COMPARISON OF THE VEGETATION OF THREE LOUISIANA SWAMP SITES WITH DIFFERENT FLOODING REGIMES

Net primary productivity and community structure were studied at three adjacent swamp sites that were all once subject to seasonal flooding. Because of man-related activities one is now permanently flooded, and a second is managed on a summer-fall dry period and a winterspring flooded period. In the permanently flooded area such flood-intolerant trees as green ash are dying while shrubs and aquatic plants are increasing. Competition among overstory trees is reduced, enhancing growth of existing baldcypress and water tupelo trees. Total areal production, however, is low (887 gIm2/yr) because of fewer trees. Productivity in the managed area is high (1,780 gIm2/yr). The baldcypress-water tupelo community is being replaced by a swamp maple-ash community typical of bottomlands. In the natural swamp forest, baldcypress and water tupelo have remained dominant. Canopy closure has limited understory and aquatic growth but has also increased competition among overstory trees, resulting in a slow increase in basal area. Productivity is about 1,166 gIm2/yr. It is suggested that the flooding regime is an important controlling factor in each of the areas. SWAMP FORESTS are recognized as integral parts of wetland ecosystems, serving as habitats, spawning and nursery grounds, and sources of food for many organisms (Brown, Brinson and Lugo, 1979; Wharton and Brinson, 1979). Seasonal flooding and draining of swamps are vital to the maintenance of these functions. Recent studies have illustrated the high productivity of seasonally flooded swamp forests (Conner and Day, 1976; Mitsch and Ewel, 1979; Carter et al., 1973) and the export of large amounts of organic and inorganic nutrients from the swamp to downstream lake and marsh environments (Day, Butler and Conner, 1977; Kemp, 1978; Wharton and Brinson, 1979). In many wetlands the construction of canals, spoil deposits, embankments and impoundments associated with mining, timber, highway activities, and, in Louisiana, crawfish culture, has altered the flow of water. The purpose of this study was to compare plant community structure and productivity of three swamp areas with different flooding re1 Received for publication 20 February 1980; revision accepted 11 September 1980. Contribution No. LSU-CEL-79-17 of the Coastal Ecology Laboratory, LSU Center for Wetland Resources, Baton Rouge, La. This research was supported by the LSU Sea Grant College Program, a part of the National Sea Grant College Program maintained by the National Oceanic and Atmospheric Administration, U.S. Department of Commerce. We would like to thank Ron Paille, Lynne Bowers, Paul Kemp, Steve Whipple, and John Day whose assistance and comments helped make this paper possible. gimes. Three major tree species, baldcypress (Taxodium distichum), water tupelo (Nyssa aquatica), and ash (Fraxinus sp.) are emphasized. MATERIALS AND METHODS-Study sitesThe Lac des Allemands swamp is located at the upper end of the Barataria Bay estuary, between the Mississippi River and Bayou Lafourche, La. (Fig. 1). During a routine examination of 1972 infrared false color imagery of this swamp, we noticed a 970-ha area that appeared white in comparison to the red and bluegreen of the other forested areas. Earlier imagery (December 1940 and March 1952) showed that extensive logging operations had occurred throughout the swamp, but the distinctive pattern of the 1972 imagery was not evident. Natural water flows through this swamp forest are northeast to southeast. However, the 970-ha site was found to be impounded and permanently flooded by water up to 1 m deep. Impoundment began when Louisiana Highway 20 was completed in 1930 along the western boundary, blocking overland flows from upstream. A natural ridge runs along the southeastern side of the area. The Vacherie Canal was dredged in 1955 along the northern boundary, and Bayou Chevreuil along the southern boundary in 1959, impounding the area with spoil deposits so that natural water flow has been altered for at least 20 yr. North of the permanently flooded area is a dense, healthy forest (320 ha). This area was isolated by construction of a board road in the