Effects of Elevated Root Zone pH and NaCl on the Stress Responses and Recovery of Selected Boreal Forest Tree Species

• Author / Creator

  Nathan Lauer

• Prior to surface mining of bitumen in the Athabasca oil sands region of northern Alberta, all vegetation and soil is removed from the boreal forests where the mines are located. Revegetation following mine closures is challenging, partly due to high soil pH and elevated Na levels present in many reclamation sites. My thesis addresses some of these revegetation concerns. In the first study, trembling aspen (Populus tremuloides), green alder (Alnus viridis), tamarack (Larix laricina), and white spruce (Picea glauca) were subjected to three levels of pH (5, 7, 9) and three levels of NaCl (0, 30, 60 mM) in a factorial design in hydroponic culture. Aspen was relatively tolerant of 30 mM NaCl treatments at pH 5 and 7, but showed decreases in dry weights, leaf chlorophyll concentrations, photosynthesis, and transpiration. Green alder was sensitive to elevated pH and NaCl because moderate increases of pH and NaCl caused significant physiological decline. Tamarack exhibited declines in dry weights, chlorophyll concentrations, photosynthesis, and transpiration as pH and NaCl levels increased. White spruce showed no changes in dry weights from elevated pH and NaCl; however, elevated NaCl levels caused declines in photosynthesis and transpiration. All species showed decreases in foliar nitrogen, dry weights, foliar chlorophyll concentrations, photosynthesis, and transpiration as a result of increased stress. I hypothesized that seedlings exhibited stunting of growth and downregulation of metabolism as a result of elevated pH and NaCl. In the second study, aspen and white spruce seedlings were maintained at two pH levels (5 and 8) with two levels of NaCl (0 and 30 mM). Another group of seedlings was subjected to the same pH and NaCl levels, but N supply was increased by 4x. Supplementation with 4x N caused a partial recovery of photosynthesis and transpiration in aspen exposed to elevated NaCl but had no effect on seedlings exposed to elevated pH. White spruce seedlings exposed to elevated pH and NaCl exhibited decreases in net photosynthesis. Supplementation with 4x N had no effects on photosynthesis in seedlings exposed to elevated NaCl but caused further declines in net photosynthesis of seedlings exposed to elevated pH. The study suggests that supplemental N can partially recover photosynthesis and transpiration in aspen exposed to elevated NaCl. In the third study, recovery from NaCl stress was investigated in aspen, tamarack, and white spruce seedlings subjected to 0, 50, and 100 mM NaCl treatments in soil. Most seedlings treated with 50 mM NaCl showed a return to non-stress levels for dry weight, foliar chlorophyll, photosynthesis, and transpiration after 30 days of recovery. Recovery after 60 days from 100 mM NaCl varied between species. Some aspen seedlings completely defoliated during the stress period and re-flushed during the recovery period. After 60 days of recovery, the new leaves exhibited higher levels of chlorophyll, photosynthesis, and transpiration compared to untreated controls. Following 60 days of recovery from 100 mM NaCl treatment, the dry weight, chlorophyll, and photosynthesis values in tamarack were lower compared with control seedlings, whereas white spruce showed no changes. Both aspen and tamarack exhibited increased foliar necrosis and K in response to NaCl stress, suggesting that both processes are important for NaCl stress and recovery. In the fourth study, NaCl stress was applied to aspen, tamarack, and white spruce seedlings, which were grown in soil and were first subjected to non-lethal NaCl stress followed by overwintering. Seedlings were then subjected to NaCl stress in the second year. Plants of all three species exhibited some form of salt injury in the first year. Aspen and tamarack seedlings treated with 50 mM NaCl in year one exhibited lower dry weights compared to non-treated control in year one. Both species exhibited elevated foliar K in response to NaCl treatment in year two. Tamarack seedlings exposed to NaCl in year one exhibited increases in photosynthesis and water use efficiency when exposed to NaCl in year two compared to control seedlings that were not treated with NaCl in year one. For white spruce, NaCl treatment in year two had no effect on any measured parameters. Taken together, the results suggest that aspen, tamarack, and white spruce can tolerate moderate levels of elevated pH and NaCl and they can also recover from moderate levels of NaCl stress.

• Subjects / Keywords

  • Plant stress physiology
  • oil sands reclamation

• Graduation date

  Fall 2020

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-6ve1-9a27

• License

  Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.