4.4: Greenhouse Studies
Twilley is responsible for the overall experimental set-up and logistics of the experiment as well as participating in data collection. Mendelssohn (LSU) and McKee's (NWRC) team is responsible for tracking biomass, with Mark Hester (UNO), in particular, tracking photosynthesis responses of the vegetation. Stephen Faulkner, a biogeochemical ecologist with NWRC, and Robert Gambrell, a wetland biogeochemist with LSU, are monitoring changes in soil chemistry. In addition, Ray Schneider (LSU) is looking at pathogenic fungi that may be enhanced by the experimental stresses.
"The whole point is to re-create the climate conditions we weren't in place to observe, then link these to hydrology, link the hydrology to the soil conditions, and, ultimately, link the soil conditions to plant vigor."
The investigators have moved 180 5-gallon buckets of smooth cordgrass and its surrounding soil from three differing soil-type sites: Lake Felicity (organic), Bay Junop (clay), and Grand Isle (sand). In what amounts to a study in forensic, after-the-fact ecology, Twilley states, "The whole point is to re-create the climate conditions we weren't in place to observe, then link these to hydrology, link the hydrology to the soil conditions, and, ultimately, link the soil conditions to plant vigor."
More than simply meeting the broader goal of recreating drought conditions, this research group's multidisciplinary study has to be considered another one of the project's cornerstone tasks, one that harbors a complex experimental design that puts all 180 buckets the team collected to good use. For example, each soil type is exposed to two distinct salinity regimes, one of which is held at 15 ppt while the other is held at 30 ppt.
Stephanie Cogburn takes a porewater sample for chemical analysis.
Each of these soil type-salinity pairings is then treated to three varied water deficit intervals representing the range of tidal pulses they would see in the marshes, that is, a pulse each day, a pulse every seven days, and a monthly pulse. Finally, given the fact that the drought was measured in terms of both a hydrologic drawdown and a lack of rainfall, these 18 different soil type-salinity-tidal pulse groups are each treated to two sets of precipitation conditions: one with simulated historic rainfall and one with none whatsoever.
All told, then, there are 36 separate sets of unique conditions to observe, and although the study is ongoing, a pattern is beginning to emerge. According to Twilley, the smooth cordgrass plants in the sandy soils are having the toughest time in the low-pulse, no-rain scenarios. It is just this sort of observation of factors working in aggregate that the greenhouse research group had hoped to re-create. The results of the long-term greenhouse experiment, which will provide detailed information about changes in soil chemistry and plant mortality under native soil conditions, will be important in interpretation of the experiments the McKee-Mendelssohn team is conducting.