The effects of site preparation on native forb establishment in a wet prairie, Willamette Valley, Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/ff365945j

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  • Wetlands and wet prairies are economically and environmentally valuable ecosystems, but many have been degraded or converted to other uses. As human understanding of wetlands' value has increased, restoration efforts have grown correspondingly. Restoration attempts use a diversity of methods, which often include seeding with native plant species. This thesis reports on early results from a restoration project that was conducted in the Willamette Valley of western Oregon on a study site that historically supported wet prairie but had been converted to agricultural use. The site was withdrawn from agricultural use 11 – 12 years prior to the work reported here, contoured to reestablish wetland hydrological conditions, and planted with the native perennial, tufted hairgrass (Deschampsia cespitosa), in a preliminary attempt to recreate wet prairie conditions. The subsequent management goal, addressed in the work reported here, was to increase native species richness without increasing exotic plant cover by seeding the site with a mixture of 27 common wetland natives. In each of three blocks at the site, four units were treated either by burning (N = 2 per block) or mowing (N = 2 per block), then seeded with a mixture of 27 common native wetland species. One each of the burned and mowed units from each block was treated with glyphosate herbicide. Thus burning, burning + herbicide, mowing, mowing + herbicide, all with seeding of native species, and an unseeded control each occurred in one unit in each of three blocks at the site. The specific objectives of this study were to determine: 1. If treatments varied in their effects on plant communities, 2. Whether treatments differed in the degree to which they enhanced establishment of seeded species or other native species, 3. If so, which treatment was most effective at increasing cover or richness of native or seeded species, and 4. Consequences of treatments in terms of exotic species cover or richness. Although the low number of replications (N = 3) limited statistical inference, it appeared post-treatment plant communities differed among treatment groups (including controls) (Blocked Multi-Response Permutation Procedure; A-value 0.249 to 0.312; p-value < 0.005). While all four seeding treatments enhanced cover by native species compared to that in controls, burning without herbicide appeared to be particularly effective. Burning and burning + herbicide enhanced cover of seeded species more than did mowing treatments; while all four seeding treatments appeared to increase richness of native species overall, relative to that in unseeded controls, differences were not statistically significant. However, treatments also tended to increase cover and richness of exotic species relative to those in controls. While the increases in exotic species richness post-treatment were generally smaller than the increases in native species richness, changes in cover tended to be similar between the two species groups. Further monitoring of this site will be necessary to determine longer term consequences of these treatments for the plant communities.
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