Salt marsh reestablishment following dike breaching in the Salmon River estuary, Oregon

Graduation date: 1982Presentation date: 1981-08-21A 22 ha, 1S yeas old, diked permanent pasture in the Salmon River estuary in Lincoln County, Oregon, was chosen for a study to determine the potential for natural salt marsh restoration with dike breaching. Two undiked fragments of relatively undisturbed salt marsh, one at each end of the study site, were used as "control" marshes. A permanent plot-permanent transect system was set up for vegetation and soils sampling before and after dike breaching. Three pasture communities, the Potentilla pacifica community, the Agrostis aiba-Holcus lanatus community, and the Holcus lanatus community, were described before dike breaching. Elevational means for each community were not significantly different, and spatial distribution appeared to be most influenced by cultural practices. Vegetative cover in\ud 49-1.0 m2 permanent plots was also reccrded for comparison with post-dike breaching data.\ud Half the dike was leveled in September 1978 and tidal creeks reopened. Vegetation resampling in 1979 and 1980 has shown that\ud upland pasture-type species suffered close to 100% mortality by 1979. Somewhat salt-tolerant residual species, mainly Agrostis albs and Potentilla pacifica, either expanded into areas previously occupied by\ud upland species or died back depending on degree of flooding and/or drainage within a local area. Permanent plots were grouped into four persistence classes (P.C.) based on cover of residual species. They ranged from P.C. I with 100% cover in 1979 and 1980 to P.C. IV with 30% mean cover of residual species by 1979. P.C. I permanent plots\ud average 10-15 cm higher than P.C. IV plots, and are furthest from tidal creeks and/or locally better drained. Intensive sampling of colonizing species was done in 1979 and 1980\ud on over 3000 m2 using the permanent plot system. By 1980, in areas of high residual species cover (P.C. I and II), the most successful colonizers were Atriplex patula (up to 18% cover locally), Hordeum\ud brachyantherum (to 11%), and Carex lyngbyei (to 6%). In P.C. III and IV areas, where there was mostly bare mud, Spergularia marina (to 11%) and Salicornia virginica (to 12%) have colonized most rapidly. Because of 0.30-0.40 m subsidence of the diked site below the elevation of the "control" marshes, intertidal to low-transitional salt marsh is expected to develop. Future net primary productivity is projected to be in the 1200-1800 g/m2/yr range, compared to 800-1200\ud g/m2 /yr for the higher transitional marshes on undisturbed Salmon River sites. Soils changes on the study site include increase in interstitial soil water salinity from zero on the diked site to 18 to 30 ppt summer salinity (5-10 ppt higher than "control" marshes). Soil pH which was lower (4.7) on the diked site than the "control" marshes (5.5) has returned to the level of the "control" marshes. Higher concentrations\ud of NH4 on the breached site indicate more anaerobic conditions with increased flooding and poorer drainage