Impacts of climate-induced mangrove expansion on the ecological functions of salt marshes

Dr. Irving A. Mendelssohn

Department of Oceanography & Coastal Sciences
3263 Energy, Coast and Environment Bldg., School of the Coast and Environment
Louisiana State University, Baton Rouge, LA 70803
Tel : 225 578-6425
Fax : 225 578-6423
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The advancement of species poleward, presumably due to global climate warming, has recently been documented in several regions of the world. In coastal ecosystems, one unique distributional shift is the movement of Avicennia germinans (black mangrove) northward into temperate Spartina alterniflora salt marshes.  In the Mississippi River Delta Complex of the northern Gulf of Mexico, black mangroves were historically restricted to the southernmost barrier islands and beaches by winter freeze events; however, in recent years a noticeable expansion has been observed.  Given the sparse documentation of the ecosystem-level effects of black mangroves expansion within coastal salt marshes, our goal was to quantify the impacts of this expansion on ecological processes including surface sediment accretion, organic matter production, organic matter decomposition, and carbon assimilation, as well as several edaphic characteristics.  Our results indicate that presently black mangrove expansion has had no major impacts on the ecosystem processes we measured.  Sediment accretion rates, belowground production, decomposition rates, and carbon assimilation were similar between Avicennia and Spartina stands located within the same hydrologic setting.  While decomposition rates were similar between habitats, mangrove leaves and roots decomposed more quickly than Spartina leaves and roots.  Some differences were documented in edaphic parameters between habitats.  Elevation, redox potential, bulk density, and soil ammonium were slightly higher, while soil moisture and porewater salinity were somewhat lower, where black mangroves had expanded into the surrounding salt marsh.  Other researchers working in different regions of the northern Gulf of Mexico have reported similar findings, which will be reviewed. Even tropical cyclone-generated sedimentation did not significantly differ between mangrove and salt marsh habitats located over large geographic distances. Because the expanding black mangrove stands are small in stature as well as areal coverage, significant effects on ecosystem processes may presently be muted.  However, if stands continue to grow in vertical and horizontal extent, effects on ecosystem processes may occur in the future.  This research provides a baseline from which future ecosystem responses may be evaluated as mangrove populations continue to develop.

Keywords

Climate change, ecotone, function, structure

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