Impacts of stressors on the emergence and growth of wetland plant species in Pennsylvania, USA

Agriculture and urbanization in the upland landscape often generate stressors, such as sedimentation and eutrophication, that may negatively impact wetland plant communities. Individual plant species responses to stressors are also likely influenced by the hydrologie and geomorphologic characteristics of different wetland hydrogeomorphic (HGM) subclasses. The goal of this study was to determine to what extent the two stressors listed above influence the growth and development of wetland plant species and how these responses vary across HGM subclasses. The impacts of the two stressors on wetland plant species emergence and growth were examined in a 2 × 2 factorial greenhouse experiment, using soil moisture and organic matter content to simulate three HGM subclasses. Both stressors elicited significant responses in species establishment and growth, but overall trends in stressor responses differed by wetland type. For instance, sedimentation reduced the emergence of four species in simulated riparian depressions but only affected one species in simulated slope wetlands and none in simulated headwater floodplains. Sedimentation had little impact on any species establishment or growth in headwater floodplains. Approximately half of the species showed an increase in at least one growth variable in response to nitrogen enrichment. Additionally, the conditions used to simulate HGM subclasses appeared to impact a species performance and sensitivity to stressors. Therefore, further investigation of the impacts of HGM subclass conditions on species performance is needed. We expected differences in species responses to be related to their natural distribution across pristine and impacted wetlands; however, no obvious relationships were found. Our results suggest that it will be difficult to make generalizations about species responses to stressors because responses varied by stressor type, stressor interactions, and HGM subclass. Therefore, perhaps the next step in formulating broad generalizations and predictions about wetland plant community composition will require a shift from species-based approaches to a trait-based approach for examining how species composition may change in response to future disturbances.