Effects of flooding, salinity and herbivory on coastal plant communities, Louisiana, United States

Flooding and salinity stress are predicted to increase in coastal Louisiana as relative sea level rise (RSLR) continues in the Gulf of Mexico region. Although wetland plant species are adapted to these stressors, questions pe rsist as to how marshes may respond to changed abiotic variables caused by RSLR, and how herbivory by native and non-native mammals may affect this re sponse. The effects of altered flooding and salinity on coastal marsh commu nities were examined in two field experiments that simultaneously manipulat ed herbivore pressure. Marsh sods subjected to increased or decreased flood ing (by lowering or raising soda, respectively), and increased or decreased salinity (by reciprocally transplanting sods between a brackish and fresh marsh), were monitored inside and outside mammalian herbivore exclosures fo r three growing seasons. Increased flooding stress reduced species numbers and biomass; alleviating flooding stress did not significantly alter specie s numbers while community biomass increased. Increased salinity reduced spe cies numbers and biomass, more so if herbivores were present. Decreasing sa linity had an unexpected effect: herbivores selectively consumed plants tra nsplanted from the higher-salinity site. In plots protected from herbivory, decreased salinity had little effect on species numbers or biomass, but co mmunity composition changed. Overall, herbivore pressure further reduced sp ecies richness and biomass under conditions of increased flooding and incre ased salinity, supporting other findings that coastal marsh species call to lerate increasingly stressful conditions unless another factor, e.g., herbi vory, is also present. Also, species dropped out of more stressful treatmen ts much faster than they were added when stresses were alleviated, likely d ue to restrictions on dispersal. The rate at which plant communities will s hift as a result of changed abiotic variables will determine if marshes rem ain viable when subjected to RSLR.