Rebecca J. Howard

Salinity as a constraint on growth of oligohaline marsh macrophytes. I. Species variation in stress tolerance

The effects of increased salinity on plant growth were examined in a greenhouse experiment with four species common to oligohaline marshes of the northern Gulf of Mexico: Eleocharis palustris, Panicum hemitomon, Sagittaria lancifolia, and Scirpus americanus. Effects of final salinity reached (6 or 12 g/L), salinity influx rate (3 d or 3 wk), and duration of exposure (1, 2, or 3 mo) were investigated. Sagittaria lancifolia was the first species to show visible signs of stress, with browning and curling of older leaf edges. The salt effect was delayed for 6-8 wk in P. hemitomon, but this species had the highest aboveground tissue mortality rate at 12 g/L as exposure continued. Final salt concentration affected all species to a greater degree than did salinity influx rate. No aboveground mortality occurred at 6 g/L, but growth suppression was apparent and varied with species. The magnitude of growth suppression in response to salinity increased for all species as the duration of exposure increased. Overall, we ranked the species as follows, in order from least to most salt tolerant: Panicum hemitomon < Sagittaria lancifolia < Eleocharis palustris < Scirpus americanus. This ranking reflects the field occurrence of these species along a gradient of increasing salinity in northern Gulf of Mexico coastal habitats from freshwater wetlands through oligohaline areas to mesohaline wetlands.

Salinity as a constraint on growth of oligohaline marsh macrophytes. II. Salt pulses and recovery potential

The ability of common oligohaline marsh macrophytes of the northern Gulf of Mexico coast to recover from pulses of increased salinity was investigated in a greenhouse experiment with Eleocharis palustris, Panicum hemitomon, Sagittaria lancifolia, and Scirpus americanus monocultures. Components of salinity pulses applied were final salinity reached (6 or 12 g/L), salinity influx rate (3 d or 3 wk), and duration of exposure (1, 2, or 3 mo). After each exposure period, we placed plants into freshwater until the end of the 120-d experiment to determine recovery potential. The four species varied in their ability to recover from the salinity pulses. Within a species, recovery varied with final salinity level and duration of exposure, and to a lesser extent with salinity influx rate. Scirpus americanus, growth of which was stimulated by <3 mo of exposure to 6 g/L, was able to recover even under the most extreme conditions of exposure to 12 g/L salinity for 3 mo. Ability to recover decreased with increased salinity and increased duration of exposure for the remaining three species. Recovery of specific aspects of growth was also suppressed in these species by a rapid salinity influx rate compared to a slow influx rate. The complex variations in recovery patterns displayed by the different species may lead to changes in species dominance following the short-term salinity pulses that can occur during storm events, which in turn may affect marsh plant community composition and structure.

Effect of increased water depth on growth of a common perennial freshwater-intermediate marsh species in Coastal Louisiana

The response ofSagittaria lancifolia to increased water depths of 7.5 and 15 cm was examined in this field study. Water-depth treatments were achieved by digging sods containing one or two individual plants or ramets ofS. lancifolia from the marsh, removing sediment from the resulting hole, and replacing the sods in their original location at the appropriate lower elevation. Plants subjected to increased water depth of 15 cm had higher mean and maximum leaf heights than disturbed control plants. Aboveground biomass was not affected by water-depth treatment; however, 15-cm treatment plants had reduced root biomass and lowered leaf tissue concentrations of Ca, Cu, Fe, Mg, and Zn. Marsh sods at 15 cm below the marsh surface had the lowest redox potential and highest interstitial water sulfide concentration, indicating that this treatment created the most stressful belowground environment.Sagittaria lancifolia plants responded to the level of stress imposed by the experimental conditions with an altered growth form of increased leaf height.

Assessing coastal wetland vulnerability to sea-level rise along the northern Gulf of Mexico coast: Gaps and opportunities for developing a coordinated regional sampling network

Coastal wetland responses to sea-level rise are greatly influenced by biogeomorphic processes that affect wetland surface elevation. Small changes in elevation relative to sea level can lead to comparatively large changes in ecosystem structure, function, and stability. The surface elevation table-marker horizon (SET-MH) approach is being used globally to quantify the relative contributions of processes affecting wetland elevation change. Historically, SET-MH measurements have been obtained at local scales to address site-specific research questions. However, in the face of accelerated sea-level rise, there is an increasing need for elevation change network data that can be incorporated into regional ecological models and vulnerability assessments. In particular, there is a need for long-term, high-temporal resolution data that are strategically distributed across ecologically-relevant abiotic gradients. Here, we quantify the distribution of SET-MH stations along the northern Gulf of Mexico coast (USA) across political boundaries (states), wetland habitats, and ecologically-relevant abiotic gradients (i.e., gradients in temperature, precipitation, elevation, and relative sea-level rise). Our analyses identify areas with high SET-MH station densities as well as areas with notable gaps. Salt marshes, intermediate elevations, and colder areas with high rainfall have a high number of stations, while salt flat ecosystems, certain elevation zones, the mangrove-marsh ecotone, and hypersaline coastal areas with low rainfall have fewer stations. Due to rapid rates of wetland loss and relative sea-level rise, the state of Louisiana has the most extensive SET-MH station network in the region, and we provide several recent examples where data from Louisiana’s network have been used to assess and compare wetland vulnerability to sea-level rise. Our findings represent the first attempt to examine spatial gaps in SET-MH coverage across abiotic gradients. Our analyses can be used to transform a broadly disseminated and unplanned collection of SET-MH stations into a coordinated and strategic regional network. This regional network would provide data for predicting and preparing for the responses of coastal wetlands to accelerated sea-level rise and other aspects of global change.