Watershed urban development and riparian vegetation community composition in the Portland-Vancouver metro area

Background/Question/Methods

The purpose of this study was to identify relationships between specific urban development features and the community composition of riparian vegetation.  Urban structures and processes can dramatically change riparian areas, altering vegetation communities and resulting riparian processes.  While general trends in vegetation response to urbanization have been suggested, it is unclear how specific urban features within a watershed contribute to those changes.  In this study, we hypothesized that non-native cover in riparian areas would be directly correlated with high impervious surface cover in the watershed, and that wetland obligate species would be inversely correlated with impervious surface cover and positively correlated with forest cover.  Vegetation was surveyed along three transects in each of 31 randomly-selected riparian sites on public and private property in the Portland-Vancouver metro area during the summer of 2011.  Transects ranged from five to 60 meters in length.  Species were categorized by native status, structure class, and USDA wetland indicator status.  Land cover data from 2006 in the National Land Cover Database and GIS were used to determine the percentages of different cover types in the 31 urban watersheds.

Results/Conclusions

We identified a total of 82 species, including 65 native and 17 non-native species.  Linear regression results indicated that the type of land cover within urban watersheds may affect near-stream riparian vegetation community characteristics.  Within the first five meters from the stream, relative cover of non-native species and shrubs was positively associated with watershed impervious surface and medium to high development cover, and negatively associated with watershed forest cover (p<0.05).  Cover of herbaceous and native species was positively associated with watershed forest cover and negatively with impervious surface and medium to high development cover (p<0.05).  Shannon diversity was also positively associated with watershed forest cover (p<0.05).  No relationship was found between relative cover of different wetland indicator types in the near-stream area and watershed land cover type. These preliminary results support the hypothesis of increased non-native cover in basins with high development, and suggest that high forest cover in urban watersheds may help to buffer against non-native invasions of near-stream riparian communities.  Additionally, watershed forest cover may be important for the maintenance of riparian herbaceous species in the near-stream area.