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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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