Reclaiming to Brackish Wetlands in the Alberta Oil Sands: Comparison of Responses to Sodium Concentrations by Carex atherodes and Carex aquatilis

The variation in sodium concentrations in waters of natural fens and marshes on the western Canadian landscape provides a background for choosing the appropriate plants for wetland reclamation. Broad tolerances to salinity are especially important for reclamation trials on saline-rich 'in-pits' that were left from open-pit oil sands mining. One such species, Carex aquatilis, has been identified as a key species in early reclamation attempts; however, at the Sandhill Wetland on the Syncrude Canada oil sands lease, this species has aggressively colonized, dominating parts of the wetland and limiting species diversity. A second species, also widespread on natural lake shores and marshes, is Carex atherodes, with field observations suggesting a broad tolerance to salinity. Here, we examine the responses of this species to a series of sodium concentrations and compare these to those of C. aquatilis. In particular, we addressed three questions: (1) How do structural attributes of C. atherodes respond to a series of Na+ concentration treatments? (2) Are different structural responses related to the functional attributes of photosynthesis, stomatal conductance, and/or transpiration rate? (3) How do these responses compare to those of C. aquatilis? We implemented a phytotron experiment to test the responses of these two species to either five or six concentrations of sodium, ranging from 20 to 3000 mg Na+ L-1. In general, structural responses of C. atherodes did not differ between 50 and 789 mg Na+ L-1, while performances of all attributes were reduced at 1407 mg L-1. Physiological attributes had high variation, but also had reduced performances at similar treatment levels. In comparison, a clear threshold was present for structural attributes in Carex aquatilis between 1650 and 2148 mg Na+ L-1, while physiological attributes were reduced between 1035 to 1650 mg Na+ L-1. These responses from C. aquatilis were similar to those previously reported. Na+ concentrations in porewater at the Sandhill Wetland in 2019 reached as high as 1200 mg Na+ L-1, with natural subsaline and sodic sites ranging much higher. Although all of the plants in the treatments remained viable at the end of the experiment, these results indicate that Na+ concentrations above 1500-2000 mg Na+ L-1 may inhibit the growth of these two species and decrease their competitive abilities.

Automated summary

The study examined the responses of two wetland plant species, Carex atherodes and Carex aquatilis, to different sodium concentrations. Structural and physiological attributes of the plants varied with sodium levels, with Carex atherodes showing reduced performances at 1407 mg Na+ L-1, while Carex aquatilis had a clear threshold between 1650 and 2148 mg Na+ L-1. High sodium concentrations above 1500-2000 mg Na+ L-1 may inhibit the growth of these species in wetland reclamation.