Responses to salinity in the freshwater marsh plant, Iris hexagona: Local adaptation or plasticity?

Increasing salinity and subsidence are major threats to freshwater marsh habitats along the northern Gulf Coast.  These forces are particularly acute in Louisiana, where saltwater intrusion has destroyed and damaged numerous freshwater plant populations. The fragile coastal ecosystem is a tremendously diverse and valuable natural resource, and restoration of this rapidly disappearing habitat has become an ecological and economical priority. For over a decade we have studied a native Louisiana iris (Iris hexagona), as a model to understand the effects of salinity on freshwater plants.  Iris hexagona is the only salt-tolerant iris species native to Louisiana.  Here we describe results of a common garden experiment testing the local adaptation hypothesis.  Wild irises collected from freshwater, intermediate, and brackish populations were exposed to different levels of salinity.  Results indicate that moderate levels salt reduced plant biomass and increased seed production in all populations.  In addition, plants growing in brackish populations produced more seeds than plants growing in lower salinity environments.  There was no significant interaction between population and salinity, thus no evidence that populations were adapted to their local salinity regimes.  Salt has positive and negative effects on the fitness of I. hexagona, which exhibits substantial plasticity in tolerance to a range of environmental salinities.