Graduate Thesis Or Dissertation


Plant community dynamics in remnant and restored Willamette Valley wetland prairies Public Deposited

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  • Invasion by exotic species can pose a major challenge for developing native plant communities in wetland restoration projects. Often native plant communities do not develop as anticipated in restored wetlands due to colonization by exotic species that dominate the native plant community. Despite the time and expense to restore wetlands, there has been little long term research to compare plant communities in restored and natural wetland sites. Research into plant community diversity across several wetland sites over several years can provide a broader perspective into how these ecosystems recover from long-term disturbance. The objective of this study was to compare plant community change from 2000 to 2005 between restored and remnant wetland prairie sites in the southern Willamette Valley in Oregon to determine if exotic species abundance was consistent between these groups. Specific objectives included 1) comparing the diversity and abundance of all species in remnant and restored wetland prairie sites, 2) evaluate the trajectory of community change between remnant and restored wetland prairie sites to determine if there was rapid change in restored sites, and 3) describe the plot level heterogeneity of the plant community in all sites to determine how microsites influence diversity. In 2005, species abundance was re-measured in four remnant wetland prairies and four restored wetland prairies that had been selected for an unrelated vegetation survey in 2000. Species were characterized by life form, origin, and wetland indicator status. Species abundance between groups of remnant and restored sites were compared using a multi-response permutation procedure (MRPP). The plant community trajectory was evaluated with nonmetric multidimensional scaling (NMS) and tested for significance with multivariate analysis of variance (MANOVA). Species area curves were compared between sites and within remnant and restored groups of plots. Within-year and between-year significance tests indicated that remnant and restored sites were similar in exotic species abundance, graminoid abundance, and wetland species abundance with no significant difference between these remnant and restored wetland prairie sites. Individual sites in both groups experienced changes in exotic species abundance which confounded the statistical results. Species heterogeneity was no more spatially diverse across the remnant site plots than restored site plots. Species area curves did not show significant differences between remnant and restored plots but individual plots did show homogeneous community characteristics at smaller spatial scales. Restoration sites had developed high graminoid cover by the 2000 survey which was conducted two to three years after restoration was initiated. All sites were equally likely to contain exotic species. Exotic species common across all sites included Centaurium umbellatum, Holcus lanatus, and Hypericum perforatum. Native species common across sites included Deschampsia cespitosa, Danthonia californica and Juncus tenuis. These results suggested that differences between remnant and restored Willamette Valley wetland prairie sites were not generalizable at the landscape scale and were more dependent on site specific management activities and local barriers to colonization. Five years may not be enough time to see evidence that suggests if restored plant communities will develop spatial characteristics of the remnant sites. This research does suggest that multi-site comparisons help distinguish individual sites that are not developing characteristics of remnant wetland plant communities.
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