PARENT SESSION
Oral Session #20: Urban Ecosystems: Comparisons Across Biomes, Patterns and process.
Presiding: M. Carreiro
Monday, August 5. 1:00 PM to 3:45 PM. Grand Ballroom West, Radisson.

Patch area effects on urban riparian native plant assemblages.

Youngman, J.^*,1, Sharp, M.¹, Yeakley, J.¹, ¹ Portland State University, Portland, Oregon

ABSTRACT- We hypothesized that urban riparian plant assemblages have higher native plant species richness, diversity and cover with increasing patch area and decreasing patch edge. We randomly selected sites within the urban growth boundary of the Portland, Oregon metropolitan area using digital aerial photos. Selection criteria included: no vegetation management for at least 10 years, slope less than 100%, at least 7.5 m of uninterrupted vegetation along stream, no road-crossing within 10 m, no built structures and the presence of a permanent stream. Thirty sites (17 public, 13 private) were selected. Using the line-intercept method, we sampled three transects per site, identifying all herbaceous and woody species at 1cm intervals from the streambank to the upland edge of the vegetation. Using a geographic information system, we determined landscape variables including patch area (A) and perimeter to area ratio (P:A). We compared landscape variables to native plant species richness, diversity (Shannon) and percent cover using regression analysis. We found significant (p<0.05) positive correlations between patch area and both native species richness (r²=0.189) and percent native cover (r²=0.147). P:A was significantly negatively correlated to native species richness (r²=0.25), to percent native cover (r²=0.222) and to native species diversity (r²=0.168). These results suggest that both total patch area (A) and the ratio of perimeter to area (P:A) are related to native plant species richness and percent cover in urban riparian areas.

KEY WORDS: biogeography, edge, native plant cover, native plant diversity