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Coastal Louisiana: Impacts of Hurricanes on Salt Marsh and Mangrove Wetlands
Marsh and Mangrove Stands
Narration: In this study, scientists are examining how different types of coastal vegetation capture and hold sediment delivered by Hurricane Gustav in the storm surge. The dark green vegetation in this photograph is the black mangrove, a tropical tree. The brown vegetation is a temperate salt marsh grass found throughout the Atlantic and Gulf coasts, but is now dormant because it’s winter here.
Dr. Karen McKee, USGS Research Ecologist
Karen: This is a wetland that is dominated by two saline species, that is plants that can tolerate salt water, and what we have here is the black mangrove, which is a tropical tree and it occurs at its northernmost limit here in North America in Louisiana at this location. And it is growing intermixed with a salt marsh species, which is a temperate plant, which is called Spartina alterniflora, or smooth cordgrass. And in this location the two species, their distributions converge and here the two species are competing for space.
Measuring Vegetation Structure
To better understand how marsh and mangrove vegetation differ in their ability to capture storm sediment, scientists assess the characteristics of the vegetation structure including canopy height, stem density, leaf area, and other important features that might influence sediment capture.
Dr. Mark Hester, University of Louisiana-Lafayette
Mark: Here we’re measuring average height, so I’m trying to get my eyeballs lined up with the average canopy height below the inflorescence. We’ve got 66 centimeters.
Dr. Irv Mendelssohn, Louisiana State University
Irv: I am measuring plant canopy leaf area index. First thing you do is take a reading of the light intensity above the canopy.
Leaf Area Index
Irv: And then four below. And so you get a measure of light interception by the canopy.
Narration: This diagram illustrates the differences in leaf area index between salt marsh and mangrove vegetation. On the left, less light is intercepted by the plant canopy due to less leaf area. On the right mangroves intercept more light due to the higher leaf area per ground area.
Irv: Computing. And what we can do is… It has a number of different indices that are provided. But it gives you the leaf area index and this is 1.67.
Karen: OK. And what have you been getting in the mangrove? As high as 3.6, 3.7 and as low in the Spartina as 0.6.
Irv: So there’s quite a large range.
Narration: So the mangrove canopy shown here, which is taller with a greater leaf area, might be expected to slow water movement as the storm surge passes, causing more sediment to be deposited. The marsh grass, on the other hand, is shorter and less dense and thus might be expected to capture less sediment than the mangroves.
Karen: And what are you estimating for here?
Mark: Well, so for cover on this one, I’m estimating 35 percent. We have 66 centimeter average height, and we have 19 stems per tenth of a square meter.
Mark: I guess we’ve to get a photo of this one.
Storm Sediment Depth and Composition
Narration: At each sampling site, cores of soil are collected to measure the depth and composition of storm sediments that were deposited in marsh and mangrove stands. In this case, there were several centimeters of mineral material added to the soil surface, which is important in helping these marshes build vertically and keep up with sea-level rise.
Dr. Karen McKee, USGS Research Ecologist
Karen: The black mangrove is very interesting because it reproduces by a method called vivipary, which means that the seeds germinate while they’re still attached to the parent plant. This propagule, which looks like a small lima bean, is actually a developing seedling and it’s carried on the tidal waters until it becomes stranded on the soil surface. And ultimately it becomes a larger plant, which you see here. Plants in the tropics become the size of trees, but here they remain in a bushy stature because they are periodically killed by freezes, which kills the shoots and causes stunting of the trees.
Title: Coastal Louisiana: Impacts of Hurricanes on Salt Marsh and Mangrove Wetlands
Description: This video describes research conducted by Dr. Karen McKee, USGS Research Ecologist, and her university partners, Dr. Irv Mendelssohn (Louisiana State University) and Dr. Mark Hester (University of Louisiana). They are studying the effects of hurricanes on marsh and mangrove wetlands in the Mississippi River Delta, which contains over 40% of the U.S. wetlands in the lower 48 states. Although hurricanes can have devastating effects on human communities, they may provide nourishing sediment to subsiding wetlands. This video describes an investigation to determine how much storm sediment from Hurricane Gustav (2008) was deposited in marsh and mangrove stands and if these different types of vegetation influence sediment capture. McKee and colleagues carried out their research by helicopter, which is the only way to sample the vast wetlands in coastal Louisiana. The results of their investigation will lead to a better understanding of how hurricanes may benefit coastal wetlands.
Location: LA, USA
Date Taken: 12/31/2008
Video Producer: Karen McKee , U.S. Geological Survey
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