COS 104-4 - Evaluating the impacts of multiple stressors on community structure of a plant ecotone

Abstract: Changes in environmental conditions can affect the distribution of, and interactions between, plant species. However, stressors rarely occur in isolation, and their interactive effects can have unpredictable impacts on the structure and functions of plant communities. Therefore, experimental studies on the impacts of multiple stressors on plant communities are critical to predicting the potential outcomes of global environmental change. Tidal wetlands are characterized by ecotones comprised of functionally distinct plant species, making them optimal environments for conducting such studies. To investigate the interactive effects of multiple stressors on plant community structure along a tidal marsh ecotone, we conducted an 8-month-long greenhouse study in which we manipulated flooding and sedimentation in mesocosms and evaluated their impacts on species-specific plant growth. In April 2022, we randomly assigned both flooding (drained, saturated, flooded) and sedimentation (none, medium, high) treatments to 45 intact vegetation sods containing both Spartina alterniflora, the more stress-tolerant species, and Juncus roemerianus, the stronger competitor (n = 5 per treatment combination). From April-December 2022, we administered daily water-level manipulations and monthly sedimentation treatments. Additionally, we made monthly measures of porewater chemistry and species-specific stem densities and heights, and we conducted a species-specific end of study above- and belowground biomass harvest.

While flooding did not have a significant impact on either Spartina or Juncus aboveground growth (i.e., percent changes in biomass, stem densities, and stem heights), sedimentation did, but only for Spartina. Changes in Spartina stem density were greatest in the high and no sedimentation treatments, while changes in Spartina aboveground biomass were greatest in the no sedimentation treatment. Preliminary analysis suggests that both flooding and sedimentation may have had significant impacts on belowground biomass and that those impacts may be species-specific. For example, we found that total belowground biomass in the sediment-addition layer was greater for the medium sedimentation treatment than the high sedimentation treatment for both species. However, the impacts of flooding differed between the two, with flooding stimulating belowground production in Spartina (stress-tolerator) and having minimal impacts on Juncus (competitor). These preliminary results suggest that multiple stressors can have distinct impacts on the above- and belowground components of plant communities, and the magnitude and type of these effects may depend on plant functional traits (e.g., stress tolerance and competitive ability).