Significant amounts of SO2 and NOx have been emitted from the Athabasca oil sands region (AOSR) in Alberta, Canada, in the past several decades. The impact of acid deposition on forest ecosystems and the risk of soil acidification were assessed in jack pine (Pinus banksiana) and trembling aspen (aspen) (Populus tremuloides) stands in acid-sensitive watersheds (NE7 and SM8) in the AOSR. NE7 has been exposed to greater rates of deposition. A simulated N and S deposition experiment was also conducted. Nitrogen deposition increased N availability in NE7, indicated by increasing Diff_N (Diff_x: the difference of parameter x in tree rings between NE7 and SM8) and decreasing Diff_δ15N over time in aspen stands. Sulfur deposition increased in stemflow due to interception deposition, which decreased pH and base cations in soil towards jack pine but not towards aspen releasing enough base cations through canopy leaching. The Ca/Al ratio did not reach the critical limits of 1.0 for soil solution (range: 1.0 to 4.1) or 0.5 for fine roots (range: 0.7 to 7.9) while Al concentrations in soil solutions (range: 0.1 to 8.5 mg L-1) achieved the level inhibiting seedling growth of aspen and Picea glauca (a common species succeeding aspen). Critical loads of S deposition ranged from 223 to 711 molc ha-1 yr-1, and S deposition did not exceed critical loads; N deposition was not considered due to N limitation in boreal forests. Exceedances were underestimated when only bulk deposition was considered as compared to those that use total deposition because intercepted SO42- deposition made up approximately 60% of total deposition. The H+ budgets of soils were negative in NE7 and SM8, implying that soils were recovering from previous acidification, reflecting the reducing trends of S emission. Simulated N deposition provided beneficial effects on tree growth but not for understory. Simulated S deposition increased leaching loss of base cations and reduced exchangeable base cations in the surface soil. I concluded that acid deposition has changed soil and tree chemistry, and tree growth in the AOSR, which could give adverse effects on understory plants sensitive to Al toxicity and cationic nutrient deficiency.