How do clay dunes support thornscrub islands in salty lagoons, and what could help restore them?

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Clay dunes are unusual geological features near playas, lagoons, or flats that are periodically wet but dry out each year. When the sediment contains clay and strong prevailing winds are present, flakes or granules of clayey material are carried during the dry season and trapped by vegetation at the edges. Dew or rain moistens the particles, which stick together and eventually form dunes that support vegetation.

Locally called Jomas, these dunes occur along the Gulf coast of Texas and Mexico and reach their greatest stature near the mouth of the Rio Grande River, where this study took place. They form island-like features in hypersaline lagoons and support ecologically unique plant assemblages. Sharp environmental gradients separate salt-tolerant plants, or halophytes, from typical coastal thornscrub vegetation. Endangered animals such as ocelots live in the thornscrub. Development pressures along the border threaten their existence, and constructing artificial Jomas has been proposed.

This observational field study characterized four Joma plant communities. The adjacent hypersaline Flats community was limited to halophytes. The Edge community occurred at the margins of the Jomas, where salinity was lower than in the Flats but still elevated. At low salinities on top of the Jomas were a dense Thornscrub community and a Mixed Halophyte and Thornscrub community. The mixed community was hypothesized to result from disturbance; this was not presented as an established explanation.

The researcher analyzed elevation and salinity in plots along transects through the Jomas to correlate individual plant species with salinity preferences and community membership. A number of species showed fidelity to one community type, but quite a few bridging species occurred in two community types. The study concludes that this information has important implications for the degree of precision required when restoring or creating the clay dune ecosystem.