Abstract
We examined whether differences in biogeographic factors in riparian areas were associated with changes in plant species diversity, contrasting rural and urban riparian zones in northwest Oregon. A total of 35 sites were established: 17 in rural watersheds and 18 in urban watersheds. At each site, vegetation was identified and mapped at 0.03 m intervals along each of 4 transects established perpendicular to stream, from streamside to the outside edge of the vegetated strip. Biogeographic factors were analyzed from digitized aerial photographs, and included riparian width, length, area and perimeter. A total of 138 plant species were identified, including 102 native and 36 non-native species. We found significant differences between urban and rural sites, with greater riparian area and length in rural riparian areas. Diversity (Shannon) of native plant species was greater in rural sites. In rural sites, native and non-native plant species diversity were significantly related to riparian perimeter-to-area ratio and to riparian width. In urban areas, however, no biogeographic factors were found related to plant species diversity. We suggest that more research is needed on factors including connectivity of riparian sites at the landscape scale, on disturbance history and on dispersal and establishment characteristics of riparian plants.