The ecological implications of temperature and salinity for a native and non-native iris in coastal Louisiana

Background/Question/Methods: Iris pseudacorus is native to Eurasia and is listed as an invasive species in 43 of the lower 48 states.  It is a freshwater plant that tolerates high salinities and is known to grow in salt marsh habitats in both its native and invasive ranges.  I. pseudacorus shares habitat requirements and environmental tolerances with a Louisiana native, I. hexagona, found in Louisiana’s coastal marsh and woodland fringe habitat.  Despite its popularity as a landscape plant over the past 200 years, I. pseudacorus does not appear to have escaped or established in more saline environments of Louisiana.  A previous experiment found that salinity negatively impacted the performance of I. pseudacorus seedlings.  Later, a pilot study conducted in Summer of 2005 showed that when I. pseudacorus rhizomes established in a common garden experienced a salinity treatment of 8 ppt, they experienced strong mortality.   These findings are at odds with the known ability of I. pseudacorus to survive in saline habitats, and led to the question of whether elevated salinity in combination with Louisiana’s high summer temperatures may negatively effect I. pseudacorus.    Growth chambers were used to determine if salinity and high temperature combined to have negative impacts on the early establishment of rhizomes.   Average summer temperature of the San Francisco Bay Delta, a region in the U.S. where this species is known to survive in salty environments, was used as the “low temperature” and average summer temperature for Carencro Louisiana, where the pilot study took place, was used as the “high temperature”.  Plants were potted in soil and watered with freshwater, 4 ppt, and 8ppt salinity and allowed to grow for one month  in warm and cool temperatures. 

Results/Conclusions: We found that neither temperature nor salinity affected the early establishment of rhizome fragments.  At low salinity, we found a significant species effect, where Iris pseudacorus grew faster than I. hexagona.  We also found a significant time effect as well.  At higher salinity we found a species by time interaction.  These results were unexpected and suggest that the timing of a disturbance event leading to the fragmenting and dispersal of rhizome fragments could determine the establishment success of fragments of native and invasive iris in coastal wetlands.