Graduate Thesis Or Dissertation
 

Plants, plant communities, net production and tide levels; the ecological biogeography of the Nehalem salt marshes, Tillamook County, Oregon

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  • Nehalem Bay is located on the northern coast of Oregon in Tillamook County and contains approximately 243 ha of salt marsh. The Nehalem marshes occur as islands and land-tied units. West Island is the largest marsh island with an area of 82.9 ha and a maximum elevation of 3.079 m above mean lower low water. Three divisions on West Island based on inundation period have been identified: intertidal marsh, below mean high water; transitional marsh, from MHW to 2.76 m above MLLW; and extratidal marsh, above 2.76 m. West Island may be further subdivided into four topographic units based on the steepness of the elevation gradient: edge marsh, from.the lower margin to 2.00 m; low marsh, 2.00 to 2.36 m; transitional marsh, 2.36 to 2.76 m; and high marsh, 2.76 m and above. Creek density is low for the lower intertidal marsh, for the edge marsh, and.for the higher extratidal marsh; and is high for the upper intertidal and transitional marsh. Salt marsh vegetation-on West Island was sampled by harvesting aboveground biomass along seven transects at two-month intervals from May of 1972 to September of 1972. Plant species diversity increases with elevation and each species displays a particular elevation range. Analysis of dry-weight data by an ordination routine suggests the presence of 11 plant communities along the elevation gradient on West Island. Plant communities named by dominant species are Triglochin, Scirpus, Carex (tall and short phases), Carex-Deschampsia-Triglochin, Triglochin-Deschampsia, Carex-Deschampsia-Triglochin-Agrostis, Juncus- Agrostis, Juncus-Agrostis-Festuca, Aster-Potentilla-Oenanthe, Carex- Aster-Oenanthe, and Picea-Salix. Plant communities are readily identified by signatures on aerial photographs. Net aerial production on West Island increases with elevation and varies from a minimum of 518 g/m2/yr for the Triglochin community to a maximum of 1936 g/m2/yr for the Aster-Potentilla-Oenanthe community. The community mean net aerial productivity is 1388 g/m2/yr. More than 90 percent of the intertidal marsh net aerial production is removed to the estuary as organic detritus, and net aerial production in the high marsh is rapidly incorporated into marsh soil. It is estimated that approximately 56 percent of all net aerial production on West Island is transported to the estuary by the tides. It is recommended that salt marshes be preserved in their natural state and new salt marshes be created to ensure the continued high productivity of estuarine ecosystems in Oregon.
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