Green tree reservoir

Green tree reservoirs (GTRs) consist of bottomland hardwood forest land which is shallowly flooded in the fall and winter.

Prior to modern industrialization and commercial farming, the Southeastern United States was home to more than 10 million hectares of bottomland hardwood forest. Today, there is only about 2.8 million hectares remaining (King et al. 2008). Not only are southeastern hardwood forests disappearing, but there has been a huge effort to control water flow so that natural forest flooding is less frequent. Bottomland hardwood forest is an important wetland habitat for many species of waterfowl, as well as other animals. In order to support migrating waterfowl, both private land managers and public agencies implement green tree reservoirs. In a GTR, a forest stand is flooded to create a seasonal wetland. These seasonal wetlands provide habitat and food for waterfowl, amphibians, and many other species.

The water level on green tree reservoirs are manipulated by flooding and “draw down,” a process in which water is drained off of the reservoir. There are many methods for flooding a forest stand (Fredrickson and Batema 1992), many of which use levees or dykes. Water is either diverted from a natural source or pumped onto the landscape. This usually occurs in early winter, when trees are dormant and less likely to be damaged by standing water. The water should be drawn down before the tree dormancy stage has ended. This dormant season coincides with waterfowl migration, providing habitat for migrating birds.

In most cases, GTRs are flooded to a depth of 2-10 inches. Waterfowl in the order Anatinae are dabbling ducks that require only a shallow water depth. Diving ducks are less common on the Mississippi Flyway, but they also can utilize a water depth of only 10 inches. The water depth in a GTR will likely not be uniform. Various water depths allow for different types of food to be available at one time (Fredrickson and Reid 1988). Every species of animal that uses a GTR has slightly different food and habitat needs. Knowing the life history of the target species is critical to proper manipulation of a GTR.

There are plant and tree species specifically beneficial to waterfowl (discussed below), and the abundance of these species and forest composition can be managed by adjusting flood and draw down on the reservoir (Young et al. 1995). Bottomland hardwood forests can also be effectively manipulated by removing timber; in some cases it is required to maintain viable forest stands (Kellison and Young 1997).

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