Using transplants and salt addition to test the effects of sea-level rise on salt marsh communities

Background/Question/Methods

While there are many challenges to projecting the effects of future sea-level rise, even a small increase in sea level could have dramatic impacts on salt marsh plant distribution and survival.  Impacts could result from increases in salinity or increases in water level.  To examine the effects of sea-level rise and salinity, we employed transplants to simulate gradual flooding and increased salinity exposure associated with decadal sea-level rise.  We moved four upland species seaward across three distances, representing a 10, 20, or 30mm tidal increase both in the presence and absence of neighbors, to quantify physiological stress responses in response to future sea-level rise and assess the role of competition in defining salt marsh zonation.  To ascertain whether reduced performance and survival were primarily due to increased salinity exposure, salt addition treatments were applied to the four species in their natural upland habitat both in the presence and absence of neighbors.  We ask three questions: 1) Will accelerated sea-level rise potentially result in reduced survival of terrestrial salt marsh species?  2) If so, would this survival also be influenced by competitive pressure from existing low marsh species? 3) Is reduced survival attributable to increased soil salinity?

Results/Conclusions

All transplants experienced reduced growth and photosynthetic inhibition even 1 vertical centimeter below the lower distributional limits of their current zones, both in the presence and absence of neighboring vegetation.  Survival and growth were even more impacted at 2 and 3 vertical centimeters lower.  Plants receiving salt addition in their natural positions experienced reduced growth and photosynthetic inhibition.  Our findings suggest that the performance of species is strongly influenced by physical stress limitations, primarily increased soil salinity.  Neighbor removal actually increased some vegetative growth, suggesting additional effects of competition in this system.  The interplay between physical stress and competition, coupled with the physiological tolerance of species to changing environmental conditions, will likely be key determinants of salt marsh plant fates in response to sea-level rise.  Our results showed that in this system, species at the marsh-terrestrial interface are negatively impacted by even the small changes in environmental conditions and may only be capable of tolerating changing abiotic conditions associated with short-term sea level rise.