Stand composition and structure across a changing hydrologic gradient: Jean Lafitte National Park, Louisiana, USA

Abstract

We report the results of an intensive study of forest structure and composition across a 1.4-m elevation gradient from the top of a natural levee into the backswamp of Bayou Des Familles, Jean Lafitte National Park, Jefferson Parish, Louisiana, USA. At the southernmost edge of the great bottomland hardwood forest of the Lower Mississippi Alluvial Valley (LMAV), forests of the Bayou Barataria-Des Familles distributary are undergoing rapid subsidence with resulting increased flood frequency, depth, and duration. We used data from a 4.6-ha permanently marked plot to examine patterns of distribution and regeneration in forest trees. Non-metric multidimensional scaling ordination of 23 quadrats (20×100 m) from this plot showed variation in forest composition across this 1.4-m elevation gradient corresponding to bottomland hardwood forest Zones III (semipermanently flooded) through V (temporarily flooded). A comparison of the size-frequency distributions of common species in upper, middle, and lower sectors of the gradient revealed deficient and poor recruitment in Quercus virginiana, Acer negundo, Celtis laevigata, and Salix nigra and episodic regeneration in Liquidambar styraciflua, Taxodium distichum, and Quercus nuttallii. Recruitment of the exotic species, Sapium sebiferum, is occurring at the low end of the gradient, as well as in canopy gaps throughout the gradient. Logistic regressions of sapling ( -10 cm dbh) size classes as a function of elevation showed that saplings of L. styraciflua, Q. nigra, and U. americana occur at higher elevations than do adult trees of the same species, evidence of the rate of hydrologic change in this forest. A fourth species, Acer rubrum, resprouts vigorously under rising water levels and may be an effective competitor with more light-demanding, flood-tolerant species at low elevations.