily flooded palustrine marsh (PEM1A) and at some locations PEM1C. Vegetation may include three-square bulrush, spikerush, gulf cordgrass, marshhay cordgrass, sea-oxeye, penny-wort, and others.

Fresh and Non-Tidal Ponds

Depressions that pond water in interior areas that are not affected by tides and have little to no emergent vegetation are designated as ponds or interior water bodies. The NWI classification for these areas is usually palustrine unconsolidated bottom with a semi-permanently flooded water regime (PUBF) or permanently flooded water regime (PUBH).

Salt- and Brackish-Water Marsh

Salt- and brackish-water marshes generally occur in back-island areas where the land is low enough to be inundated periodically by tides. Low salt- and brackish-water marshes are vegetated by various vascular plants including smooth cordgrass (Spartina alterniflora), saltwort (Batis maritima), prennial glasswort (Salicornia virginica), shoregrass (Monanthochloe littoralis), black mangrove (Avicennia germinans), and others (photograph, photograph, photograph, photograph). The USFWS NWI classification designates these areas as Estuarine Intertidal Emergent Wetlands with persistent vegetation and regularly flooded tidal water regimes (E2EM1N). Topographically higher marshes commonly have vegetation characterized by sea-oxeye, marshhay cordgrass, gulf cordgrass, seashore dropseed, shoregrass, annual glasswort, bigleaf sumpweed, marsh fimbry, Carolina wolfberry (Lycium carolinianum), and locally three-square bulrush. The NWI classification for these higher marshes is estuarine intertidal emergent wetlands, persistent vegetation, with irregularly tidal flooding (E2EM1P).

Wind-Tidal Flats

Wind-tidal flats are tidal flats that are inundated by estuarine waters elevated by astronomical tides and by wind induced and storm tides. Lower tidal flats that are regularly flooded by tides typically contain algal mats that produce darker signatures on aerial photographs (photograph, photograph). These flats may be vegetated annually by scattered annual glasswort but usually are barren of vascular plants. These topographically low lands have an NWI classification of estuarine intertidal unconsolidated shore with a regularly flooded tidal water regime (E2USN). The topographically higher flats are flooded less frequently, and are usually brighter white on aerial photographs. They are classified as E2USP (P = irregularly flooded) in the NWI classification. There has been a loss of wind-tidal flats on Mustang Island since the 1950's (White and others, 1998).

Estuarine Water Bodies

Corpus Christi Bay, which borders Mustang Island, is the source of tides and is mapped as estuarine open water. Smaller tidally-influenced water bodies that contain salt to brackish water in back island areas are also mapped as estuarine open water (photograph). In the NWI classification, Corpus Christi Bay and other salt- and brackish-water bodies are classified as estuarine subtidal unconsolidated bottom with a subtidal water regime (E1UBL).