Biogeographic and Land Cover Effects on Urban Riparian Plant Assemblages

Jessica Lynn Youngman

Master of Science in Environmental Science

Portland State University

Presented July 11, 2002.

Abstract

I hypothesized that differences in biogeographic factors and upland land cover types would affect native plant assemblages in urban riparian areas. Thirty sites were randomly established in publicly and privately owned riparian areas within the Portland, Oregon metropolitan area. Total, native, and non-native species richness, Shannon diversity, and percent cover were compared to riparian patch size, perimeter-to-area ratio, area of conifer, mixed canopy, grasses, and built-up area.

A total of 123 plant species were found in the urban riparian sites, 87 native and 36 non-native species. For the entire study, 58% of the vegetation coverage was native, and 42% was non-native. Using linear and forward stepwise regression I found the amount of edge, measured by perimeter-to-area ratio, is important in determining native species composition. In forward stepwise regression analyses, edge was the only variable to significantly (p<0.05) predict native species richness, diversity, and percent cover with R² values of 0.252, 0.224, and 0.169 respectively.

Increased edge, may reduce native species richness, diversity, and percent cover because non-native invasive (shade intolerant) species are often favored by the microclimatic differences created by an increase in edge. Studies have found that edge-influenced microclimatic changes may occur 30-240 meters into a forested patch and include changes in light regime, wind speed, soil and litter moisture content, humidity, and air temperature. Urban disturbance along these edges may also be particularly destructive to native plant assemblages because, as opposed to more natural environments which may recover, urban disturbance is often permanent. Newly disturbed areas may not have the opportunity to replace lost species because the disturbed area was converted to pavement, housing, or lawn, hence halting the natural succession process.

Management efforts should attempt to preserve remaining native species diversity in urban riparian areas in order to promote high future native diversity and preserve the important ecological functions of riparian areas. When managing urban riparian areas for native plant species, habitat patch characteristics must be considered. Patch size should be maximized and edge minimized when determining riparian preservation and restoration throughout the metropolitan area.