The Research
- Southeastern Louisiana University Wetlands Restoration Laboratory (SWRL) has conducted numerous experiments in the Manchac/Maurepas area in an attempt to isolate the factor(s) responsible for the lack of regeneration of baldcypress.
- Founded in 1989, the lab has compiled an impressive database, from years of experimentation, which offers evidence that specifically implicates certain factors, while at the same time directly challenging some long-held doctrine regarding these Louisiana wetlands.
First, the specific factors:
Nutria Herbivory:
Cute, huh? Nutria are native to South Americaand were accidentally released from a fur ranch in south Louisiana during the 1920s. These animals are supremely well adapted to life in Louisiana's marshes and have greatly proliferated. Nutria have no real natural predators besides humans, and currently their market value is minimal. This results in huge numbers of the animals in our wetlands. Nutria are herbivores and eat many wetland plant species. They especially like to chew young baldcypress seedlings in half at the base.
If protection from nutria is not provided to young baldcypress seedlings, the cypress
trees will experience 100% mortality.
Entangling Vegetation:
Our research shows that even if a protection tubeis placed around saplings to protect from nutria, entangling vines such as deer pea and marsh morning glory (right) will completely carpet the young trees, outcompeting the saplings for available sunlight. We originally tested this hypothesis by hand managing the seedlings using ordinary hedge clippers... whew! It's a big marsh out there. Okay, so while this approach works, it's not technically feasible. So we decided to test the entangling effect with commercially available weed mats cut into 1 meter squares. This method works just as well from a statistical standpoint and is much easier to implement in the field.
Nutrient Deprivation:
Historically, the entire Lake Pontchartrain basinwas fed by the Mississippi River from the west; silt and nutrients were slowly deposited over Louisiana's wetlands. This resulted in the formation of fertile and productive wetlands all across southern Louisiana. With the advent of our modern lifestyle, the necessity arose for levees to protect the burgeoning urban centers along the banks of the Mississippi River.
During the 1800s a levee system was begun which would effectively protect cities along the Mississippi from flooding all the way from Minnesota to Louisiana. Did they work? Hmmm. One thing is for sure: they prevented the natural deposition of sediment onto Louisiana's wetlands. Sediment that could have been used for crucial marsh-building processes is instead dumped off of the continental shelf to settle into the deep waters of the Gulf of Mexico. Along with this sediment go the nutrients crucial to wetland productivity. It is hard to imagine that a marsh like the one pictured above could be nutrient poor, yet our research indicates that baldcypress saplings that were fertilized had significantly better growth rates than those trees which were not fertilized.
What long-held doctrines were challenged?
Quite often an assumption gets going and builds upon its own momentum. Although such an assumption may very well be true for other areas under other sets of circumstance, this assumption does not necessarily hold true for other, similar situations.
Saltwater Intrusion is one such assumption. The infiltration of saltwater into the
interiors of formerly brackish or freshwater wetlands is a serious problemacross southern
Louisiana. There are considered to be two primary culprits for this encroachment:
1) the dredging of canals (navigation and oil exploration) into less salty marshes
allows the salt water to penetrate deep within these marshes, and 2) the building
of levees prevents the influx of fresh river waters from regularly flushing these
habitats.
Saltwater intrusion has been shown to be responsible for the death of many fresh and
brackish plant species and the ensuing loss of vegetation has resulted in the dramatic
fragmentation of formerly solid marsh. As you can see from these photographs, canals
dug for oil exploration and access have dramatically opened up this interior marsh
south of New Orleans...
In fact, in the lower photo the only vegetation left is on thespoil banks of the access canals. These spoil banks are created by heaped mud dredged from the bottom during canal construction. The results of mucking about in fragile ecosystems before we know exactly what the results will be are amply demonstrated here. Okay. Back to assumptions. Some people automatically believed (and many still do) that this saltwater intrusion was directly responsible for the lack of baldcypress regeneration in the Manchac/Maurepas swamps.
Our research shows no evidence to support such a claim. In fact, in situ saltwater intrusion experiments indicate that baldcypress are remarkable tolerant to saltwater, at least for short-term periods. Short-term being the sort of duration that might be caused by a hurricane or tropical storm (~2 weeks).
This is what science is for - testing assumptions.
Let us not forget that science can be used not only to explore new frontiers in learning, but also to rigorously examine long-held, but largely untested doctrine.
(Many thanks to Dr. Seuss for reminding us to test those long-held doctrines.)
