Response and Recovery of Low-Salinity Marsh Plant Communities to Constant and Pulsed Saline Intrusion

The effects of climate change on environmental conditions will be manifested as both chronic changes and pulse changes of varying durations. In estuaries, future variation in sea level, freshwater withdrawal and drought will lead to saline intrusion into low-salinity tidal marshes. I sought to understand the responses to various salinization scenarios at individual, community and ecosystem levels. First, I used a mesocosm experiment to investigate the responses of individual low-salinity marsh macrophytes to different durations of saline water exposure, without interspecific interactions. Second, I used another mesocosm experiment to assess the response of plant communities to saline water pulses of three salinities for five durations, and then assessed subsequent community recovery when the saline pulses were withdrawn. Third, I used a field experiment to explore the impacts on a freshwater marsh plant community of short- and long-term changes in salinity. I ranked the freshwater marsh plant species in order from least to most salt tolerant as follows: L. peploides, P. hydropiperoides, P. cordata and Z. miliacea, followed by S. lancifolia and E. palustris in a tie. I found that community composition was increasingly affected by the more-saline and longer-duration treatments. In the second mesocosm experiment, most but not all of the plant species were able to recover from low-salinity, short-duration saline pulses in less than one year. Because not all the species recovered, post-disturbance community structure diverged among salinization treatments. In the field experiment, both presses and pulses of saline water addition caused the loss of L. repens, resulting in community composition diverging away from the controls. In the press treatment, plant biomass and diversity were strongly suppressed. In the pulse treatment, community composition did not recover to the baseline conditions in between pulses of saline water, as L. repens failed to fully recover from pulses of salinity. In the second mesocosm experiment, shifts in community composition prevented long-term reductions in productivity. In the field experiment, however, salt-tolerant plants could not immigrate, and salinity presses caused a significant decrease in aboveground biomass. In contrast, salinity pulses had no effect on plant production. These results show that species composition is more sensitive to environmental changes than overall ecosystem processes. However, changes in ecosystem structure due to pulse changes may not limit recovery in ecosystem function.