Oregon Coast Dunes

The coastal forest stops at the edge of the dune field, except for islands of conifers and other vegetation. The area between the forest and the sea is comprised of high oblique and transverse dunes (photograph below, top) and low deflation plains (photograph below, bottom), where the wind has removed much of the sand down to the water table. These deflation plains have a more stable sand substrate that supports communities of plants adapted to wet soils (summary by Wiedemann et al., 1999:13-18).

To survive in the shifting sands of the dunes, plants typically have extensive rhizomes, stolons, or root systems, and many plants are low-growing to lessen the impact of the blowing sand (summary by Schultz, 1990:234-237). Although some grasses are native to the dunes (photograph below, top), European beachgrass (Ammophila arenaria) was introduced to stabilize sand dunes around harbors and roads. This grass has an extensive root system and seems to grow best where wind-blown sand accumulates quickly enough to bury and kill most plants. These are the conditions found above the high-tide line along the beach, where the ridge-like foredune has formed. Beachgrass also can form hummocks (mounds of stabilized sand; photograph below, bottom) behind the foredune (summary by Schultz, 1990:239-241).

The light-colored ridge across the center of the photograph below is the foredune that runs parallel to the ocean. The grassy foredune blocks the supply of new sand from the ocean that would otherwise feed the active dunes. Without the fresh supply of sand moving in from the ocean, the wind increasingly removes the sand behind the foredune and greatly expands the deflation plains, where a variety of plants become established (summary by Schultz, 1990:239-241). This process eventually would lead to stabilization of the entire dune field by vegetation that could cover all of the areas once occupied by shifting sand (summary by Schultz, 1990:329-330). In partnership with the Oregon National Guard, the U.S. Forest Service is conducting an experiment in which segments of the foredune are cleared in an attempt to restore the dynamics of shifting sand moving onto the dune field. The photograph below shows a cleared segment of the foredune at the Oregon Dunes Overlook, 16 km south of Florence. The process is long-term, but initial results suggest that new sand is beginning to move inland.

The transition forest of the dune field is comprised of many of the same species of trees and shrubs that we see in other forest areas along the coast. It also includes several small lakes that provide homes for ospreys (Pandion haliaetus) and many aquatic and semiaquatic organisms. Because of the high relief along much of the rocky coast, most natural lakes are found in the dune fields. Gradually, these lakes will fill with sand, soil, and detritus, which will allow aquatic vegetation to creep in from the margins. Eventually they will become freshwater wetlands that will later be claimed by the forest (summary by Schultz, 1990:244-248).

Among the unique plants growing in the wetlands of the Oregon dunes are cobra lilies, also known as pitcher plants (Darlingtonia californica). The pitcher-shaped leaves are partially filled with an enzymatic fluid that aids in the "digestion" of small organisms, such as insects, that are trapped within the tube. From these organisms, the cobra lilies obtain nitrogen that is unavailable in the bog soil they inhabit. Sundews (Drosera rotundiflora) also grow along the Oregon Coast, but they capture insects with a sticky fluid secreted by stalked glands on their round leaves.

Literature

• Schultz, S. T. 1990. The Northwest Coast: A Natural History. Timber Press, Portland, Oregon.
• Wiedemann, A. M., L. J. Dennis, and F. H. Smith. 1999. Plants of the Oregon Coastal Dunes, new edition. Oregon State University Press, Corvallis.

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