Environmental Monitoring & Assessment Program

Floristic Data for 96 Palustrine Emergent Wetlands in Portland, Oregon

1999. Floristic Comparison of Freshwater Wetlands in an Urbanizing Environment. Wetlands 19(3):517-534.

We evaluated the floristic condition of freshwater palustrine wetlands dominated by wet meadow, emergent marsh, aquatic vegetation, or open water within the rapidly urbanizing area of Portland, Oregon, by (1) characterizing plant species richness (presence/absence) and composition of naturally occurring wetlands (NOWs) and mitigation wetlands (MWs), and (2) identifying relationships between floristic characteristics and variables describing land use, site conditions, and mitigation activities. Data were collected on 45 NOWs and 51 MWs. Overall species richness was high (365 plant taxa), but more than 50% of the species present on both NOWs and MWs were introduced. Only 14 species occurred on more than half the sites and nine of these were invasive nonindigenous species. The mean number of native species per site did not differ between land use categories (ANOVA, p=0.6031), however, wetlands surrounded by agricultural and commercial/industrial/transportation corridor uses had more introduced species per site than wetlands surrounded by undeveloped land (p<= 0.05). Although overlapping in floristic composition, NOWs and MWs had significantly different (MRPP, p < 0.0001) species assemblages that were identified using TWINSPAN. MRPP analyses for all sites showed that watershed, land use, HGM class, percent cover of water and MW age were significantly related to the floristic composition of the study wetlands. Canonical correspondence analyses further revealed that the primary gradient for species distribution in NOWs was related to moisture; the secondary gradient was related to land use. The primary gradient also described a strong relationship between percent cover of water and HGM class. For MWs, the primary gradient was related to watershed location and surrounding land use; the secondary gradient was related to percent cover of water and MW age. Most MWs (44 out or 51 sites) were depressions in various settings, so while HGM class separates NOWs from MWs, it does little to distinguish MW assemblages. Our results show wetlands in the urbanizing study area are floristically degraded. Further, current wetland management practices are replacing natural marsh and wet meadow systems with ponds, resulting in changes in the composition of plant species assemblages.

¹ Dynamac Corporation, 200 SW 35 th Street, Corvallis, OR 97333
² United States Environmental Protection Agency, NHEERL-WED, 200 SW 35th Street, Corvallis, OR 97333
² Correspondence author: e-mail: kentula@.epa.gov
³ Present address: Department of Fisheries and Wildlife, Oregon State University, Corvallis, OR 97331

biodiversity, canonical correspondence analysis(CCA), hydrogeomorphic classes (HGM), introduced species, mean similarity dendrograms, multiple response permutation procedures (MRPP), native species, nonindigenous species, Oregon, USA, TWINSPAN, wetland mitigation, urban ecosystems.