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Maurepas Swamp Restoration

The Maurepas Land Bridge is a critical line of defense in reducing risk from storm surge for communities around Lake Maurepas as well as East Baton Rouge. Historically, the land bridge was covered with swamp forest dominated by bald cypress trees. The entire land bridge was clear cut in the late 1800’s and early 1900’s when logging of cypress peaked in the region. While some natural regeneration occurred, it was limited by the introduction of nutria, lack of nutrient freshwater and sediment input due to the leveeing of the Mississippi River, and subsidence. The construction of the MRGO, which introduced higher salinity to Lake Pontchartrain and Lake Maurepas, also had an effect on the remaining cypress stands. Currently, on much of the land bridge, natural cypress regeneration is not possible because the swamps are permanently flooded and seedlings need a period of draw down to germinate.  In other areas, the swamp forest has already converted to marsh. Additionally, a severe drought in 1999 and 2000 in the region caused salinity in the area to spike and killed many trees, both naturally occurring and planted.

Logging Historic logging operations using a pullboat in a Louisiana bayou. The huge steam engine drove a drum which hauled the logs out of the swamp, creating deep grooves (left, photo courtesy New York Public Library). These operations created scars that can still be seen on the landscape as narrow parallel or wheel spoke patterned canals (right, photo from 2010).

In the past, swamp restoration projects on the landbridge have failed, mostly due to high salinity. During the drought of 1999 and 2000, salinity in the Maurepas area reached 10 ppt, and tens of thousands of re-planted trees were killed. LPBF believes that with the closure of the Mississippi River Gulf Outlet (MRGO), the risk for high salinities has diminished. We have observed a freshening of the basin since the closure. With lower baseline salinities, and the construction of the proposed West Maurepas Freshwater Diversion, the risk to swamps trees during drought may be diminished.

Swamp Restoration on the Maurepas Landbridge

Swamp regeneration often does not occur in southeast Louisiana because of nutria herbivory, high interstitial soil salinity, altered hydrological regimes and slow germination rates. Under existing conditions, without human intervention, bald cypress seedling mortality is quite high.

LPBF has begun a planting program on the Maurepas Landbridge modeled after our program at the Caernarvon Freshwater Diversion. The goal planting program is to contribute to large-scale, long-term restoration in the Pontchartrain Basin. LPBF and our partner, Restore the Earth Foundation (REF), are committed to swamp reforestation on a large enough scale to restore the habitat for animals and storm surge benefits for the surrounding communities.

LPBF has conducted various swamp tree plantings on the Maurepas Landbridge since 2013 in partnership with the REF and with the help of many dedicated community volunteers. Our volunteers help transport, space and plant the trees, as well as outfit them with nutria protectors. Nutria protectors are a critical component of tree survival as trees without protection are readily eaten or destroyed by the prolific rodent. In addition to volunteer plantings, there has also been commercial plantings, supported by REF, where thousands of trees are planted in one week. Through the 2016/2017 planting season 24,600 trees have been planted on the Maurepas Landbridge, resulting in 127 acres of swamp restoration.

Volunteers having fun in the mud, helping to restore the beautiful swamps on the Maurepas Landbridge.

We have been solely responsible establishing and conducting the monitoring of the planted trees. Under this program, a percentage of all the trees that are planted are tracked over time for survival and growth rates. This allows for accurate estimations of success of the plantings over time. The monitoring includes scientific data collection and analysis. For each tree, the survival is assessed and diameter at breast height (DBH) and height are measured. The data is analyzed to produce growth rates for trees by planting (over time), by species and by location. To date, survival of the planted trees is high at 87%, and growth rates are low. The data provided from the monitoring program has proven to be invaluable for showing success of this project and providing lessons learned for future plantings to improve success.

Monitoring the growth rates and survival rates of the tress is important to show success and learn lessons that make future restoration plantings more successful. Each tree in the monitoring program is tagged with a unique number (top left), measure for height (top right) and diameter at breast height (bottom left). If the tree is not tall enough to get diameter at breast height, then the basal diameter is measured (bottom right).

Natural Swamp Regeneration on the Maurepas Landbridge

While we were scouting for planting areas in July of 2015 we found a large swath of the land bridge where we believe natural swamp regeneration is occurring. We spotted adult swamp species, such as water tupelo and cypress seeding, and younger trees of different ages growing and thriving.  There was also a swamp community understory developing, such as palmetto and swamp red maple.  In addition, the sounds of the swamp were alive. Insects were buzzing, birds were chirping and life of all kind abounded. We confirmed our previous observations and discovered a possible gradient of regeneration with more regeneration occurring on the southern end of the land bridge and less regeneration moving northward.  We also tested the soil salinity and found is was lower than the 3.0 ppt threshold for cypress. This is very exciting since this area was hit hard by the drought in 1999 and 2000 and high rates of mortality for natural and planted trees were reported in the area after the drought. LPBF would like to gain an understanding of natural regeneration in swamp habitats and study the conditions and processes under which it occurs. LPBF obtained funding to set up long term plots to study the natural regeneration over time. A report, entitled "Potential Occurrence of Natural Swamp Regeneration on the Maurepas Land Bridge, Southeast Louisiana" has been produced, outlining the methods, results and discussion for the first year of the study. This research is intended to be a multi-year study and therefore, no major conclusions have been reached.

