A 5-year mesocosm study on 11 species of wetland plants common to coastal Louisiana: The effects of water quality, hydrology, sediment addition, and hurricanes on above- and belowground production

Background/Question/Methods

Louisiana’s wetlands are in desperate need of restoration, yet the approach to be used for this restoration is hotly debated.  These approaches hinge upon the influences of coastal processes, such as river diversions and hurricanes, on plant zonation. Some research suggests that nutrient addition will be beneficial, while others argue that this will negatively impact belowground production.  The purpose of this study was to use a massive factorial design to build a set of assembly rules to be used in implementation of successful management and restoration strategies.  This study utilized a 5-year mesocosm experiment to investigate (1) the relationships between 11 woody and herbaceous plant species inhabiting 48 habitat combinations common to coastal Louisiana, (2) the reaction of a species-habitat relationship to Hurricane Demetra, and (3) the effects of nutrient loading on the (a) above- and belowground biomass production of wetland plants, (b) depth of root foraging activity, (c) overall effect on root to shoot ratio (R:S) and (d) soil elevation.

Results/Conclusions

The most biomass produced of all habitat types was in a freshwater river diversion scenario (fresh water with nutrients, throughput hydrology, and sediment addition).  Peltrandra virginica and Sagittara lancifolia were found to be the weakest competitors overall, whereas Panicum hemitomon and Typha domingensis were the two most dominant species.  P. hemitomon dominated fresh water with nutrient augmentation while Typha domingensis dominated all other habitat types.  After 5 years of growth, root to shoot ratio decreased, even though belowground biomass doubled with nutrient additions. Moreover, absolute amount of belowground biomass increased in deeper soil sections (>30 cm) over the other water quality treatments. Soil elevation was positively related to the amount of belowground biomass. Further, only nutrient treated mesocosm vessels produced enough root material to more than offset soil subsidence of 5mm/year. The results of these experiments should be used to promote beneficial use of river diversions and treated wastewater to restore the wetlands of coastal Louisiana.