| [Image: Bottomland hardwood forests] | | | | | | | |
The largest concentration of bottomland hardwoods in the United States occurs in the Lower Mississippi River Alluvial Valley.
Sammy L. King
Bottomland hardwood forests are distributed along rivers and streams throughout the central and southern United States. These wetland forests contain numerous oxbow lakes, sloughs and other wetlands embedded in a mixture of oaks, sweetgum, hickories, ash, cottonwood, sycamore and other species. These forests are economically valuable for timber production, and they also support diverse wildlife populations including waterfowl, songbirds, black bear, and a variety of reptiles and amphibians. The types and abundance of tree species at a given site depend upon the frequency of flooding, soil types and other site characteristics.
The largest concentration of bottomland hardwoods in the United States occurs in the Lower Mississippi River Alluvial Valley (LMAV). The LMAV consists of the floodplain of the Mississippi River and extends from Cairo, Ill., through parts of seven states to the Gulf of Mexico. Historically, there were about 25 million acres of bottomland hardwood forests in the region. However, extensive harvesting along with the building of levees altered the processes that structure these forests.
In the 1980s, the Louisiana Department of Wildlife and Fisheries and other state and federal agencies became concerned about the effect of widespread forest clearing on wildlife dependent on these forested wetlands. They began experimenting with replanting the forest on sites that had been cleared. These initial trial-and-error experiments have now developed into one of the largest reforestation and wetland restoration efforts in the world. Broad-scale conservation goals have been developed for migrant songbirds, waterfowl, shorebirds, other waterbirds and the threatened Louisiana black bear.
These restoration efforts include management of existing forests to improve forest conditions for wildlife as well as reforestation of former agricultural fields. Reforestation on public lands, and particularly on private lands through the U.S. Department of Agriculture’s Wetland Reserve Program, has begun to restore many wildlife and forest-related functions to the region. In Louisiana, the Wetlands Reserve Program has resulted in the restoration of more than 157,000 acres. In 2006, about $9 million will be spent through the Wetlands Reserve Program in Louisiana.
Recent research through the LSU AgCenter and the U.S. Geological Survey’s Louisiana Cooperative Fish and Wildlife Research Unit has focused on management and restoration of bottomland hardwood forests to benefit wildlife species in the LMAV. Researchers and land managers are working cooperatively to develop forest management guidelines for public lands that will enhance wildlife habitat, particularly for migratory birds and the black bear.
Most bottomland hardwood forests are relatively young because of extensive harvesting at the turn of the 20th century, and they lack characteristics of older stands. For example, ongoing research indicates that younger stands generally have less downed and standing dead wood, and other studies indicate that younger stands may also have fewer tree cavities. Although downed and standing dead wood may seem like an economic loss, it is highly valuable for many species of wildlife, including many species of songbirds, reptiles and amphibians.
There is also concern about the current stress levels of trees and the ability for some forest stands to regenerate. Large-scale flood control and navigation measures have altered surface and subsurface hydrologic patterns, resulting in widespread changes in forest composition. In some cases, sites have become much wetter, whereas in others, sites are much drier. Thus, overstory trees became established under very different conditions than presently exist, and the ability to regenerate a valuable stand is often compromised. For example, recent studies indicate sites along channelized streams had significantly higher sediment deposition than those along unchannelized streams with as much as 32 inches of sediment deposited in one year. Channelized streams also supported less economically valuable species, such as maple and box elder, whereas unchannelized streams supported a greater number of oaks and hickories.
Restoring bottomland hardwood wetlands is also challenging because of the long time frame necessary for these forests to mature and because of altered flooding patterns. Initial research results, however, are encouraging. Research was conducted to evaluate the effectiveness of restored wetlands in providing habitat to amphibians, particularly frogs. Amphibian populations have experienced dramatic declines worldwide as a result of habitat loss, diseases and other factors.
This loss is of concern not only because of their overall contribution to biodiversity and ecosystem function, but also because amphibians are an important food resource for many other species of wildlife. Studies indicate that restored wetlands are rapidly colonized by most species of frogs. In Avoyelles Parish, 11 frog species of 12 that occur in the area were observed in wetlands restored through the Wetlands Reserve Program. Studies indicate that dense shoreline vegetation, abundant vegetation in the pond, and mature forest cover in upland areas near the wetland are important to amphibian habitat in restored wetlands. Similarly, other researchers have found substantial use of restored areas by songbirds and waterbirds. It is expected that the types of animals using these sites will change as the forests mature.
As the forests mature, new challenges are expected. How do we manage these stands to get a desirable mix of high quality species beneficial to wildlife and economically valuable? Because open grown oaks are generally of poorer quality than oaks grown in partial shade, can we expect the first rotation to be a high quality stand? Researchers have found that cottonwood interplanted with oak results in accelerated development of forest structure, rapid colonization by songbirds, and increased economic returns on some sites. However, is this method suitable on all sites? Are there alternatives? There are many questions to answer now and in the future.
Sammy L. King, U.S. Geological Survey’s Louisiana Cooperative Fish and Wildlife Research Unit and Adjunct Associate Professor, School of Renewable Natural Resources, LSU AgCenter, Baton Rouge, La.
(This article was published in the spring 2006 issue of Louisiana Agriculture.) |