Wetted bliss: in a Louisiana refuge, different degrees of moisture create distinctive woods

Natural History, March, 2008 by Robert H. Mohlenbrock

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Just about any which way you cross Louisiana, you will pass through or beside wetlands, from swamps and marshes to bayous and oxbow lakes. Interstate 10, the main highway that runs through the low-lying southern portion of the state, including New Orleans, had to be elevated for eighteen miles just to bridge the swampy Atchafalaya River basin. Even in the north, where the land is higher, wetlands are abundant. One northern wetland that is a botanist's delight is Black Bayou Lake National Wildlife Refuge, about six miles north of Interstate 20. It was established in 1997 with the purchase of four and a half square miles of land surrounding Black Bayou Lake. Another two and a half square miles of property, comprising mostly the lake itself, has a free lease for ninety-nine years from the nearby city of Monroe.

Much of the land surrounding the lake used to be farmland, but it is now being restored as woods and, in one place, as an example of Louisiana prairie. Amenities include a visitor center, an environmental education building, a wildlife observation deck, photography and bird-watching blinds, boat ramps, hiking trails, and a boardwalk over the deepest swampland. A list of birds that may be observed-including waterfowl, wading birds, and neotropical migrants--is available from the visitor center.

Close to the visitor center are three small ponds with typical aquatic and shoreline vegetation, as well as the recreated patch of prairie. North and east of there, refuge manager Kelby Ouchley has planted an arboretum of trees and shrubs native to Louisiana. A hard-surface, handicapped accessible trail has recently been completed through it. The various species and varieties, including some that do not occur naturally within the refuge, are carefully labeled. But the botanical gems of the refuge are the three woodland habitats that depend upon different degrees of wetness.

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Near the visitor center, where the land is slightly elevated, is a mesic (moist) woods. That grades down into a wet woods and finally an extensive swampy woods. Drier than the other two but still moderately wet, the mesic woods is distinguished by a forest floor carpeted with herbaceous plants. The wet woods, in contrast, has several shallow, bare depressions where water stands for considerable time following a rain. Merging with the wet woods, the swampy woods has standing water during the winter and spring. At those times of year it can be up to a few feet deep in places.

A remarkable characteristic of the three habitats is how the different degrees of wetness influence the distribution of closely related species. White ash, usually considered an upland species, occurs in the mesic woods, for instance, but is replaced in the wet woods by green ash, which in turn gives way to Carolina ash in the swampy woods. Similar tree progressions include: cedar elm to American elm to water elm; black gum to swamp tupelo to tupelo gum; water oak (not an aquatic tree, as the name would imply) to cherrybark oak and willow oak to overcup oak. A three-part habitat pattern is also evident in vines, as graybark grape gives way to frost grape and then catbird grape, and saw greenbrier gives way to bristly greenbrier and then round-leaved greenbrier.

As a rule, the reason the species tend to be naturally segregated is a twofold one. The roots of some species simply cannot tolerate substantial periods in standing water. And those species versatile enough to grow in more than one zone may lose out to species that are more finely tuned to a particular habitat.

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HABITATS

Mesic woods Dominant trees are black gum, black hickory, cedar elm, persimmon, sugarberry, sweet gum, and water oak. Less common trees include honey locust, pecan, white ash, wild black cherry, and winged elm. Vines include Carolina snailseed, graybark grape, ladies'-eardrops, peppervine, saw greenbrier, and the invasive Japanese honeysuckle. Among the shrubs are black elderberry, Louisiana blackberry, and saltbush. Herbaceous plants include butterweed, cleavers, spring forget-me-not, Virginia wild rye, and white panicle aster.

Wet woods American elm, cherrybark oak, eastern cottonwood, green ash, swamp tupelo, and willow oak dominate. Small trees and common shrubs include deciduous holly, green hawthorn, and swamp privet. Common vines are bristly greenbrier, frost grape, rattan-vine, trumpet creeper, and wisteria. Herbaceous plants include Carolina buttercup, erect dayflower, marsh elder, pale smartweed, and woolly rose mallow, as well as some relatively rare members of the carrot family--finger dogshade, mock bishop's-weed, and white-nymph.

Swampy woods Bald cypress dominates, but Carolina ash, overcup oak, swamp cottonwood, tupelo gum, water elm, water hickory, and water locust also appear. Climbing hemp vine winds its way to the top of many trees, while catbird grape and round-leaved greenbrier are lower-growing vines. Buttonbush is the dominant shrub in the standing water. Few herbaceous plants live in the deeper waters, but shallow water provides a home for three kinds of beggar's-ticks, cutleaf water-milfoil, lizard's-tail, two kinds of pennyworts, two kinds of smartweeds, soft rush, spongeplant, and sting-less nettle. The swamp also hosts large stands of southern wild rice.