The interactive effects of hurricanes and fire on plant productivity, accretion and elevation of a saltwater Marsh at Grand Bay Nerr, Mississippi

Coastal wetlands are subject to large-scale disturbance by fire and hurricanes, which are predicted to increase in frequency and intensity with climate change. A field study and mesocosm experiment examined effects of fire and hurricanes on a Juncus roemerianus dominated marsh at Grand Bay NERR, MS. Field plots were established along transects, which spanned low, mid and high marsh (July 2008). Bimonthly measurements of accretion, plant biomass and porewater physicochemistry were collected from plots. Study plots were affected by Hurricanes Gustav and Ike (September 2008), which deposited 15.4 +/- 2.4 mm of sediment in low marsh. Half of the transects were subjected to a low-intensity burn (January 2009). Accretion was influenced by initial input of hurricane sediment, particularly in the low marsh. Following fire, above-ground biomass was significantly reduced, and recovered to pre-burn levels over the subsequent year, with low marsh recovering more readily than mid or high marsh. Distance from shoreline and elevation determined flooding and sulfide stress, which likely influenced plant recovery. Mesocosms were established with sods of soil and vegetation from the study marsh. In a controlled experiment, hurricane and fire disturbances were simulated through removal of biomass, burning, and deposition of sediment and ash on soil surfaces. Data was collected on elevation change, above- and below-ground biomass, and bioavaliable nutrients for a year. Initial die-off of mesocosms masked many impacts of disturbance, as treatment effects were not significant for biomass variables. Control mesocosms had the greatest biomass, suggesting vegetation's intolerance to disturbance. Although many mesocosms lost all vegetation, sod elevations did not collapse completely, suggesting that these sediment driven ecosystems may be more resistant to subsidence from root zone collapse. As climate change contributes to sea-level rise, understanding impacts of multiple disturbances on ecosystem processes will be imperative for the long-term persistence and management of coastal ecosystems.