There are swamps on the Maurepas Landbridge, south of Pass Manchac that seem to be naturally regenerating. A combination of dead cypress, live adult cypress and cypress sapling can be seen in many areas (top left, young cypress is the lighter colored tree in the foreground), areas further south have healthier swamps with denser canopies, the goal for the entire landbridge (top right), small, healthy looking cypress seedlings were seen (bottom left), and tupelo seedlings and saplings were found among the bull tongue and other understory fresh marsh species (bottom right, water tupelo sapling is the small tree towards the rear of the photo, in the middle).

Natural Swamp Regeneration Research Program

In order to determine if natural regeneration is indeed occurring, LPBF set out to set up a study that would investigate the extent and density on natural swamp regeneration in the Maurepas land bridge. Our specific questions are:

  1. Is natural swamp regeneration occurring in the region, indicated by the presence of swamp tree species in the sapling layer?
  2. Is there a gradient of increased natural regeneration moving from north to south on the land bridge?
  3. If any natural regeneration is observed, is regeneration sustainable towards a trajectory of healthy natural swamp?

The documentation of natural regeneration occurring in this area since the closure of the MRGO would show that large-scale hydrologic restoration projects can have rapid and far reaching benefits, including the return of historic ecosystems.

The study was conducted on the Maurepas Land Bridge, west of I-55 and south of Pass Manchac. Plots were set up along a gradient from minimal observed regeneration (north) to healthier forest (south). Eight (50 x 50 meter) plots were randomly chosen in eight sub-areas on the land bridge. Plots had to be located more than 50 meters from the water’s edge to avoid edge effects and within 400 meters of the water’s edge to be logistically feasible. Elevation measurements were taken at all four corners and the center of each plot. A soil salinity measurement was taken at the center of each plot. Herbaceous vegetation was measured at two, 1m2 quadrat vegetation plots for percent cover by species at the northwest and southeast corners of each plot. Woody vegetation was surveyed by tagging and measuring all saplings and seedlings that could be found and all adult trees in each plot, identifying to species and recording location coordinates using a handheld gps. Trees were measured for diameter at breast height (DBH). Young trees (saplings) encountered in the plot that were too small to measure DBH were measured for height.

Plot locations along the Maurepas Land Bridge and location of the Maurepas Land Bridge in the greater New Orleans Region (inset).

Pole marking the center of one of the plots and trees that have been tagged and flagged after they were measured (left) and a different plot showing the trees with tags, flags and tree paint.

Results

In total 647 trees were measured across all plots, representing nine species. The species observed were red maple (Acer rubrum), green ash (Fraxinus pennsylvanica), water tupelo (Nyssa aquatica), blackgum (Nyssa sylvatica), black willow (Salix nigra), live oak (Quercus virginiana), baldcypress (Taxodium distichum), Chinese tallow (Triadica sebifera), and American elm (Ulmus americana). Water tupelo had the most individuals across all plots (237) and red maple had the second most (121). The remaining species had fewer individuals with Green ash at 85, baldcypress at 73, Chinese tallow at 67, blackgum at 28, black willow at 13, and American elm and live oak at one. In general, there was a trend of more individuals and more species found at lower soil salinity. The species composition, in general, consisted of many small individuals of red maple and green ash, medium sized individuals of black willow and blackgum and larger individuals of water tupelo and baldcypress. However, bald cypress was found across a range of sizes with both small and large individuals present. It seems that trees that are wind dispersed (red maple, black willow, green ash) are colonizing the area first. These same species are common understory species in mature swamp.

Mean DBH (cm) by species, across all plots is shown on the left and mean DBH (cm) by seed dispersal method is shown on the right.

What does this mean for natural regeneration?

Although this data set is currently limited, it does suggest that the swamp forest on the Maurepas Land Bridge is in the beginning stages of natural regeneration. Pioneer species have colonized and climax species are present in lower numbers. The future of natural regeneration in the region is uncertain but there a few factors that increase the chance of successful regional swamp regeneration. The closure of the MRGO has decreased both the surface and soil salinity experienced in the region and also seems to have decreased the large fluctuations in salinity experienced when the MRGO was open. Whether or not salinity spikes will be less severe during drought than previously experienced in the region (during 1999-2000) remains to be seen. In addition to decreasing salinities from the MRGO closure, there is also freshwater and sediment diversions proposed for the region, that would help maintain fresh salinities. The 2012 Louisiana State Master Plan proposes the West Maurepas Diversion which is a freshwater diversion into the southern end of the land bridge. The final design of this project has been funded. In the 2017 Draft Louisiana State Master Plan, there are three diversions in the region proposed, including the East Maurepas, Manchac Land Bridge and Union Diversions, all proposed for implementation within the first 10 years. Therefore, there seems to be an emphasis on salinity control in the region which would promote natural regeneration and guard against future salinity spikes. Over time, this study will reveal the dynamics of natural swamp regeneration, including shifts in species composition, mortality, and the next generation of saplings that would become the future tree canopy.

Resources

Maps

Swamp Restoration Suitability Analysis Map

Reports

Swamp Restoration Suitability Analysis Report
Swamp Restoration Potential in LaBranche Wetlands
Potential Occurrence of Natural Swamp Regeneration on the Maurepas Land Bridge