Vegetation and elevational gradients within a bottomland hardwood forest of southeastern Louisiana

Twelve belt transects were used to sample woody Vegetation in a 110-y-old r egrowth forest on a natural levee ridge bordering Bayou Sauvage, Orleans Pa rish, Louisiana. Elevation was measured along each transect and species flo od tolerances, based upon measured elevational ranges, were assessed. Codom inant native hardwoods included Celtis laevigata, Quercus virginiana and Sa lix nigra; palmetto (Sabal minor) was the principal understory species. An invasive exotic species, Sapium sebiferum, was found throughout the study s ite (2485 adults >2 m tall per ha), and showed high flood tolerance. Among saplings (0.3-2 m tall), S. sebiferum abundance (3916 per ha) far exceeded that of all native species combined. The forest showed complex species zona tion along an elevational gradient of 163.5 cm across a forest-marsh ecoton e. Correspondence analysis delimited at least three plant communities, two dominated by single woody plant species and a third with 15 species. These communities were: (1) a Salix nigra community characteristic of bayou margi ns, (2) a ridge-forest community of 15 hardwood species with overlapping fl ood tolerances and (3) a Daubetonia drummondii community giving way to fres hwater marsh. The analysis tentatively resolved species associations within the ridge-forest community into "low-" and "high-ridge" subcommunities tho ugh insufficient data existed for their formal recognition. Elevational dis tributions of individual species compared reasonably well with published re gional assessments of their flood tolerances and value as wetland indicator s.