Master of Science in Environmental Science

Portland State University

Presented November 1, 1999.

Many cities are developing management plans to address preservation and protection of plant communities adjacent to urban streams. Ecologically sound management decisions depend upon an accurate understanding of changes that occur in riparian plant communities as urbanization continues. This study examined the distribution of native and non-native plant species, contrasting rural and urban riparian zones in northwestern Oregon. Objectives were: 1) to determine if there was a difference in composition of plant species between urban and rural riparian zones; and 2) to determine if there was a relationship between riparian corridor width and the proportion of native to non-native plant species.

A total of 35 sites were established: 17 in rural watersheds and 18 in urban watersheds. At each site, vegetation was identified and mapped along each of 4 transects established perpendicular to stream, from streamside to the outside edge of the vegetated strip. A total of 138 plant species were identified.

Greater total plant species diversity (Shannon) and greater non-native plant species diversity were found in urban sites (p<0.05). The number of non-native plant species was greater in urban sites ( >7 per site) than in rural sites ( < 5 per site). Diversity of native plant species was greater in rural sites (p<0.05).

Relative cover of native plant species was higher in rural (76%) than in urban (51%) sites. A positive correlation existed between width of the riparian area and both native and total plant species richness for rural and urban sites (p<0.02). Relative cover of native plant species increased with riparian width in rural sites (r2=0.35). Non-native plant species diversity decreased as riparian width increased in rural sites (r2=0.48).

Differences between rural and urban sites may be due to changes in the natural disturbance regime associated with increasing urban development and to increasing fragmentation and isolation of riparian habitats as urbanization continues. Management for ecologically diverse plant communities along urban streams should include measures that: 1) minimize disturbance in or near riparian areas; 2) preserve remaining riparian habitats; 3) reestablish connectivity among those habitats; and 4) increase diversity through control of non-native species and replanting with native species.