Fall 2006 | Munson Conservation Lecture Series

This lecture addressed the sustainability of Louisiana’s Coastal Wetland Forests, comprised mostly of swamp cypress and tupelo tree species. Keim presented the cultural and historical significance of these forest ecosystems, their ecological and hydrological importance, the impacts and effects of human development, catastrophic storms and land subsidence, as well as needs for effective ecosystem management. While there are still several unknowns regarding the survival of these forest ecosystems with rapidly changing environmental conditions, Keim’s most recent studies provide data that will help to inform future management processes.

These forested wetlands, which were mostly located in the Louisiana Delta, are a symbol of Louisiana and the southern United States. Prior to 1492 these forest swamps were expansive across the south. The settlement of humans to the area resulted in a conversion of almost all the swamp forests to farm land. In addition, extensive logging has occurred throughout the forests. There are approximately 1 million acres of cypress and tupelo forest remaining in this region.

Throughout geologic history, marsh ecosystems have dominated at the lowest elevations closest to the coast, but rapidly changing geology has pushed these swamp forests closer to the coast, exposing them to new elements such as storm surge, wind, and saline conditions. The delta is characterized by lobes of sediment that form and degrade over short time periods. With the rapid degradation, young sediment deposited in the river delta never condenses fully to make stable ground, and instead subsides and drops down. Overtime, this process, along with storm flooding, has eliminated much of the low level marshlands and as a result has pushed the swamp cypress and tupelo forests coastward.

In addition to the natural subsidence processes influencing the forest ecosystems, human development, such as levee building for flood protection has diminished the amount of sediment flowing from the Mississippi River into the delta and has forced water levels to rise in the forests. From the years 1935 to 2005 water levels in the forests rose over one meter or two centimeters each year.

Poor regeneration and reduced productivity are the most significant effects of increased flooding on these forest ecosystems. While these tree species do not need the flood conditions to grow, they are the only species that can tolerate sitting in standing water for years. Although they can survive these stressful conditions they rely heavily on pulsating floods in order to regenerate. Cypress and tupelo need a two week dry period to become tall enough to withstand the following year’s floods. With the increased water levels and more frequent flooding, the amount of times available for natural regeneration and succession to occur has diminished significantly.

As a result of continuous flood conditions, the present health of the swamp forests ranges dramatically. The healthiest forests are located on higher ground and closer to a brown sediment-rich river. In the summer, the healthiest forests are not inundated with water. Trees are dense and have smaller crowns because they are competing with each other. During the growing season a healthy forest floor includes terrestrial plants that are not flood tolerant. In contrast, where flooding is greater during the growing season, understory vegetation is sparse. In extreme flood conditions, the forest floor becomes entirely aquatic. The lack of detritus, deep water flood conditions, and dominant aquatic life causes the canopy to thin. Over time, where flooding persists permanently, the forest ecosystem will transition to floating marsh with just a few reminent trees that are unable to regenerate.

There are other contributing factors to swamp forest degradation in addition to flooding. However, comprehensive data on their survival under these stressful conditions does not exist. Although conclusions regarding these factors have not yet been scientifically proven, speculation as to their impacts is possible. The correlation between rising seas and water level in the forests is strong. Salt water intrusion is likely to have a devastating effect on the cypress and tupelo as they are not salt tolerant species. Essentially the entire marsh and some of the forests are inundated with salt water from hurricane Rita. Because of the lack of data on salt water intrusion, it is unclear how often this has occurred throughout history and thus the long term impacts of salt water stress are not completely understood. Further, since sea level rise will occur on a shorter time scale than the forest life cycle (cypress and tupelo species grow to old ages), the ability for the forests to respond to rapid change is unlikely. Low region marshes will likely float up with sea level rise encroaching on the forest ecosystems. As the marsh degrades and forests become coastal, the tree species could serve as effective wind break protection against hurricane surge. In addition, the changing forest could have significant impacts on bird abundance as the great majority of near tropic birds use the swamp forests as a stop over point during their northern migration.

Better data and comprehensive studies about the status of the forest will help to inform decision makers in developing plans for restoration and management. Keim’s most recent study develops spatial data that ties satellite data into ecological health to show what is occurring spatially in the forest. Keim uses landsat to classify forests into undegraded, transitional and degraded classes, illustrating which forests are sustainable, degraded, and open to ocean exposure. The classification helps to determine which forests experience dry enough conditions for regeneration. The analysis shows that two-thirds of the study area is in some sort of degraded state.

While much of the focus in addressing the consequences of land subsidence and catastrophic storms in coastal Louisiana is on coastal marshes, these unique swamp forests are extremely important to the health of the region, both ecologically and culturally. Studies like the one that Keim has conducted are essential to the preservation and restoration of these unique forests. There is a great opportunity to make a difference in the survival of these forests and it is critical that more comprehensive studies of the swamp forests are conducted to better understand them and to plan for their future.