Dominant environmental factors in wetland plant communities invaded by Phragmites australis in East Harbor, Ohio, USA

Elevation, standing crop, disturbance and soil fertility often emerge from studies of freshwater plant communities as the dominant environmental factors determining both species richness and species composition. Few studies in North America have investigated the relationship between these factors and species abundance (standing crop) and species composition in the context of invasion by Phragmites australis. This study explores the influence of key abiotic and biotic variables on species abundance and composition across three Lake Erie wetlands differing in hydrology and Phragmites abundance in East Harbor, Ohio, USA. Standing crop for 92 species was related to standard sediment analyses, wave exposure, distance to shoreline, elevation, light interference, species density, and Phragmites standing crop in each of 95 1 × 1 m quadrats by using canonical correspondence analysis (CCA). Elevation (Axis I) and Phragmites standing␣crop-soil fertility (Axis II) explained 35.7 and 26.2%, respectively, of the variation in the species–environment relationships. Wave exposure was not a primary component of the first four canonical axes. Axis I was instrumental in describing species composition, separating wet meadow species from marsh species. Axis II was inversely related to species density for both wet meadow and marsh species. These findings generally support prevailing models describing the distribution of wetland plants along environmental gradients. Two discrepancies were noted, however: (1) species density was highest in the most sheltered sites and (2) wave exposure was directly associated with Phragmites standing crop-soil fertility gradient. The structural integrity of Phragmites stems, topographic heterogeneity and differential responses to anthropogenic disturbance may contribute to departure from prevailing multivariate models. This information has direct implications for local and regional wetland managers.