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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).〔King, S. L., D. J. Twedt, and R. R. Wilson. 2006. The role of the wetland reserve program in conservation efforts in the Mississippi river alluvial valley. Wildlife Society Bulletin. 34: 914-920.〕 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. == Management == 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),〔Fredrickson. L. H. and D. L. Batema. 1992. Greentree reservoir management handbook. Gaylord Memorial Laboratory, University of Missouri.〕 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).〔Fredrickson, L. H. and F. A. Reid. 1988. Waterfowl management handbook. USFWS Fish and Wildlife Leaflet 13.2.1. Washington D. C.〕 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).〔Young, G. L., B. L. Karr, B. D. Leopold, J. D. Hodges. Effects of greentree reservoir management on Mississippi bottomland hardwoods. Wildlife Society Bulletin. 23: 525-531.〕 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).〔Kellison, R. C. and M. J. Young. 1997. The bottomland hardwood forest of the southern United States. Forest Ecology and Management. 90: 101-115.〕 Older research reflects the view that standing water in GTRs permanently damages trees and acorn production. (Young et al. 1995, Wigley and Filer 1989). However, more current research (King et al. 2006)〔King, S. L., D. J. Twedt, and R. R. Wilson. 2006. The role of the wetland reserve program in conservation efforts in the Mississippi river alluvial valley. Wildlife Society Bulletin. 34: 914-920.〕 shows that when managed properly, green tree reservoirs can be very productive. Many researchers have found that production in GTRs is highest when managers try to emulate natural hardwood wetlands (Fredrickson and Reid 1988,〔Fredrickson, L. H. and F. A. Reid. 1988. Waterfowl management handbook. USFWS Fish and Wildlife Leaflet 13.2.1. Washington D. C.〕 King and Allen 1996).〔King, S. L. and J. A. Allen. 1996. Plant succession and greentree reservoir management and restoration of bottomland hardwood wetlands. Wetlands. 16:503-511.〕 In 1985, 95% of GTRs were flooded annually and for longer than the dormancy season of the forest (Wigley and Filer 1989).〔Wigley, T. B. and T. H. Filer. 1989. Characteristics of greentree reservoirs: a survey of managers. Wildlife Society Bulletin. 17:136-142.〕 Many plant species are adapted to a higher water tolerance; working with those plants rather than trying to force a dry-adapted plant to function in a wetland has increased GTR success. For example, pin oaks are a wetland oak and have small acorns suitable for waterfowl. In one study, researchers found that pin oak acorns even suffered less insect infestation (and therefore produced more usable food items for waterfowl) in flooded areas than in non-flooded forests (McQuilkin and Musbach 1977).〔McQuilkin, R. A. and R. A. Musbach. 1977. Pin oak acorn production on green tree reservoirs in southeastern Missouri. 41: 218-225.〕 Encouraging wetland plants suited for seasonally flooded areas supports the natural wetland model and results in better food sources. In another example, it was found that fall flooding decreases grain availability for spring-migrating waterfowl (Greer et al. 2007). Now that more research has emerged detailing the natural life history of wetland plants and trees, GTR managers are better prepared to manipulate the resource. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「green tree reservoir」の詳細全文を読む スポンサード リンク
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