Academic journal article The American Midland Naturalist

Vegetation and Elevational Gradients within a Bottomland Hardwood Forest of Southeastern Louisiana

Academic journal article The American Midland Naturalist

Vegetation and Elevational Gradients within a Bottomland Hardwood Forest of Southeastern Louisiana

Article excerpt

ABSTRACT.-Twelve belt transects were used to sample woody vegetation in a 110-y-old regrowth forest on a natural levee ridge bordering Bayou Sauvage, Orleans Parish, Louisiana. Elevation was measured along each transect and species flood tolerances, based upon measured elevational ranges, were assessed. Codominant native hardwoods included Celtis laevigata, Quercus virginiana and Salix nigra; palmetto (Sabal minor) was the principal understory species. An invasive exotic species, Sapium sebiferum, was found throughout the study site (2485 adults >2 m tall per ha), and showed high flood tolerance. Among saplings (0.32 m tall), S. sebiferum abundance (3916 per ha) far exceeded that of all native species combined. The forest showed complex species zonation along an elevational gradient of 163.5 cm across a forest-marsh ecotone. 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 margins, (2) a ridgeforest community of 15 hardwood species with overlapping flood tolerances and (3) a Daubentonia drummondii community giving way to freshwater marsh. The analysis tentatively resolved species associations within the ridge-forest community into "low-" and "high-ridge" subcommunities though insufficient data existed for their formal recognition. Elevational distributions of individual species compared reasonably well with published regional assessments of their flood tolerances and value as wetland indicators.

INTRODUCTION

Bottomland hardwood forests are a characteristic plant community of the Mississippi River deltaic plain of southeastern Louisiana with species belonging to the genera Quercus, Celtis, Liquidambar, Ulmus, Fraxinus, Carya, Acer and Sabal (Penfound and Howard, 1940; Braun, 1950; Montz, 1972). Such forests typically develop on natural levee ridges and narrow streambanks built up by periodic depositional activity of river distributaries (Spearing, 1995).

In southeastern Louisiana, forested levee ridges delimit drainage basins and, in coastal regions, protect interior wetland marshes from erosion (Spearing, 1995). Levee ridges bordering inactive distributaries are subject to gradual erosion and subsidence; this results in more frequent and prolonged flooding and promotes a gradient of plant communities from bottomland forest to swamp and then marsh.

Several authors have described examples of bottomland vegetation, attempting to characterize an apparent zonation of tree species caused by slight changes in ground elevation and concomitant hydroperiod (e.g., Penfound and Hathaway, 1938; Penfound, 1952; Shelford, 1954; Hosner and Minkler, 1963; Bell, 1974; Bell and del Moral, 1977; Nixon et al., 1977; Hupp, 1982, 1983; Hupp and Osterkamp, 1985; White, 1983, 1993). However, few studies have attempted a quantitative correlation between species distribution and elevational gradients (for examples see Bell, 1974; Bell and del Moral, 1977; Smith, 1996).

The present study characterizes a bottomland (old regrowth) forest bordering an inactive distributary in southeastern Louisiana. Although our principal objective is to describe the local distribution of mature woody species over an elevational gradient, we also quantify the distribution of woody saplings and understory. We examine the proliferation of a recently introduced exotic species (Sapium sebiferum), and whether understory native species affect its sapling growth and regeneration. Invasive species have damaged and severely altered native habitats and species compositions around the world, and our study site may be no exception. We hope to determine whether or not natural conditions (e.g., shade competition from native understory species) will eventually inhibit the continued invasion and increasing density of Sapium sebiferum at our site. We also compare the elevational distributions of tree species with expected distributions based upon regional wetland indicator data, and briefly assess the predictive power of such data at the local level. …

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