Riparian vegetation assemblages and associated landscape factors across an urbanizing metropolitan area

Abstract

While diverse, native riparian vegetation provides important functions, it remains unclear to what extent these assemblages can persist in urban areas, and under what conditions. We characterized forested riparian vegetation communities across an urbanizing metropolitan area and examined their relationships with surrounding land cover. We hypothesized that native and hydrophilic species assemblages would correlate with forest cover in the landscape. For each of 30 sites in the Portland—Vancouver metro area, we recorded vegetation at 1-cm intervals along 3 transects using the line-intercept method. Land cover was characterized at 2 scales: within 500 m of each site and across the entire watershed. Multivariate analyses were used to evaluate relationships between species composition and land cover patterns. A classification tree was created to determine landscape predictors of riparian community type. Results indicated a strong relationship between watershed land cover and vegetation diversity and structural complexity. Our hypothesis of native species association with landscape forest cover in urban riparian areas was supported, but we found no clear relationship between land cover and wetland indicator status. Our results suggest that high watershed forest cover (at least 15%) may enable the persistence of functionally diverse, native riparian vegetation communities in urban landscapes.