In the dry climate of the Western United States, water is a limited resource not only for the wildlife but also for the human inhabitants. Although wetland areas comprise a very small portion of total land area (i.e., less than 2%), over 80% of wildlife is dependent on their presence. Rainfall in this region varies from less than 15cmyr_1 in the desert regions to greater than 140 cm in the mountains. In the mountainous regions, rainfall and snowmelt are greater than losses and, therefore, wetlands rarely dry out. However, evapotranspiration in the basin areas is 3-4 times greater than precipitation and, consequently, soil salinization is a stress to which vegetation must adapt. In the driest regions soil salinization prevents vegetative establishment. Ephemeral drains are prevalent in the intermountain west with snow melt and high rain contributing to their flow.
At higher elevations in the United States, where soils are semipermanently inundated or saturated, associations of Populus, Salix, and Acr are found. Floodplain areas flooded or saturated 1-2 months during the growing season are comprised of a wide array of hardwood trees. Common species in the United States include Fraxinus spp., Tilia spp., Ulmus spp., Liquidambar spp., Celtis spp., Acer spp., Plantanus spp., and some Quercus spp. At the highest elevations, flooding occurs for less than a week to about a month during the growing season. Typical tree species include a variety of Quercus spp. and Carya spp., with some Pinus spp.
Floodplains of the southeastern United States occur within three physiographic regions: (1) coastal plains, (2) piedmont, or (3) Appalachian Mountains. Rainfall is sufficiently prevalent during all seasons except for brief periods of drought. Successional patterns of southern forested floodplains are often dictated by hurricanes, tornados, catastrophic ice storms, and extended drought. Soils are typically acidic, with the exception of near neutral pH soils across much of the Southern Mississippi Alluvial Floodplain and the Selma Chalk geologic region of Alabama and Mississippi. In many floodplains, as one moves in a direction perpendicular to the river, soil textures range from coarse sands near stream channels, fine sands in natural levees, to loams and clays in backwater areas. This separation pattern is a result of particle size and sheet flow velocity.
The lowest elevation, nearly always flooded sites on floodplains in the southeastern United States are occupied by Taxodium-Nyssa swamps. In other parts of the world, it appears there are no similar tree species that can survive permanent or long periods of inundation. As long as the floodplain channel remains stable and flooding frequency remains constant, these species should dominate the stands indefinitely.
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