Modelling Short and Long-term Effects of Soil Cover Depth on Tree Growth and Productivity in Reclaimed Landscapes of Northern Alberta

• Author / Creator

  Welegedara, Nilusha

• Success in establishing productive upland forests on landforms reconstructed using mining waste mainly depends on the cover materials and depth that determine water and nutrient availability for plant growth while mitigating the potential risks or limitations of the substrate that is being reclaimed. In this study, a process based ecosystem model, ecosys was used to forecast short and long-term effects of soil cover depth on soil moisture and nitrogen (N) availability, salinity and thereby plant productivity in different reclamation covers on a reclaimed overburden substrate landform which has elevated salinity and sodicity. The modelled outputs were tested against measured soil moisture content (, rooting depth, sap flow, leaf area, salt redistribution, soil and foliar N concentrations, plant biomass production with three soil cover depths differing in thickness (35, 50, and 100 cm). The study site was a 17-year-old forest reclamation site on a slope constructed on saline sodic overburden material, capped with a cover soil, and planted with trembling aspen and white spruce in the Athabasca Oil Sands Region, Fort McMurray, Alberta, Canada. The modelled changes in soil moisture, rooting depth, tree water-use, salt redistribution, soil and foliar N concentration and aboveground tree biomass with soil cover depth followed the same trends as independent measurements. Greater , plant water-use, N mineralization, N uptake and consequently greater aboveground biomass were modelled in 100 cm cover vs. 35 cm and 50 cm covers particularly during dry and intermediate years, after the sites had reached over-story crown closure (2011 - 2015). However, a clear relationship between root zone salt concentrations, driven by upward salt migration from underlying overburden, and aboveground biomass growth was not apparent. The relative limitations on net primary productivity (NPP) from water vs. nutrient uptake depended on slope position in the reclaimed landscape. Thus lower plant productivity modelled in upper slope positions was mainly controlled by  rather than N availability and salinity.Modelled NPP increased linearly with modelled transpiration (R2 > 0.9) and N uptake (R2 > 0.8) that increased with AWHC and total soil nitrogen (TN) that in turn increased linearly with cover depth. However, non-linear relationships between transpiration and AWHC and between tree N uptake and TN indicated that there is a threshold cover depth (100 cm for the current study) where further increases in AWHC and TN would have little effect on NPP, according to site conditions. After running ecosys with seven hypothetical covers in addition to constructed landforms (35 cm, 50 cm and 100 cm), results also indicated that there is little effect of cover depths greater than 100 cm on plant growth. The 100 cm cover achieved NPP similar to that of natural boreal mixed-wood forests of the region earlier than did the 35 cm and 50 cm covers. However the long-term (100 yrs.) modelling without any ecosystem disturbances indicated that all the reclamation covers reached a similar NPP (~400 g C m-2 y-1) during wet years after ca. 25 years from start of reclamation, comparable to the NPP modelled for a similar age regenerating natural site after stand replacing fire within the region. Long-term modelling (1999 - 2099) with climate change (RCP 8.5) increased NPP in the 100 cm cover (22%) more than in the 50 cm cover (15%) and the 35 cm cover (14%) because NPP in the shallower covers declined during dry years due to lower AWHC. Also aspen vs. white spruce growth tended to increase with increasing cover depth and climate change due to improved water and N uptake associated with greater AWHC as well as N mineralization from increased total soil nitrogen. However, the NPP gains modelled in all the reclaimed sites under warming climate were lower than that of the regenerating natural site (45%) due to decreased soil nutrient availability, particularly phosphorus, over time. Overall, this study demonstrates the importance of cover soil depth as a modulator of soil available moisture, nutrient availability and salt redistribution particularly during early forest development. It also demonstrated that a terrestrial ecosystem model such as ecosys can be a useful tool in forecasting short and long-term hydrological patterns, salt redistribution, N cycling, NPP and thereby land capability for reclamation soil covers of different depths and properties.

• Subjects / Keywords

  • Land reclamation
  • Soil cover depth
  • Plant productivity
  • Plant growth
  • Salt redistribution
  • climate change
  • South bison hills
  • Soil moisture content
  • Nitrogen availability
  • Northern Alberta

• Graduation date

  Spring 2019

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-kc1w-qb63

• 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.