Dune ecosystems may be viewed as a series of gradients related to various environmental factors, which operate on different spatial and temporal scales. If we view a profile from the sea landward, we first have the beach (near-shore and back-shore), the embryo or incipient dunes, and the foredune. The first dune ridge (the next inland from the foredune) is normally the highest and forms a continuous sand structure. The second is an older dune ridge, frequently lower because of the reduction in sand supply and the gradual loss of sand. This formation occurs when we have a series of parallel ridges, formed by onshore winds, each ridge lower than the previous. Sand is trapped by vegetation and saltation cannot be initiated beneath the vegetation, unless a blowout forms. Older dune ridges become fragmented when blowouts and parabolic dunes develop. Parabolic dunes are formed when prevailing winds blow at right angles to the dune ridges. Poorly stabilized regions are rapidly eroded but the more vegetated areas on either side remain covered by plants for a longer time. As the bare sand of the central region moves inland, the two horns or tips of the parabola remain attached to the relatively stabilized sand of the trailing ridges. A slack (a dune depression where sand has been blown away until the water table is exposed) may be formed in the middle, between the parabola arms. Parabolic dunes can also be formed in transverse dunes.
Throughout the dune field, there are gradients in salinity, sedimentation, nutrients, flooding, and shelter. Dune vegetation forms a complex spatial mosaic, mainly because of variations in physical gradients which depend on the distance to the sea and topography. Disturbances also result in temporal successions that add another dimension of complexity to the spatial mosaic described.
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