The advancement of species poleward, due to global warming, has recently been documented worldwide. In Louisiana, Avicennia germinans (black mangrove) is moving northward into Spartina alterniflora salt marshes. Mangroves were historically restricted to southernmost islands and beaches by winter freezes; however, recently a noticeable expansion has occurred. The influence that mangroves have on ecosystem processes within Louisiana salt marshes has not been documented. Thus, this research examined the effects of mangrove expansion on sediment accretion, organic matter production and decomposition, and carbon assimilation, as well as edaphic parameters. Our results indicate that presently mangrove expansion has had no major effects on the ecosystem processes measured. Sediment accretion, belowground production, decomposition, and carbon assimilation were similar between Avicennia and Spartina. Where mangroves expanded into the surrounding salt marsh, elevation, redox potential, bulk density, and soil ammonium were slightly higher, while soil moisture and porewater salinity were somewhat lower. Because the mangroves are small in stature and areal extent, significant effects on ecosystem processes may presently be somewhat muted. However, if stands continue to grow, noted effects may occur in the future. Our research provides a baseline from which future ecosystem responses may be evaluated as mangroves in Louisiana continue to develop.