Hydrologic and vegetation responses associated with restoration of wetlands in the Willamette Valley, Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/k3569658b

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  • I examined hydrological and plant community changes associated with the implementation of a restoration management plan in two riparian meadows located within an agricultural landscape of the central Willamette Valley, Oregon. I established exclosure fencing (a form of passive restoration) in one agricultural field and established fencing and plugged a drainage ditch (active restoration) in a separate agricultural field. Permanent transects 15 m in length were established within two plant communities associated with hydrological regimes within these restored agricultural fields. Plant communities were classified as wetland meadow (inundated for more than 4 weeks/year) and mesic meadow (saturated within the upper 30 cm but not inundated) for at least 4 weeks/year. Four transects were randomly established within the wet meadow community and 6 transects were randomly situated within the mesic meadow community. Two shallow subsurface piezometers were installed to a depth of 1 m at 5 m and 10 m along each of these 15 m transects. Additionally, two shallow sub-surface piezometers were established at the outer perimeter of the agriculturally excluded fields. Shallow sub-surface and surface water table levels were measured at each piezometer after wetlands were inundated and continued until water table dropped below the piezometers (Dec. - June) for one pre-treatment and two post treatment years. The actively restored wet and mesic meadows demonstrated increased water table elevation and a decrease in water table fluctuation during both post treatment years. Increases in water table elevation were greatest in areas closest to active restoration but were significant up to 102 m. from restoration. Results indicate that filling drainage ditches induce hydrologic effects at great distances across floodplain soils. Plant community composition (species response) was quantified in both restored sites as well as the adjacent agriculturally managed (untreated) sites one year before treatment and two post-treatment years. I sampled two plant community types: wet meadow and mesic meadow. I calculated species richness and the relative abundance of wetland indicator species, nuisance weeds, and native plants. Nuisance weeds increased and native plant abundance decreased in agriculturally managed mesic meadows. Wetland plant species abundance tended to increase in agricultural sites with light grazing, and decreased in areas that were plowed and re-seeded. Native plants increased and nuisance weeds decreased in the actively restored mesic meadow. The passively restored mesic meadow exhibited no change in native plant abundance and decreases in all other categories. In the actively restored wetland there were increases in plant species richness and nuisance weed abundance with a decrease in native plant abundance. Agriculturally excluded wetlands dominated by Reed canary grass (Phalaris arundinacea) exhibited no changes for the entire study period. Results suggest that for the first few years following agricultural exclusion, nuisance weed species do not increase, but active restoration may result in increases (due to disturbance). Additionally, results indicate restored agricultural landscapes dominated by introduced grasses demonstrate minimal short-term plant community change unless initiated by intense land management practices (e g., plowing, re-seeding, or removal of dominant plant communities). Based upon results of this study, I conclude that restoration plans should repair damaged hydrological features before planting riparian plant species. Following this chronological sequence will minimize the potential destruction of planted communities by future shifts in water table elevation caused by hydrologic restoration. Furthermore, any active restoration that initiates a direct or indirect removal of the dominant plant community should be accompanied by aggressive plantings of desirable plant species and prolonged site maintenance.
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