Hurricane disturbance and recovery of carbon and energy balance in a tidal mangrove forest

The goal of this presentation is to determine mangrove ecosystem function following a disturbance. Hurricane Wilma made landfall as a Category 3 storm along the western coast of the Florida Everglades in October 2005. The storm deposited large amounts of marine calcitic sediments. At a site 2 km inland from the center of landfall, 5 cm of sediments were deposited, and 30% of mature trees were defoliated and killed. At this site, tower based measurements of carbon dioxide eddy covariance and energy balance were started in 2004. Several years of repeated measurements of tree basal area, litterfall, root turnover, and peat surface elevations also pre-date the storm. Measurements resumed in 2006, providing an opportunity to examine the effects of hurricane disturbance on coastal ecosystem functioning. Following defoliation sensible heating decreased and latent heating increased. Reductions in sensible heating are attributed to decreased energy exchange at the forest crown and increased solar energy reaching the peat surface. Increases in latent heating are attributed to increased turbulent transport of latent heat fluxes from the saturated soil and water beneath the forest. Soil temperature and soil heat fluxes increased after the storm. Reductions in net ecosystem carbon assimilation rates (NEE) are apparent 4 years after the storm, though the rates are now approaching pre-storm values. The increased soil temperature following the storm presumably led to increased soil respiration which, along with the tree mortality, branch loss, and defoliation, explained the decreases in ecosystem-level carbon assimilation rates following the storm. Minimum half-hourly NEE values reached -25 μmol m-2 s-1 and -22 μmol m-2 s-1 before and after the storm, respectively. Daytime ecosystem respiration rates (Re) estimated using the intercept of light-response curves increased by an average of 1.2 μmol m-2 s-1 at a reference temperature of 20 oC during low tide periods. Nightime Re at 20 oC increased by an average of 0.6 μmol m-2 s-1 during low tide periods. Temperature response functions suggest the higher soil temperature after the storm resulted in an average increase in Rs of 1 to 2 μmol m-2 s-1. This is correlated with a general decline in peat surface elevations, suggesting significant carbon loss from the soil has occurred through carbon efflux and dissolved fluxes to the tide. However, the rate of changes in peat surface elevations has decreased over the last year with canopy closure, suggesting the soil and overall ecosystem carbon balance are approaching a new dynamic equilibrium following the storm.