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.
Karen: OK.
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.
Irv: Good.
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.