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Abstract: Unfavorable conditions of reclaimed soils for vegetation establishment have been a concern for successful oil sands reclamation. Growth limitations of planted trees have been observed in the Athabasca Oil Sands Region (AOSR) in Alberta, Canada, but the factors limiting tree growth were poorly understood. This thesis research identified limiting factors for growth of lodgepole pine (Pinus contorta, Pl) and white spruce (Picea glauca, Sw) in reclaimed sites in the AOSR and tested these identified limiting factors through manipulative experiments. In a field study, lodgepole pine growth was found to be significantly correlated with water availability and Sw growth was positively correlated with soil nitrogen (N) concentrations and negatively correlated with salinity in the reclaimed soil, suggesting growth limitations of low water availability for Pl, and low N availability and high soil salinity for Sw. Both N (200 kg N ha-1) and complete fertilizer (200 kg N, 20 kg P and 19 kg K per hectare plus micronutrients) similarly increased the height, diameter at breast height (DBH) and aboveground biomass growth for Sw, but not for Pl. Foliar N concentration, but not other nutrients, in needles of Sw increased after fertilization, confirming the N limitation for the growth of Sw. Among the unfavorable conditions, the effects of soil salinity on N cycling in a reclaimed soil were evaluated in a laboratory incubation study and the interactive effects of N and salinity on Pl and Sw seedling growth were studied in a greenhouse experiment. The short-term incubation experiment indicated that soil salinity significantly reduced gross N mineralization, nitrification, and N immobilization rates, but those rates were not completely inhibited even under high salinity level (with soil electric conductivity of 8 dS m-1). Gross N immobilization rates in all salinity levels were greater than gross N mineralization rates, resulting in negative net N mineralization rates, which could partly explain low N availability in saline reclaimed soils in the studied sites. However, the greenhouse experiment demonstrated the effectiveness of N fertilization in improving the growth of seedlings in a saline growth media, in which N fertilization significantly increased net photosynthetic rates and N concentrations in needles and roots, resulting in improved seedling height and root collar diameter growth and total seedling biomass for both Pl and Sw seedlings. The N fertilization aggravated the inhibitory effects of soil salinity on seedlings treated with high salinity level (150 mmol L-1 NaCl). The research findings will help develop proper management practices to correct growth limiting factors in reclaimed sites with low nutrient availability, some of those sites may be affected by high salinity levels, and thus ensuring the success of reclaiming disturbed lands in the AOSR.