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July 2011 cover image Every Drop Counts

Flora Fact: Multiplying Mangroves

Rising warmth and salinity increase black mangrove numbers.

By Sheryl Smith-Rodgers

Whenever he’s out collecting data, Eric Madrid often hears old-timers describe how favorite fishing spots along the Texas Gulf Coast have changed.

“They tell me that black mangrove are everywhere now, and they didn’t used to be nearly as common,” says Madrid, a botanist with Texas A&M University who’s studied the species since 2009.

What’s up? “Warmer temperatures are a primary reason,” he explains. “We hypothesize that changes in water salinity brought about by the construction of the Intracoastal Waterway in the ’40s have also played a role in the expansion of black mangrove populations in Texas. Today, our state’s largest population of these shrubby trees grows in northern Corpus Christi Bay near Aransas Pass.”

Madrid, who’s part of an international team monitoring the species in the Gulf of Mexico, can’t yet predict how larger populations of mangroves will affect Texas coastal ecosystems. But they are important.

“Mangroves are part of the base of the food chain in Texas wetlands, and they also help to create habitat for fish, crabs, insects, small invertebrates and birds,” Madrid says.

Black mangroves — named for the flaky, black bark — occur in wet soils dampened by high tides. To survive occasional submersions, mangrove roots send up hordes of pencil-like structures (called pneumatophores) that emerge from the ground and absorb oxygen. Another survival trick: Seeds sprout into seedlings (propagules) while still on the tree! After falling off, they can float up to a year before rooting.


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