Black mangroves may promote longevity of Pass A Loutre

Linda F. Benedict, Nyman, John A.

News Release Distributed 09/02/09

The Pass A Loutre Wildlife Management Area, also known as the Bird’s Foot Delta because of its shape, lies at the end of the Mississippi River and protrudes about 40 miles farther into the Gulf of Mexico than the rest of Louisiana.

Despite the location, freshwater vegetation prevails because of massive freshwater inflows from the Mississippi River. Saline waters intrude only during tropical storms and extreme low flow of the river during continental drought.

The most salt- and flood-tolerant plant species native to Louisiana, black mangrove, currently does not occur in the Bird’s Foot Delta. In the salt marshes of Louisiana, however, black mangroves are expanding. In fact, from 2002 to 2009, sites with black mangroves doubled.

Black mangroves serve several valuable functions for the environment. They provide habitat for juvenile fish and even more so for juvenile shrimp and crabs. They are the preferred nesting habitat for the brown pelican. They also reduce storm surge and wave energy even more than marsh grasses because they are taller and stronger even when they are small shrubs.

Although black mangroves do not now occur in the Bird’s Foot Delta, an LSU botany professor, Claire Brown, wrote in 1936 that many of the islands and lower bayous there had rank growths of shrubby black mangroves.

By 1967, a professor at LSU’s School of Renewable Natural Resources, Robert Chabreck, noted that black mangroves were no longer there. The loss of the black mangrove from the Bird’s Foot Delta during the mid-1900s happened when navigation and flood control confined the river within artificial levees, which probably increased freshwater flow through the Bird’s Foot Delta and allowed less salt-tolerant species to out-compete black mangroves.

In recent decades however, salinity has increased at Pass A Loutre as indicated by the death of virtually all baldcypress and many willow trees since the 1980s. Factors that probably increased salinity since the 1980s include these:
– Reduced river flow caused by sediments dredged from the main navigation channel and dumped where the Mississippi River enters Pass A Loutre.
– Freshwater diversions constructed miles upstream to restore river flow to marshes there, which reduced freshwater flow through the Bird’s Foot Delta.
– More frequent tropical storm surges. Tropical storm intensity is expected to increase further because of climate change, which would increase the number of storm surges affecting the Bird’s Foot Delta.

The Coastal Protection and Restoration Authority of Louisiana recently released Louisiana’s Comprehensive Master Plan for a Sustainable Coast, which calls for two new diversions that will be large enough to create new wetlands. Those land-building diversions will further reduce freshwater and sediment inflow to the Bird’s Foot Delta and, eventually, may require abandonment of the Bird’s Foot Delta.

Even if approved today, those diversions would take decades to begin creating new wetlands. While wetlands in the Bird’s Foot Delta cannot persist indefinitely, their longevity can be promoted until land building diversions have begun to create new wetlands. In other words, a delta in the hand is worth two in a plan. Justification for longevity arises from the benefits Bird’s Foot Delta wetlands including storm surge reduction, fish and wildlife habitat, and recreation and commercial fishing. A lack of salt-tolerant plant species in the face of rising salinity is one factor limiting longevity of Bird’s Foot Delta wetlands.

An LSU undergraduate student majoring in Natural Resource Ecology and Management, Matt Huber, has spent the past year lending a helping hand to Louisiana’s coastal wetlands. He began in 2008 as a student worker providing field and laboratory assistance. He left his job as a part-time student worker in the spring of 2009 when he was accepted part-time by the America’s Wetland Conservation Corps, which places members at sites across Louisiana to coordinate coastal restoration projects and education projects for volunteers and communities. This effort promotes stewardship and conservation and raises public awareness of the negative impact of the loss of Louisiana’s wetlands on state, national and worldwide ecosystems. I continue supervising Matt because I serve as a site supervisor via the LSU AgCenter’s partnership with the America’s Wetland Conservation Corps.

The purpose of Matt’s restoration project is to reestablish populations of black mangrove that can serve as a source of seed and vegetative spread in the Bird’s Foot Delta. Mike Materne, coastal restoration specialist with the LSU AgCenter’s School of Plant, Environmental & Soil Sciences, provided 396 juvenile black mangrove trees that he and his team grew from seeds collected in the fall of 2007 from just north of Fourchon Beach near Grand Isle. Matt organized a dozen volunteers, most of whom were LSU students in the School of Renewable Natural Resources, to spend their 2009 spring break planting those trees during very unpleasant weather.

In the summer of 2009, the LSU College of Agriculture awarded an Undergraduate Research Grant to Matt and me to test the effects of river water and seawater on the chemical composition of black mangrove leaves. Matt returned to the site during July with volunteers to count surviving trees (59 percent) and to collect plant samples for his research project. Matt also traveled to Grand Isle to collect samples for his research project from black mangroves growing in more saline areas. Matt will return to both sites in fall and spring to monitor survival for the restoration project and to collect plant samples for his research project.

By the time he graduates next year, he will have spent three years getting his hands dirty and lending a hand to coastal wetland restoration. Although the primary purpose of undergraduate research projects is to provide learning experiences for students, these data may lead to techniques that restoration managers can use to determine how big of an effect river diversions have on our coastal wetlands.

Andy Nyman

9/2/2009 8:41:20 PM
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