By MARY GRIZZARD

Black Mangroves are a fascinating keystone species of saltwater wetlands here in the coastal tip of Texas, and they’re on the march. Just in case you weren’t sure, these are the dense, stiff-leaved thickets that grow in intermittent clusters along the shores of the Laguna Madre and in the Rio Grande delta. In the tropics, Black Mangroves commonly grow into an impenetrable forest reaching 40 feet in height; here in the subtropics they have historically been only small scattered shrubs, being constantly pruned back, or even killed outright, by periodic hard freezes. But this is the story no longer. Because winters along the Gulf Coast have, overall, been consistently warmer in the past two decades, Black Mangroves are now growing much taller and are spreading northward up the coast.

Is this migration of the Black Mangrove a desirable affair? Yes…and no. Black mangroves, Avicennia germinans, deliver many benefits to the coastal environment. Their roots and dense, stick-like breathing structures called pneumatophores provide a sheltered nursery for the young of many varieties of game and shell fish and are a favorite protected roosting site for herons and egrets, ibises and roseate spoonbills. Mangroves filter out pollutants and purify the water, bearing the nickname “kidneys of the coast,” and their roots and pneumatophores hold coastline sediments in place, protecting them from high energy waves and storm surge erosion. But Black Mangroves aren’t simply advancing into unoccupied territory. A complex salt marsh ecosystem already exists in Texas bays and estuaries, and as Black Mangroves advance northward they are displacing these species. How this change will affect coastal wetlands and their inhabitants are questions currently being researched, and not a lot of hard data is in yet.

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