Impacts of Human-mediated Stressors on Wetland Plant Community Development

Human activities in terrestrial landscapes, such as agriculture or urban development, often result in stressors, which can subsequently alter wetland ecosystem properties and plant community development. A complication to our understanding of plant community responses to stressors is the variation in stressor impact across wetlands of different hydrogeomorphic (HGM) subclasses. HGM classification separates wetlands based on their hydrology and geomorphic setting, thereby providing a gradient of ecosystem function. This study examines whether stressors such as sedimentation, eutrophication and hydrologic modification, taken individually or in combination, can be indicated by predictable responses in the plant community. Using data from 46 wetlands in central Pennsylvania, four species indicative of relatively pristine sites and four species indicative of impacted sites were chosen to encompass a variety of life history and morphological characteristics for examination in a greenhouse experiment. The eight species were planted in both monocultures and in communities to examine variations in plant species responses to combinations of the three stressors, both with and without interspecific competition. Germination rates, aboveground biomass per plant, height and leaf area were measured and compared across treatments. Results showed that sedimentation significantly reduced the germination rates of many species, both in monoculture and community settings. Sedimentation significantly decreased species richness in riparian depressions. In communities, the aboveground biomass of the species indicative of impacted sites was significantly higher than for species indicative of pristine sites. For most species, leaf area was not significantly different between treatments. Height was not always positively correlated with aboveground biomass. Carex iii vulpinoidea provided the most consistent evidence for luxury nutrient consumption of N in the plant tissue analyses. Many species showed a significant increase in aboveground biomass per plant and/or height in the absence of neighboring species. Any Jack of an apparent effect of interspecific competition was likely a result of the high intensity of intraspecific competition in the monocultures. The importance of traits in determining community dominance varied depending on the type of stressor and wetland type. Species in riparian depressions showed a stressor-specific response: sediment stress increased the importance of germination and nutrient enrichment increased the importance of aboveground biomass per plant. Slopes and headwater floodplains did not show stressor-specific responses; all treatments separated species based on their biomass per plant. Phalaris arundinacea dominated the communities across all treatments for the three wetland types, with greater germination rates, heights, and aboveground biomass per plant. Further investigation is warranted on the implications of the dominance of aggressive species like Phalaris arundinacea on ecosystem functions.