Silviculture is the art of producing and tending a forest; the application of the knowledge of silvics in the treatment of a forest; the theory and practice of controlling forest establishment, composition and growth.
The main limiting factor on the Mississippi River that determines what species of trees can grow is the tolerance to flooding. In the annual cycle of the River, flooding occurs, sometimes for extended periods. The tree species that are present are adapted to the hydric conditions that exist. When the Nine-Foot Navigation Project was completed, the higher water elevations increased hydric conditions in the soil. Some areas that could once support species such as the water tolerant oaks and hickories can no longer support these species, and now support the more water tolerant species such as cottonwood, silver maple, willow, and green ash.
Why is Silviculture Important?
Of the species present, most are shade intolerant. This means that they will not thrive under low light levels, shaded from sunlight. As the current generation of old, mature trees die, there are no younger trees in the understory to take their place. A lot of these canopy gaps are being filled with more aggressive species such as reed-canary grass. The seed dispersion characteristics of the dominant species, silver maple, cottonwood, willow, and green ash, is wind blown. Thousands of seeds are dispersed from each tree, and cover the ground during different times of the growing season, depending on the species. These species will readily invade and populate an open, exposed-soil site in the floodplain, and are therefore referred to as pioneer species because they are the "first" to establish a site. Some of these species will pioneer on wetter sites than other species will. For example, willow will grow on the wettest site that has exposed soil.
How Long Do the Trees Last?
Mature trees in the Mississippi River's forests reach can grow to become 100 years old, but rarely over 150 years. The rich alluvial soil and readily-available water cause most bottomland trees grow quickly, and attain a large size in a relative short period of time. Trees in the surrounding uplands can grow to an older age, but will typically not become much bigger than their bottomland counterparts. Because these trees attain a large size in a relatively short period of time, the wood of these trees is typically softer, and therefore deemed not as desirable for timber as other hardwoods that grow much slower and for longer periods. These trees do form cavities quite easily due to the soft nature of the wood, and excess moisture. Cavity development is good habitat for certain wildlife, particularily wood ducks.
Hard mast trees were once more prevalent than they are today in the bottoms. Hard mast trees are ones that produce nuts, with a hard seed coat. Historical cutting practices, and the increased hydric conditions from the altered flood plain have decreased the relative abundance of these hard mast species. The hard mast species that grow in the Mississippi River floodplain include bur oak, pin oak, swamp white oak, northern pecan, shellbark hickory, with some red oak and walnut being rare. These species will tolerate the flooding that occurs on the Mississippi River. They grow on slightly elevated ridges in the floodplain. A rise of ground of one to two feet can make the difference in being able to support these mast species. There is little regeneration to take the place of the maturing mast resource that exists. These species are intermediate to relatively shade intolerant, so there are few small trees under the large ones. Also, the seed is heavy, and therefore doesn't travel far when it drops from the tree. These trees have to depend on wildlife and flooding to bury or transport the seed. Elm was once a significant component of the overstory component. Dutch Elm Disease virtually eliminated elm in the overstory starting in 1950's. Some estimates put the component of elm at up to one third of the canopy layer. Today, elm is prevelant in the understory of the rivers' forests. It is shade tolerant and reproduces prolificly. It grows to a small pole size, before the disease kills the tree, with very few trees reaching a mature age. To view the Forest Service Silvics Manual, click here.
To ensure continued forest cover on the Mississippi River, the Corps' forestry program utilizes a variety of management techniques to maintain forest cover. Even Aged Silvicultural Teqniques are used to maintain the silver maple-cottonwood forest community. It is necessary to open up sufficient areas in the canopy, so these sun-loving species will grow. The Corps utilizes small patch cuts less than fifteen acres in size in areas where the trees are over mature and there is not enough regeneration to take the place of the existing forest. With the aid of the cooperating agencies, snag trees are marked to leave for nesting and roosting. Any existing hard mast trees are also left for wildlife food source and habitat, and to regenerate a mast component in the stand. Shelterwood cuts are planned for the future to try to get a handle on the tremendous weed competition due to the direct sunlight, and to capture the site more quickly with trees. Above, a patch cut two years after cutting, showing the tremendous Cottonwood growth Most of the harvested areas are left to regenerate naturally. Silver maple and cottonwood quickly revegetate most sites, regenerating a young, healthy stand of trees. The Corps annually does regeneration surveys to ensure this happens. In areas that will support hard mast species, trees will sometimes be planted, to restore the mast component that existed at one time.