Magee, Teresa K., Ted L. Ernst, Kathleen A. Dwire, and Mary E. Kentula. 1999. Floristic comparison of freshwater wetlands in an urbanizing environment. Wetlands 19(3):517-524.
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, USA by (1) characterizing plant species richness (presence/absence) and composition of naturally occurring wetlands (NOWs) and mitigation wetlands (MW) 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 them were invasive introduced species. The mean number of native species per site did not differ between land-use categories (ANOVA,F = 0.62 at 3 and 88 df, 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 (Fishers Protected LSD at 88 df, 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. For MW, 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 of 51 sites) were depressions in various settings, so while HIM class separates NOWs from MWs, it does little to distinguish MW assemblages. Our results show that 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.