Structural and functional responses of Carex aquatilis to increasing sodium concentrations

Carex aquatilis is a widespread boreal species that is abundant in open fens and marshes. The species has broad natural tolerances to differing water levels and ion concentrations including occurrences in brackish marshes but not sodic wetlands. Sandhill Wetland, constructed on Syncrude Canada Ltd. mineral surface lease in the Athabasca Oil Sands Region, is a research site that was engineered to support a self-sustaining wetland. The site is currently in its eighth year post wet-up and is partially dominated by C. aquatilis. Increasing Na⁺ concentrations in the porewaters at Sandhill Wetland are an on-going concern for the performance of the existing plant communities. Here we examine the responses of C. aquatilis to an increasing regime of sodium carried out in a greenhouse experiment. We posed three questions for this experiment: (1) is there a threshold response for performance of C. aquatilis to increasing sodium, (2) if so, does it occur in the range of sodium that is present at current reclamation programs, and (3) is C. aquatilis resistant to high concentrations of sodium? Carex aquatilis survived all treatment concentrations of sodium including the highest treatment of 2354 mg L⁻ ¹. In general, both structural and functional attributes of C. aquatilis did not differ between the 17 and 1079 mg L⁻ ¹ treatments; however, performances of all attributes were reduced in the 2354 mg L⁻ ¹ treatment. Belowground biomass had greater decreases compared to aboveground components, including both biomass and photosynthesis. The aboveground decreases in performance were associated with exclusion of sodium from the aboveground components by the belowground components. Reduction in photosynthesis was strongly correlated with reduced stomatal conductance and lower transpiration. Although C. aquatilis demonstrated a wide tolerance to sodium concentrations, a clear threshold was present between 1079 and 2354 mg L⁻ ¹. These decreases in performance in our greenhouse trials were at levels currently present at Sandhill Wetland, and careful assessment of sodium concentrations in the near future needs to be continued.